1. No category

ON-LINE SYSTEMS FOR A U T O M A T E D S A M P L E INTRODUCTION
IN A T O M I C S P E C T R O M E T R Y
by
L E S L I E JOHN PITTS
BSC F I A P
CRSC
A thesis submitted to the University of Plymoutli
in partial fulfilment for tlie degree of
DOCTOR O F PHILOSOPHY
Department of Environmental Science
Faculty of Science
In collaboration \vitli
PS Analytical Ltd
November 1995
LIBRARY STORE
REFERENCE OMLY
to No- [qQo345710o
2 6 F E B 1998
Date
Class No.
Contl.No,
90 0345710 0
ON-LINE SYSTEMS FOR
AUTOMATED SAMPLE INTRODUCTION IN ATOMIC
SPECTROMETRY
LESLIE JOHN PITTS
ABSTRACT
A u t o m a t i o n has become i n c r e a s i n g l y common i n most a r e a s o f
human e n d e a v o u r . I n t h e f i e l d o f a n a l y t i c a l
chemistry,
p a r t i c u l a r a t t e n t i o n has been p a i d t o o n - l i n e s y s t e m s w h i c h
o f f e r h i g h sample t h r o u g h p u t . Among o t h e r a t t r i b u t e s , s u c h
s y s t e m s a l s o o f f e r a d v a n t a g e s i n t e r m s o f ease o f a u t o m a t i o n
and c o u p l i n g t o a v a r i e t y o f a n a l y t i c a l d e t e c t i o n s y s t e m s .
T h i s t h e s i s d e s c r i b e s a number o f a u t o m a t e d a p p r o a c h e s t h a t
have been developed t o f o m e i t h e r complete systems, o r u n i t s
w h i c h may be i n c o r p o r a t e d i n t o h y p h e n a t e d t e c h n i q u e s .
Two a u t o m a t e d s p e c i a t i o n s y s t e m s i n c o r p o r a t i n g a m i c r o w a v e
r e d u c t i o n s t e p f o l l o w e d by h y d r i d e g e n e r a t i o n have been
d e v e l o p e d and e v a l u a t e d . The f i r s t i s a c o n v e n t i o n a l s y s t e m
employing
atomic a b s o r p t i o n d e t e c t i o n , w h i l s t t h e second
s y s t e m employs h i g h p e r f o r m a n c e l i q u i d c h r o m a t o g r a p h y t o
separate
the
species
prior
to
detection
by
atomic
f l u o r e s c e n c e . S e l e n i u m has been u s e d t o e v a l u a t e b o t h s y s t e m s ,
t h e l a t t e r o f f e r i n g d e t e c t i o n l i m i t s o f 0.2 and 0.3 ng ml~^ f o r
s e l e n i u m ( I V ) and s e l e n i u m ( V I ) r e s p e c t i v e l y .
K i n e t i c s t u d i e s on t h e r e d u c t i o n p r o c e s s o f s e l e n i u m ( V I ) t o
s e l e n i u m ( I V ) were c a r r i e d o u t . The r e d u c t i o n t o o k 6 m i n u t e s
a t 70** C, w h i c h i s f a s t e r t h a n p r e v i o u s l y r e p o r t e d i n t h e
literature.
Thus t h e use o f o n - l i n e s y s t e m s e m p l o y i n g
a
r e d u c t i o n step are supported f o r t h i s element.
P h o t o l y s i s was a l s o e v a l u a t e d as a means o f t r a n s f o r m i n g
o r g a n o - m e t a l l i c s p e c i e s o n - l i n e . The r e s u l t s i n d i c a t e d t h a t
p h o t o l y t i c t r e a t m e n t o f s e l e n o - m e t h i o n i n e may be e f f e c t i v e i n
c l e a v i n g t h e c a r b o n - s e l e n i i a m bond, and p r o v i d e t h e b a s i s f o r
an o n - l i n e s y s t e m t o p r e t r e a t o r g a n o - s e l e n i u m
compounds.
O n - l i n e p y r o l y s i s f o l l o w e d by p r e - c o n c e n t r a t i o n on a g o l d t r a p
was
employed t o determine
mercury.
This
technique
was
i n c o r p o r a t e d i n t o a system employing
gas
chromatographyp y r o l y s i s - a t o m i c f l u o r e s c e n c e and u s e d t o d e t e r m i n e a r a n g e
o f m e r c u r y s p e c i e s . I n a d d i t i o n t h e a p p r o a c h was a l s o u s e d t o
d e t e r m i n e m e r c u r y i n s e d i m e n t s a m p l e s . Good a g r e e m e n t
was
o b t a i n e d w i t h two c e r t i f i e d r e f e r e n c e m a t e r i a l s (NIST 164 6 and
NIST 8406.
An a u t o m a t e d o n - l i n e pH a d j u s t m e n t s y s t e m was a l s o d e s i g n e d
and d e v e l o p e d . T h i s d e v i c e f e a t u r e d t h e use o f ammonia gas t o
avoid
c o n t a m i n a t i o n o f samples p r i o r t o t r a c e a n a l y s i s .
F i n a l l y , a number o f f u r t h e r a p p l i c a t i o n s o f t h e above s y s t e m s
are discussed
and
a number o f avenues f o r f u t u r e
work
suggested.
ACKNOWLEDGEMENTS
I am g r a t e f u l t o a number o f p e o p l e a n d o r g a n i s a t i o n s who h a v e
all
assisted i nthe preparation o f t h i s
contained
Dr.
t h e s i s and t h e
work
therein.
Steve H i l l ,
D i r e c t o r o f Studies,
f o r his continual help,
s u p p o r t , a s s i s t a n c e , sense o f humour, a n d f o r p u t t i n g u p w i t h
me f o r t h e l a s t
three
Professor
Worsfold
Paul
years.
f o r a l l h i s help,
guidance and
a p p r o a c h a b i l i t y i n h i s r o l e as s e c o n d s u p e r v i s o r .
All
the s t a f f
i n b o t h t h e c h e m i s t r y department and t e c h n i c a l
s e r v i c e s , w i t h very p a r t i c u l a r thanks
t o Mr I a n D o i d g e .
The E n g i n e e r i n g a n d P h y s i c a l S c i e n c e s R e s e a r c h C o u n c i l a n d t h e
sponsoring
support
company PS A n a l y t i c a l
Ltd.,
for their
financial
u n d e r t h e CASE a w a r d scheme, w i t h s p e c i a l t h a n k s t o
Professor
Peter
S t o c k w e l l a n d Dr.
Finally,
t o my w i f e
Hannah,
Holly
understanding
and
L i z a n d my d a u g h t e r s
Verity,
and s a c r i f i c e
complete t h e work.
Warren Corns.
without
I would
have
Rebecca,
who's
been
Laura,
patience,
unable t o
CONTENTS
Page number
Copyright statement
1
Author's d e c l a r a t i o n
2
T i t l e page
3
Abstract
4
Acknowledgements
5
Contents
6
List
of figures
11
List
of tables
15
Chapter 1 -
Introduction
16
1.1
-
Why
automate?
18
1.2
-
What i s a u t o m a t i o n
20
1.3
-
From s a m p l i n g s i t e
1.4
-
Batch versus continuous f l o w
24
1.5
-
Outline of thesis
26
Chapter 2 -
Hydride
to results
22
generation,instrumentation
and r e a g e n t s
28
2.1
-
Hydride generation
29
2.1.1
-
I n t e r f e r e n c e s and l i m i t a t i o n s
31
2.1.2
-
Speciation
studies
differential
employing
reduction
33
2.1.3
-
Instrumentation
35
2.1.3.1
-
Hydride generator
35
2.1.3.2
-
Detectors
37
2.1.3.3
-
Microwave h e a t i n g system
38
2.1.3.4
-
Photolysis source
40
2.1.3.5
-
P y r o l y s i s oven
40
2.1.3.6
-
Mercury p r e - c o n c e n t r a t i o n u n i t
44
2.2
-
Background and t h e o r y b e h i n d t h e
choice
o f d e t e c t i o n systems
46
2.2.1
-
Atomic absorption spectrometry
46
2.2.2
-
Atomic fluorescence
52
2.2.3
-
Atomic emission
2.2.4
-
I n d u c t i v e l y c o u p l e d plasma-mass
2.3
-
spectrometry
spectrometry
59
spectrometry
63
Reagents
68
Chapter 3 -
On-line
hydride
systems
69
3.1
-
Introduction
3.2
-
An i n v e s t i g a t i o n i n t o a p p a r e n t
70
signal
enhancement d u e t o n i t r i c a c i d when
determining
selenium
by h y d r i d e
generation
73
3.2.1
-
Background
73
3.2.2
-
Experimental
75
3.2.3
-
Further examination
3.3
-
Conditioning of hydride
o f t h e pump t u b i n g
generation
systems
3.4
-
Development
79
84
o f a microwave r e d u c t i o n
system
90
3.4.1
-
Design c r i t e r i a
92
3.4.2
-
P r i n c i p l e s o f microwave h e a t i n g
94
3.4.3
-
The b a s i c s y s t e m o n - l i n e m i c r o w a v e
system
3.5
-
97
Incorporation into
a fully
automated
system
109
3.5.1
-
Data h a n d l i n g
114
3.6
-
Speciation studies
115
3.7
-
An i n v e s t i g a t i o n i n t o
of
the kinetics
the reduction o f selenium(VI) t o
selenium(IV)
122
3.7.1
-
Experimental
123
3.7.1.1
-
Instrumentation
127
3.7.1.2
-
Reagents
127
3.7.1.3
-
Method
128
3.7.2
-
Results
129
3.7.3
-
Discussion
130
3.8
-
Photolysis
134
3.8.1
-
Experimental
135
3.8.2
-
Discussion
136
3.9
-
Summary
139
Chapter 4 -
Applications of on-line pyrolysis
141
4.1
-
Introduction
142
4.2
-
Experimental
14 4
4.2.1
-
The d e t e r m i n a t i o n o f m e r c u r y i n
sediment
4.2.2
-
147
The a t t e m p t e d d e t e r m i n a t i o n o f
mercury i n s i l v e r n i t r a t e
4.2.3
-
The a t t e m p t e d d e t e r m i n a t i o n o f
8
148
m e r c u r y i n human h a i r
4.2.4
-
150
The a t t e m p t e d d e t e r m i n a t i o n o f
mercury i n o i l
4.3
-
152
Mercury determinations employing
gas c h r o m a t o g r a p h y c o u l p e d t o t h e
pyrolysis
4.4
-
system w i t h
atomic
fluorescence detection
152
Summary
161
Chapter 5 -
Automated
pH a d j u s t i n g
system
162
5.1
-
Introduction
163
5.1.1
-
Theory
164
5.2
-
Development
5.2.1
-
Design o f t h e v a l v i n g
5.2.2
-
Development
5.2.2.1
-
The a n a l o g u e s y s t e m
5.2.2.2
-
S e t t i n g up p r o c e d u r e o f t h e a n a l o g u e
o f t h e system
166
170
of the electonics
system
173
175
system
179
5.2.3
-
The c o n t r o l s y s t e m
181
5.2.4
-
The i n t e r f a c e e l e c t r o n i c s
187
5.2.5
-
Design o f t h e r e a c t i o n
191
5.3
-
Evaluation
193
5.4
-
Discussion
201
5.5
-
Summary
202
C o n c l u s i o n s and f u r t h e r work
203
Chapter
6 -
vessel
6.1
-
Conclusions
204
6.2
-
F u r t h e r work
209
References
213
Appendix 1
223
C o n f e r e n c e s and c o u r s e s a t t e n d e d
225
Publications
227
Presentations
228
10
L I S T OF FIGURES
Page number
2.1
-
Diagram o f a f l o w
injection
hydride generator
2.2
-
36
Diagram o f t h e mercury
fluorescence detector
39
41
2.3
-
Diagram o f t h e microwave system
2.4
-
Diagram o f t h e l a y o u t o f t h e h i g h power
p h o t o l y s i s source
42
43
2.5
-
Diagram o f t h e p y r o l y s i s oven
2.6
-
Diagram o f t h e mercury p r e concentration unit
2.7
-
The o p t i c a l
45
layout of a
typical
atomic absorption spectrometer
48
50
2.8
-
D i a g r a m o f a h o l l o w c a t h o d e lamp
2.9
-
D i a g r a m o f an
electrodeless
d i s c h a r g e lamp
2.10
-
51
Transitions involved i n the
fluorescence processes
2.11
-
53
Diagram o f a b o o s t e d d i s c h a r g e
h o l l o w c a t h o d e lamp
2.12
-
Diagram o f a t y p i c a l
55
non-dispersive
atomic fluorescence spectrometer
2.13
-
Diagram o f t h e i n d u c t i v e l y
coupled
plasma t o r c h
2.14
-
57
61
Block diagram o f a t y p i c a l
11
ICP-MS
64
2.15
-
L a y o u t o f a q u a d r u p o l e mass f i l t e r
3.2.1
-
Diagram o f t h e b a s i c
generation-quartz
absorption
3.2.2
-
Diagram
-
hydride
furnace
atomic
d e t e c t i o n system
74
showing t h e e f f e c t o f s o l u t i o n
preparation
3.2.3
o n o b s e r v e d enhancement
-
80
Diagram o f t h e s i g n a l d r i f t
against
time
3.4.1
-
87
Diagram o f t h e b a s i c
on-line
microwave
r e d u c t i o n system
heating
98
3.4.2
-
Initial
3.4.3
-
Diagram o f a n o n - r e t u r n
3.4.4
-
Diagram
-
c o i l designs
100
valve
102
showing t h e c o n s t r u c t i o n
of heating,
3.4.5
Comparison
details
r e a c t i o n and c o o l i n g c o i l s
between microwave
i n j e c t i o n and c o n v e n t i o n a l l y
-
digested
108
Computer c o n t r o l l e d a u t o m a t e d
microwave
r e d u c t i o n s e l e n i u m s p e c i a t i o n system
110
112
3.5.2
-
V a l v e s w i t c h i n g and t i m i n g d i a g r a m
3.6.1
-
D i a g r a m o f t h e a u t o m a t e d HPLC-microwave
reduction-hydride
generation-AFS
selenium s p e c i a t i o n system
3.7.1
-
106
flow
standards
3.5.1
77
P h o t o m i c r o g r a p h s o f i n n e r b o r e o f pump
tubings
3.3.1
66
Typical analysis
hydride
trace obtained
generation
12
117
operating
by
under
equilibrium conditions
3.7.2
-
Trace o b t a i n e d
generation
employing
coupled
fluorescence
124
hydride
t o atomic
d e t e c t o r w i t h a one
second
sample i n j e c t i o n t i m e
3.7.3
-
Plot of fluorescence
against
3.7.4
-
time
126
detector
f o r various temperatures
P l o t o f I n ( o u t p u t maximum against
output
output)
time
131
3.7.5
-
Plot of l n ( k ) against
3.8.1
-
Plot of selenium
a g a i n s t UV
1/T
132
concentration
exposure time
for
detected
seleno-
methionine
3.8.2
-
129
137
P l o t of selenium
concentration
detected
a g a i n s t UV
exposure time
for
seleno-
methionine
to i l l u s t r a t e
reaction rate
138
4.1
-
Basic p y r o l y s i s system
145
4.2
-
Coupled GC-Pyrolysis-AFS system
154
4.3
-
E f f e c t o f column temperature
155
4.4
-
E f f e c t o f c a r r i e r gas
156
4.5
-
E f f e c t o f s a m p l e gas
flow rate
157
4.6
-
E f f e c t o f s h i e l d gas
flow rate
158
4.7
-
Response o f e t h y l m e r c u r y
flow rate
and
d i m e t h y l mercury
160
5.1.1
-
Typical combination
5.2.1
-
Process o v e r v i e w
169
5.2.2
-
Valving diagram
171
5.2.3
-
E l e c t r o n i c s overview
174
13
pH
electrode
167
5.2.4
-
Overview o f t h e analogue system
5.2.5
-
Analogue,
circuit
t i m e r s and l e v e l
176
detector
diagram
177
5.2.6
-
Logic c o n t r o l c i r c u i t
diagram
5.2.7
-
Interface electronics
circuit
5.2.8
-
First
5.2.9
-
Final reaction
5.3.1
-
Additional
183
diagram
bubble-pump v e s s e l d e s i g n
vessel design
circuitry
192
194
t o improve
c o n t r o l o f pH and sample
14
189
throughput
199
L I S T OF TABLES
Page number
3.3.1
-
Instrument operating
during
3.4.1
-
-
-
of hydrochloric
-
acid
Analysis
o f CRM
Analysis
coils
107
NIST 1643c -
o f selenium
113
o f a mixed selenium
standard
(5 ng ml"' S e ( I V ) a n d S e ( V I ) )
3.6.2
-
Optimized operating
conditions
119
f o r the
complete system
4.1
-
121
Determination o f mercury i n c e r t i f i e d
reference sediments
4.2
-
149
Instrument conditions
f o r GC-pyrolysis-
atomic fluorescence
5.2.1
-
Initial
5.2.2
-
S e t t i n g up p r o t o c o l
159
valve switching
regime
172
f o r t h e analogue
system
5.2.3
-
180
Reproducibility of the level
detection
s y s t e m s h o w i n g t h e mass o f
w a t e r sampled
5.3.1
-
104
Table o f dimensions f o r t h e heating,
determination
3.6.1
88
Absorbance r e a d i n g s f o r i n c r e a s i n g
r e a c t i o n and c o o l i n g
3.5.1
employed
hydride apparatus c o n d i t i o n i n g
concentrations
3.4.2
conditions
195
A d j u s t e d pH o f 1 % n i t r i c
under automatic c o n t r o l
15
acid
samples
200
Chapter I
Introduction
16
1.0
INTRODUCTION
Over r e c e n t y e a r s , many f a c t o r s h a v e c o n t r i b u t e d
for
an
ever i n c r e a s i n g
carried
out
quality
control
products
in
and
t o the
niamber o f r o u t i n e c h e m i c a l t e s t s t o
laboratories
checks
waste
on
of
many
incoming
effluent
government r e g u l a t i o n s
raw
all
a r e t o be met
types.
and
disease,
and
how
outgoing
monitoring
competitive
each i s a f f e c t e d
p a r t i c u l a r e l e m e n t o r compound, more and
need
to
be
government
obvious.
is
carried
and
other
Since the
often
pushed
out.
The
need
regulatory
sensitivity
to
the
of
limits,
better
t o keep p a c e w i t h demand. T h i s
has
to
a
demand
for
the
about
traces
of
a
tests
analyses
laboratories
analytical
required
led
more
body's
the
by
i f
viability
more s e n s i t i v e
for
be
industry,
materials,
require
and
In
i s t o be m a i n t a i n e d . I n m e d i c i n e , as more i s d i s c o v e r e d
health
need
in
is
techniques
instrumentation
is
increase i n workload
development
of
more
automated
systems.
Fortunately
coincided
f o r t h e i n s t r u m e n t m a n u f a c t u r e r s , t h i s demand
w i t h enormous a d v a n c e s i n t h e f i e l d s o f
c o m p u t i n g , m e c h a n i c a l e n g i n e e r i n g and
areas
that
resources
the
which
instrument
in
turn
makers
have
i n s t r u m e n t s which are c u r r e n t l y
optics.
have
electronics,
I t i s i n these
invested
produced
has
the
massive
advanced
appearing. F i f t e e n years
ago,
o n l y t h e more a d v a n c e d i n s t r u m e n t s c o n t a i n e d , o r w e r e o p e r a t e d
through,
absorption
a
computer.
Now,
s p e c t r o m e t e r has
even
an
17
the
most
humble
on-board microcomputer
atomic
which
monitors
the operation o f the instrument,
and s l i g h t l y more
advanced i n s t r u m e n t s use microcomputer i n t e g r a t e d c i r c u i t s f o r
data
handling
as w e l l
as c o n t r o l l i n g
i n s t r u m e n t . Many l a r g e r i n s t r u m e n t s
the
ubiquitous
IBM Personal
completely operated
it
the operation o f the
interface directly
Computer o r s i m i l a r ,
Why
Throughout
and a r e
and c o n t r o l l e d by i t - i n many i n s t a n c e s
i s now t h e case o f no computer - no
1.1
with
instrument.
automate?
the world,
more
automated
systems
are
being
developed i n a l l areas o f human endeavour. I n some cases, i t
c o u l d be supposed t h a t these a r e i n t r o d u c e d t o a v o i d humans
b e i n g s u b j e c t e d t o dangerous environments, w h i l s t i n o t h e r s ,
i t c o u l d be argued t h a t a u t o m a t i o n
personnel
automation
avoids t h e need f o r s k i l l e d
t o do a b o r i n g r e p e t i t i v e j o b . I n t r u t h , almost a l l
i s i n t r o d u c e d p r i m a r i l y f o r f i n a n c i a l reasons. I n
t h e case o f say t h e n u c l e a r power i n d u s t r y , t h e f a c t t h a t an
automated r a d i o - a c t i v e f u e l r o d h a n d l i n g system does a v o i d t h e
need f o r a man t o e n t e r t h e h o s t i l e environment i n w h i c h t h e
rods a r e s i t u a t e d
system
were
i s a beneficial
available,
very
much
side
effect.
higher
I f no such
wages
would
be
demanded by t h e o p e r a t o r s who would have t o handle t h e rods,
and t h e r e would be e x c e p t i o n a l l y h i g h a d d i t i o n a l c o s t s i n such
t h i n g s as i n s u r a n c e p o l i c i e s , compensation payments and so on.
18
I t i s t r u e t h a t automated systems do a v o i d t h e r e q u i r e m e n t o f
people d o i n g b o r i n g and r e p e t i t i v e j o b s , b u t a g a i n t h e prime
motive
for introducing
the automation
is financial.
Bored
people are l i k e l y t o make m i s t a k e s and m i s t a k e s c o s t money.
It
is
also
true
that
for
i n s t a l l a t i o n o f an automated
the
company
system may
concerned,
have a h i g h
the
initial
c a p i t a l c o s t , b u t once purchased t h e system s h o u l d be a b l e t o
c a r r y out i t ' s
f u n c t i o n c o n t i n u o u s l y except f o r p e r i o d s s e t
a s i d e f o r maintenance.
and t a k e
U n l i k e people, who l i k e t o e a t , s l e e p
h o l i d a y s o c c a s i o n a l l y , machines w i l l
f o u r hours a day,
work t w e n t y
t h r e e hundred and s i x t y f i v e days a year.
They do n o t get i l l ,
do not argue w i t h t h e management and i n
many cases o f f e r improved p r e c i s i o n over a n a l y s e s c a r r i e d o u t
by p e r s o n n e l .
The
social
impact
of
mechanisation
p r e d i c t e d t o be b e n e f i c i a l t o a l l ,
drudgery
and
more f r e e
t i m e t o be
and
automation
was
p r o v i d i n g a r e l e a s e from
e n j o y e d by
p o p u l a t i o n . I n t h e f i e l d o f c h e m i s t r y , i t was
the general
suggested
that
a u t o m a t i o n would f r e e t h e chemist from r o u t i n e e v a l u a t i o n s and
enable him t o c a r r y o u t more i n t e r e s t i n g work^'^.
true
i n an
ideal
world, but
employers
are
T h i s may
not
be
primarily
concerned w i t h whether o r not t h e i r employees are d o i n g work
which i n t e r e s t s them, b u t whether t h e y are e a r n i n g p r o f i t s f o r
the company. An a l t e r n a t i v e view i s t h a t machines r e p l a c e t h e
more s k i l l e d and h i g h l y q u a l i f i e d workers, and t h u s downgrade
them t o t h e s t a t u s o f machine minders, w h i l s t t h e l e s s s k i l l e d
and l e s s q u a l i f i e d p e r s o n n e l are no l o n g e r r e q u i r e d a t a l l ^ .
19
Examples
of this
t h e o r y appear
t o be more
abundant
than
examples o f t h e former, n o t o n l y i n t h e area o f c h e m i s t r y , b u t
in
most
o t h e r areas
impact o f
this
mechanical
study
-
of industry.
Fortunately,
the social
advances a r e n o t w i t h i n t h e scope o f
advances
i n the f i e l d
of
automation
are
a b s o l u t e l y i n e v i t a b l e and i t i s t h e j o b o f governments and n o t
s c i e n t i s t s t o cope w i t h t h e e f f e c t s o f these changes.
1.2
What i s automation?
I t i s i m p o r t a n t t o d e f i n e e x a c t l y what i s meant by a u t o m a t i o n
and m e c h a n i s a t i o n when a p p l y i n g these terms i n t h e f i e l d o f
c h e m i s t r y . One o f t h e main r e g u l a t o r y b o d i e s i n c h e m i s t r y i s
the
I n t e r n a t i o n a l Union o f Pure and A p p l i e d Chemistry
and
it's definitions
are t h a t
(lUPAC)
" m e c h a n i s a t i o n i s t h e use o f
mechanical d e v i c e s t o r e p l a c e , r e f i n e ,
e x t e n d o r supplement
human e f f o r t " , and t h a t "automation i s t h e use o f c o m b i n a t i o n s
o f mechanical d e v i c e s t o r e p l a c e , r e f i n e , e x t e n d o r supplement
human e f f o r t
and f a c i l i t i e s
i n t h e performance
o f a given
process, i n which a t l e a s t one major o p e r a t i o n i s c o n t r o l l e d
w i t h o u t human i n t e r v e n t i o n , by a feedback
The
b e n e f i t s o f employing
automated
system".
and mechanised
systems
have a l r e a d y been touched on. A p a r t from t h e obvious f i n a n c i a l
and m a n a g e r i a l advantages
o f n o t h a v i n g t o employ people t o
c a r r y out r o u t i n e f u n c t i o n s , t h e r e are o t h e r v e r y considerable
b e n e f i t s i n terms o f accuracy and p r e c i s i o n . Since one o f t h e
20
most a c c u r a t e l y measurable dimensions
i s t h a t o f time, timed
processes may be c o n t r o l l e d t o whatever l e v e l o f accuracy may
reasonably
be r e q u i r e d . Most c u r r e n t l y
available
computers
have c l o c k speeds i n t h e range o f 25 - 120 MHz,
so d i r e c t
t i m i n g from these c l o c k s enables t i m i n g t o be c a r r i e d o u t t o
sub-microsecond
clocks
accuracy.
are available
V a s t l y more
to scientists
accurate
i f these
and f a s t e r
are r e q u i r e d ,
a l t h o u g h except f o r one o r two n o t a b l e e x c e p t i o n s such as i n
t h e areas o f p h o t o c h e m i s t r y and r a d i o c h e m i s t r y , p r e s e n t t i m i n g
from
enough,
and
c e r t a i n l y many o r d e r s o f magnitude b e t t e r t h a n a person
with
a
computer
clocks
i s generally
accurate
stopwatch.
The
advent
o f stepper
motors
and o t h e r
electromechanical
d e v i c e s have a l s o enabled mechanised and automated systems t o
deliver
more
accurately
and w i t h
measured q u a n t i t i e s o f chemicals
greater
repeatability,
and s o l u t i o n s t o r e a c t i o n
vessels, h o s p i t a l p a t i e n t s , e t c .
Such
has been
t h e pace
o f advance
i n these
areas,
that
chemists a r e becoming t o t a l l y r e l i a n t upon t h e i n s t r u m e n t , and
t h e r e i s a g r e a t danger t h a t w i t h o u t a d e t a i l e d u n d e r s t a n d i n g
o f t h e processes
reaction,
"smart
involved i n a particular determination or
e r r o r s o f g r e a t magnitude may be made. So c a l l e d
systems"
are
being
incorporated
into
various
m a n u f a c t u r e r s s o f t w a r e i n an a t t e m p t t o m i n i m i s e such e r r o r s ,
but
i t remains
understanding
vital
that
personnel
with
sufficient
o f what t h e i n s t r u m e n t i s a c t u a l l y d o i n g and
21
what i t ' s l i m i t a t i o n s a r e , s h o u l d have o v e r a l l c o n t r o l .
The
o f t quoted example o f t h e near n u c l e a r war s t a r t i n g due t o t h e
American r a d a r system d e t e c t i n g t h e r i s i n g moon and t h i n k i n g
i t was a massive a t t a c k f o r c e , s h o u l d be r e c a l l e d by a l l who
depend on t h e r e s u l t s o f i n s t r u m e n t s .
1.3
From san?>ling
s i t e to r e s u l t s
I n most cases t h e r e i s a s e t p a t h f o l l o w e d by a sample
it's
point
of
collection,
r e q u i r e d by t h e end user.
stages
through
The
to
the
automation
d i f f e r s i n c o m p l e x i t y and v i a b i l i t y ,
final
o f these
from
results
various
and i n g e n e r a l ,
i t has been i n t h e h a n d l i n g o f samples w i t h i n t h e l a b o r a t o r y
which
has
received
manufacturers.
most
attention
from
instrument
A t y p i c a l r o u t i n e f o l l o w i n g , f o r example, a
water sample from a r i v e r , f o r p o l l u t a n t a n a l y s i s may proceed
as shown below.
C o l l e c t i o n o f sample from r i v e r
of
sample t o s t a b i l i z e i t -
-
i n i t i a l on-site treatment
r e t u r n o f sample t o l a b o r a t o r y
s p l i t t i n g o f sample f o r o r g a n i c and i n o r g a n i c a n a l y s i s
sample p r e - t r e a t m e n t
Clearly,
the f i r s t
-
a n a l y s i s o f sample by i n s t r u m e n t ( s ) .
t h r e e stages
and o n l y when t h e sample
are d i f f i c u l t
t o automate,
reaches t h e l a b o r a t o r y , can
any
a u t o m a t i o n commence. A t t h e p o i n t o f e n t r y , t h e sample may be
bar-coded,
so
that
i t ' s progression
22
through
subsequent
processes
sample
may
may
be
throughout
the
be
monitored
carried
out
laboratory
automatically.
Splitting
automatically,
as
itself.
and
the
f i n a l presentation
w h i c h most a u t o m a t i o n w i l l be
Remote o n - s i t e and
more o f t e n
advances
to the measuring
have
The
power
been
system
being
with
the
of
advent
(CMOS) i n t e g r a t e d
with
capacity,
rapidly.
This
type
i n on-line
of
instrumentation
monitoring
is
combined
to
presently
of
circuits
this,
improvements i n b a t t e r y p e r f o r m a n c e and
to
on-site
w h i c h e x h i b i t v e r y low power consumption, and
developments
in
exploited
thanks mainly
requirements
reduced
complementary m e t a l o x i d e s i l i c o n
enabled
will
possible.
than p r e v i o u s l y p o s s i b l e , again
instrumentation
transport
i s required. I t i s in t h i s
o n - l i n e m o n i t o r i n g i s now
in electronics.
the
Sample p r e - t r e a t m e n t
v a r y , d e p e n d i n g on what i n f o r m a t i o n
area,
may
of
have
proceed
restricted
however, t o a l i m i t e d number o f p a r a m e t e r s o f i n t e r e s t
-
the
p o s s i b i l i t y of a remotely deployed i n d u c t i v e l y coupled plasmamass s p e c t r o m e t e r i s s t i l l
some way
remote m e a s u r i n g s y s t e m a r e
on-board
computer
computer,
laboratory.
or
are
for
either
subsequent
transmitted
Thus w h i l s t
o f f . The
there
kept
be
via
are
t r u e t h a t most d e t e r m i n a t i o n s
f o r some time, c a r r i e d out
a
radio
memory o f
to
a
link
be
an
portable
to
numerous a p p l i c a t i o n s
the
for
t h e answer, i t i s
a r e , and w i l l c o n t i n u e
i n the
23
i n the
downloading
w h i c h remote s e n s i n g and m o n i t o r i n g may
still
r e s u l t s from t h e
laboratory.
to
1.4
Batch versus continuous
flow
Once t h e sample has been r e c e i v e d , t h e a n a l y s t has t o d e c i d e
how b e s t t o proceed w i t h t h e a n a l y s i s . When s m a l l numbers o f
samples a r e b e i n g processed f o r a l i m i t e d number o f elements,
batch
processing
i s t h e main
method
advantage o f t h e t e c h n i q u e i s • t h a t
employed.
The
chief
i t a l l o w s t h e sample t o
remain as a d i s c r e t e e n t i t y t h r o u g h o u t t h e a n a l y t i c a l p r o c e s s ,
and so c r o s s c o n t a m i n a t i o n i s a v o i d e d . Small sample s i z e s a r e
a l s o more e a s i l y handled by t h e b a t c h method. Automated b a t c h
a n a l y z e r s a r e capable o f h i g h t h r o u g h p u t r a t e s , b u t a u t o m a t i o n
i s more d i f f i c u l t ,
mechanical
r e q u i r i n g r o b o t i c auto-samplers
and o t h e r
systems t o t r a n s p o r t t h e sample f r o m c o n t a i n e r t o
instrument.
The
i d e a o f c o n t i n u o u s f l o w systems was f i r s t
i n t r o d u c e d by
Technicon w i t h t h e i r A u t o a n a l y s e r - a d e v i c e i n which sample
solutions
and b l a n k s a r e s e p a r a t e d from one a n o t h e r by a i r
bubbles,
whilst
they
travelled
through
t u b i n g ^ . Reagents a r e added o n - l i n e ,
carried
out on-line,
detector.
Such
glass
and d e t e c t i o n
capillary
i s also
as t h e now c o l o u r e d sample passes a
systems a r e now r o u t i n e l y employed i n many
l a b o r a t o r i e s which handle l a r g e numbers o f samples which have
a s i m i l a r m a t r i x , and which r e q u i r e t h e d e t e r m i n a t i o n o f t h e
same
elements
o r compounds.
Continuous
flow
r e l a t i v e l y simple t o
automate.
the
c o n t a m i n a t i o n can occur,
fact
that
cross
systems a r e
They do however s u f f e r
from
a l t h o u g h by
a t t e n t i o n t o d e s i g n and o p e r a t i n g procedures, t h i s may u s u a l l y
24
be
kept
system
to a c c e p t a b l e l e v e l s .
usually
t a k e s the
The
output
form o f
from the d e t e c t o r
sharp peaks,
quantitative
d e t e r m i n a t i o n s b e i n g b a s e d on peak h e i g h t measurements.
Hybridised
i n s t r u m e n t s which
employ both
batch
and o n - l i n e
p r i n c i p l e s a r e now a p p e a r i n g , and o f f e r a d v a n t a g e s o v e r s i n g l e
method
systems.
An a l t e r n a t i v e t o t h e a i r segmented s y s t e m d e s c r i b e d above i s
flow
injection
precise
analysis
(FIA).
In
this
volume o f a n a l y t e i s i n j e c t e d
technique,
into
a
a moving
very
stream
which i s unsegmented by a i r b u b b l e s . T h i s t e c h n i q u e was
first
employed i n e l e c t r o c h e m i c a l d e t e r m i n a t i o n s ^ , i n which a n a l y t e
samples were m a g n e t i c a l l y s t i r r e d f o l l o w i n g i n t r o d u c t i o n i n t o
a moving s t r e a m . The method was
working
mixing
independently
of
induced
sample and
of
one
another^''',
reagent
dispersion,
and
l a t e r m o d i f i e d by two
c o u l d be
that
this
who
realised
accomplished
avoided
the
by
i t was
initially
thought
that
that
flow
excessive
d i l u t i o n r e q u i r e d when u s i n g t h e m a g n e t i c a l l y s t i r r e d
Whilst
groups
system.
a i r segmentation
was
n e c e s s a r y t o p r e v e n t d i s p e r s i o n , promote m i x i n g and c l e a n t h e
walls
of the c a r r y i n g
case^.
Advantages
include
higher
tubing,
obtained
throughput,
this
from
was
not
found
not
having
quicker response
t o be
the
segmentation
times,
shorter
s t a r t up and c l o s e down p e r i o d s , and g e n e r a l l y more s i m p l e and
flexible
equipment.
publications
There
dealing
with
i s now
the
25
a
c o n s i d e r a b l e number
technique
including
of
many
reviews'''^
1.5
O u t l i n e of t h e s i s
The
work d e s c r i b e d i n t h i s
thesis
falls
into
four
distinct
s e c t i o n s , r e l a t e d by t h e coimnon t h r e a d s o f a u t o m a t i o n and
line
on-
analysis.
Much work was
particular
c a r r i e d out on h y d r i d e g e n e r a t i o n systems w i t h
emphasis
on
the
study
of
selenium
speciation.
W h i l s t e x i s t i n g t e c h n i q u e s c o u l d be o p e r a t e d i n a p a r t i a l l y
o n - l i n e way,
a f u l l y o n - l i n e system has not been d e s c r i b e d .
Since p r e - r e d u c t i o n o f s e l e n i u m { V I ) i s r e q u i r e d when employing
h y d r i d e g e n e r a t i o n , t h e development o f a microwave
r e d u c t i o n system has enabled a number o f hyphenated
t o be produced
and
induced
techniques
e v a l u a t e d . These have proved t o be
u s e f u l and r e l i a b l e
f o r t h e d e t e r m i n a t i o n and
both
speciation of
selenium.
Since t h e d e t e r m i n a t i o n o f t r a c e organo-selenium compounds by
atomic s p e c t r o s c o p y r e l i e s upon t h e d e s t r u c t i o n o f t h e carbonselenium
bonds
and
g e n e r a t i o n , work was
consequential
analysis
c a r r i e d out t o examine t h e
by
hydride
possibility
o f employing o n - l i n e p h o t o l y s i s t o achieve bond cleavage. T h i s
work i s f u l l y d e s c r i b e d i n Chapter
26
3.
Considerable a t t e n t i o n
the
environment,
biogenic fate.
pyrolysis
i s currently
both
in
b e i n g p a i d t o mercury i n
i t ' s bioavailability
Chapter 4 c o n t a i n s d e t a i l s
and
pre-concentration
d e t e r m i n a t i o n o f t h i s element.
o f work
techniques
and i t ' s
employing
for
Such methods are o f
the
interest
s i n c e mercury i s a d i f f i c u l t element t o d e t e r m i n e , due t o t h e
extreme problems o f l o s s e s o f a n a l y t e and a l s o c o n t a m i n a t i o n .
Thus any system which reduces sample p r e p a r a t i o n w h i l s t
m a i n t a i n i n g good s e n s i t i v i t y i s t o be
still
considered.
The development o f an o n - l i n e pH a d j u s t i n g system i s d e s c r i b e d
i n Chapter 5. T h i s system was
pH
adjustment
analysis.
As
of a c i d i f i e d
such,
designed
t o p r o v i d e automated
aqueous samples, p r i o r
minimal
contamination
was
to trace
a
prime
r e q u i r e m e n t , and was achieved by u s i n g gaseous ammonia as t h e
adjusting
contaminants
reagent,
thus
avoiding
the
introduction
t o t h e sample from s o l i d o r l i q u i d
27
reagents.
of
Chapter 2
Hydride generation,
imirumentation
and reamnts
28
2.1
Hydride g e n e r a t i o n
I n 1969,
Holak^^ r e p o r t e d a t e c h n i q u e f o r t h e d e t e r m i n a t i o n
arsenic
by
the
detection
generation
using
an
of
atomic
arsine,
and
absorption
of
i t ' s subsequent
spectrometer.
Holak
employed m e t a l l i c z i n c and h y d r o c h l o r i c a c i d t o g e n e r a t e t h e
hydride,
which
was
introduction into
were
also
acid^^,
then
trapped
before
t h e s p e c t r o m e t e r . Other r e d u c t i o n
systems
investigated
and
magnesium,
cryogenically
including
aluminium
zinc
titanium(III)
h y d r o c h l o r i c a c i d " ' " . The
and
-
hydrochloric
chloride
-
r e a c t i o n i s shown below i n E q u a t i o n
1.
Equation 1
Zn
+
(n+2)
2 HCl
+
-*
E-^
-
2H*
EH„
(m and n may
be
A l l of the metal - acid reactions
and
this
before
+
ZnCl^
+
(excess)
equal)
s u f f e r e d f r o m b e i n g slow,
made i t necessary t o t r a p t h e
subsequent a n a l y s i s . A
regimes
have been
plastic
bags,
hydride
into
evaluated
pressurised
either
produced,
number o f d i f f e r e n t
including
rubber
containers^"''^^,
silver
diethyldithiocarbamate-
hydrides
nitrate
and
trapping
balloons^^'^^,
bubbling
solution^^,
or
ephedrine^°. However, these
the
silver
trapping
t e c h n i q u e s a l l s u f f e r e d from one d i s a d v a n t a g e o r a n o t h e r - f o r
example t h e
a c i d vapour degraded t h e
rubber from which
b a l l o o n s were made, e t c . W h i l s t c r y o g e n i c t r a p p i n g was
29
one
the
of
t h e more t r o u b l e f r e e t e c h n i q u e s
advantage
that
i t separated
hydrogen generated,
available,
the
hydride
i t a l s o had
from
the
the
excess
so p r e s e n t i n g a g r e a t e r c o n c e n t r a t i o n o f
the h y d r i d e t o t h e d e t e c t o r .
A f t e r t h e i n t r o d u c t i o n o f sodium b o r o h y d r i d e as t h e r e d u c i n g
medium, d i r e c t t r a n s f e r became t h e method o f c h o i c e , s i n c e t h e
reduction using t h i s
reagent
i s v e r y much f a s t e r t h a n w i t h
z i n c - h y d r o c h l o r i c a c i d . I t a l s o has
generates
these
hydrides
being
from
antimony,
a l l the
t h e advantages t h a t i t
hydride
arsenic,
forming
bismuth,
elements,
germanium,
lead,
selenium, t i n and t e l l u r i u m . I t a l s o has a s u p e r i o r r e d u c t i o n
y i e l d and s u f f e r s l e s s c o n t a m i n a t i o n o f b l a n k s . The r e a c t i o n
i s shown i n E q u a t i o n
2:
Equation 2
BH4-
+
(n + 2) H**
Since t h e use
trapping,
H*
+
+ 3 H2O
E"^
-
-*
EH„
(m and n may
be
H3BO3
+
enabled
8H*
(excess)
equal)
o f sodium b o r o h y d r i d e
i t has
H2
+
obviates
the need f o r
automated h y d r i d e g e n e r a t o r s
c o n s t r u c t e d , u s i n g p e r i s t a l t i c pumps^^ o r p r e s s u r i z e d
pumping systems^^.
Systems employing p e r i s t a l t i c
t o be
reagent
pumps
may
be more e a s i l y automated, s i n c e l e s s o p e r a t o r i n t e r f e r e n c e i s
required,
and
are o p e r a t e d
f l o w modes. The
pressurized
i n flow i n j e c t i o n or
reagent
pumping
continuous
systems
allow
g r e a t e r c o n t r o l over r e a c t i o n c o n d i t i o n s ' ^ , and are m a i n l y used
30
i n t h e b a t c h mode
2.1.1
Of
Interferences
and l i m i t a t i o n s
t h e two forms o f i n t e r f e r e n c e a f f e c t i n g
using
hydride
generation,
chemical
determinations
interferences
a r e more
p r o b l e m a t i c t h a n s p e c t r a l i n t e r f e r e n c e s . Since t h e a n a l y t e o f
i n t e r e s t i s detached from i t ' s m a t r i x , s p e c t r a l i n t e r f e r e n c e s
are r a r e l y encountered.
Chemical i n t e r f e r e n c e s do p r e s e n t many problems however, and
great
care
must be employed t o o b v i a t e
their
effects.
unexpected c h e m i c a l i n t e r f e r e n c e i n which t r a c e s
acid present
signal
i n the matrix
level during
An
of nitric
produced an enhancement i n t h e
selenium determinations
i s reported i n
Chapter 3.2. A l a r g e volume o f l i t e r a t u r e has r e p o r t e d on t h e
effects
of different
conflicting
part
interferences,
are o f t e n
views on t h e s e v e r i t y o f these e f f e c t s ,
t o t h e processes
conditions
but there
employed
involved
by
i n the hydride
individual
workers.
A
due i n
generation
number
of
c l a s s i c a l studies^*'" a r e however f r e q u e n t l y r e f e r r e d t o and
these p r o v i d e much i n f o r m a t i o n on i n t e r f e r e n c e s . A new work^°
contains
an o v e r v i e w o f most o f t h e p u b l i s h e d
subject o f hydride
apparently
generation,
conflicting
and c l e a r l y i l l u s t r a t e s many
r e s u l t s obtained
field.
31
work on t h e
by workers i n t h i s
The
causes
twofold.
of
The
interferent
chemical
first
interferences
is
the
are
thought
p r e f e r e n t i a l reduction
to
be
of
the
i o n e i t h e r t o a d i f f e r e n t o x i d a t i o n s t a t e or
t h e f r e e m e t a l . T h i s can
to
g i v e r i s e t o slow r e a c t i o n k i n e t i c s
f o r the species o f i n t e r e s t , c o - p r e c i p i t a t i o n , adsorption
of
the h y d r i d e or i t ' s c a t a l y t i c decomposition. I n t e r f e r e n c e s
due
t o t h e t r a n s i t i o n group m e t a l s f a l l i n t o t h i s c a t e g o r y . I t has
been suggested^'"^^ t h a t these i n t e r f e r e n c e s c o u l d be reduced by
t h e use o f a low c o n c e n t r a t i o n
with
a
high
postulated
concentration
t h a t the
high
o f sodium b o r o h y d r i d e combined
of
hydrochloric
acid concentration
acid.
It
would keep
is
the
i n t e r f e r i n g element i n s o l u t i o n , and i t would be reduced l e s s ,
thus a l l o w i n g
the
hydride forming r e a c t i o n
t o proceed more
favourably.
The
second
formation
flame,
cause
of
chemical
interference
i s due
to
o f compounds i n t h e r e l a t i v e l y c o o l argon-hydrogen
in
the
cases
of
atomic
absorption
and
atomic
f l u o r e s c e n c e d e t e c t i o n . T h i s mode o f i n t e r f e r e n c e e x p l a i n s
m u t u a l i n t e r f e r e n c e most h y d r i d e s have on one
A
number o f
precautions
to
avoid
the
problems
caused
have been suggested by d i f f e r e n t workers,
these may
summarised
be
as
v a r y i n g the c o n c e n t r a t i o n
hydrochloric
acid,
elements
the
the
use
of
standard
such
as
of
masking
iron,
32
agents,
mercury
etc.
by
and
addition,
r a t i o s o f t h e sodium b o r o h y d r i d e
use
the
another^**.
interferences
releasing
the
the
use
and
and
of
pre-
treatment
to
separate
the
analyte
from
the
interfering
compounds.
2.1.2
As
S p e c i a t i o n studies enploying d i f f e r e n t i a l reduction
more
has
been
p r o c e s s e s which
d i s c o v e r e d - about
occur i n nature,
the various
i t h a s become
r e c o g n i s e d t h a t t h e form i n w h i c h t h e element
a vital
of
role.
T h i s has important
and
t r a n s p o r t a t i o n o f t h e element
system
which
critically
Pb(IV),
bio-accumulation
through
the
ecological
i s presented^^.
Some o f t h e h y d r i d e
state,
i n many a r e a s
depends upon t h e p h y s i c o - c h e m i c a l form i n
t h e element
oxidation
increasingly
i spresent plays
implications
i n t e r e s t . The t o x i c i t y , b i o - a v a i l a b i l i t y ,
chemical
foinning e l e m e n t s
these being
As(III)
exist
i n more t h a n one
and A s { V ) ,
Pb(II)
and
S b d I I ) and S b ( V ) , S e ( I V ) and S e ( V I ) and T e ( I V ) and
T e ( V I ) . Even though sodium b o r o h y d r i d e p r o v i d e s a much f a s t e r
reduction
than
differences
zinc-acid,
i n the rate
there
are
of hydride
still
formation
noticeable
between t h e
d i f f e r e n t s p e c i e s . Indeed, t h e o x i d a t i o n s t a t e o f t h e a n a l y t e
p l a y s a c r u c i a l r o l e i n t h e f o r m a t i o n o f t h e hydride^^. I n t h e
cases
of selenium
and
tellurium
f o r example,
i t i s only
p o s s i b l e t o d e r i v e t h e h y d r i d e from t h e l o w e r o x i d a t i o n s t a t e ,
and
pre-reduction
content
be
used
must
be
i s t o be d e t e r m i n e d .
to
advantage
employed
i f the t o t a l
T h i s apparent
disadvantage
i n the determination
33
element
of
may
individual
s p e c i e s , by f i r s t a n a l y s i n g f o r t h e more reduced m o i e t y ,
and
then c a r r y i n g out a r e d u c t i o n s t e p and r e - a n a l y s i n g f o r t o t a l
c o n c e n t r a t i o n . The
c o n c e n t r a t i o n o f t h e more h i g h l y o x i d i s e d
form i s o b t a i n e d by d i f f e r e n c e .
I n t h e case o f a r s e n i c , t h e r e i s a two
conversion
of
stage process i n t h e
arsenic(V) t o arsine",
and
this
p o t e n t i a l which i s dependant upon pH.
At
a pH
has
a
redox
o f 4.5,
only
a r s e n i c ( I I I ) produces a r s i n e , subsequent r e d u c t i o n i n 5 mol 1'^
h y d r o c h l o r i c a c i d y i e l d s t o t a l arsenic. Again, arsenic(V)
be d e t e r m i n e d by d i f f e r e n c e . The
selective determination
antimony may
be c a r r i e d out i n t h e same way^*.
Thus i t may
be
species p r e s e n t
generation,
seen t h a t
the
i n a sample may
determination
be
of
i n the
literature^^"".
34
of
individual
achieved using
hydride
and t h e r e are a number o f examples o f t h e use
t h i s technique
may
of
2.1.3
Instrumentation
During t h e course o f t h i s s t u d y , a number o f instriaments have
been employed.
2.1.3.1
Hydride g e n e r a t o r
E a r l y work was c a r r i e d o u t u s i n g a c o n t i n u o u s
generator
and
(Model 10.003, PS A n a l y t i c a l L t d . , O r p i n g t o n , Kent)
t h i s was l a t e r upgraded t o a f u l l y
system
flow hydride
(Model 10.004,
s i m i l a r mechanically,
PS A n a l y t i c a l
computer
Ltd.).
controlled
Both u n i t s a r e
employing a p e r i s t a l t i c pump t o s u p p l y
r e d u c t a n t , a c i d b l a n k and sample v i a a s w i t c h i n g v a l v e t o a
g a s - l i q u i d s e p a r a t o r . The b a s i c l a y o u t o f t h e system i s shown
i n F i g u r e 2.1. The d i f f e r e n c e between t h e two models i s t h a t
i n t h e e a r l i e r model, t h e t i m i n g i s s e t from t h e f r o n t p a n e l ,
using r o t a r y
switches
which a l l o w o n l y a l i m i t e d
number o f
p r e - s e t t i m i n g p e r i o d s . The l a t e r model d e r i v e s i t ' s t i m i n g
from t h e computer s o f t w a r e , and has a much g r e a t e r f l e x i b i l i t y
over t h e range o f t i m i n g p e r i o d s
which may be s e t by t h e
o p e r a t o r . T h i s f a c i l i t y became e s s e n t i a l when u s i n g t h e atomic
f l u o r e s c e n c e d e t e c t o r i n " f l o w i n j e c t i o n " mode, as d e s c r i b e d
i n Chapter 3.7.
With b o t h
systems, t h e o p e r a t i o n i s t h e same and f o l l o w s a
f o u r stage p r o c e s s . The f i r s t stage i s t h e d e l a y , d u r i n g which
35
Fiquro
2 1
Diaqram o f a flow
injection
hydride
generator
\ .1(11
HI .nk
^ -KO^
BLANK
IK............
ViCOH C MItCI
O.Ai Kv>l.lilK-lvl
POSITION
C.\^I.I<|UKJ
S\-|>.U.»IOf
< A 1 \ pxM
Nihil
I ..IN l i t
MviiihijiK
Xicon C .unci
Koi.iitu-i.-i
SAMPLE
POSITION
C . V I H|IIUI
Svp.ll.llO*
36
t i m e t h e sample reaches t h e s w i t c h i n g v a l v e . T h i s i s f o l l o w e d
by t h e r i s e p e r i o d , d u r i n g which t h e s i g n a l reaches a steady
s t a t e . There t h e n f o l l o w s t h e a n a l y s i s t i m e , d u r i n g which t h e
d e t e c t o r makes measurements, and
finally
t h e decay p e r i o d ,
d u r i n g which t h e s i g n a l r e t u r n s t o t h e b a s e l i n e . Each o f these
p e r i o d s may
be
v a r i e d , depending upon t h e
r e a c t i o n b e i n g i n v e s t i g a t e d . The
kinetics
l a t e r model
of
the
(Model 10.004,
PS A n a l y t i c a l L t d . ) p r o v i d e s t o t a l v e r s a t i l i t y i n t h e c o n t r o l
o f these t i m i n g p e r i o d s .
The same g e n e r a t o r may be employed as a c o l d vapour g e n e r a t o r
f o r use i n t h e d e t e r m i n a t i o n o f mercury, a l t h o u g h a s l i g h t l y
d i f f e r e n t g a s - l i q u i d s e p a r a t o r i s t h e n recommended.
2.1.3.2
Detectors
The atomic a b s o r p t i o n s p e c t r o m e t e r used t h r o u g h o u t t h i s s t u d y
was a model SP 9 (ATI, Cambridge). The b a s i c t h e o r y r e g a r d i n g
atomic a b s o r p t i o n s p e c t r o m e t r y i s g i v e n i n S e c t i o n 2.2.1, and
the
functioning
of the
instrument
i s more f u l l y
described
there.
The
atomic
fluorescence
instrument
used
is
known
as
the
E x c a l i b u r (PS A n a l y t i c a l , O r p i n g t o n , K e n t ) . I t ' s o p e r a t i o n and
t h e o r y i s g i v e n i n more d e t a i l i n S e c t i o n 2.2.2.
37
The
mercury f l u o r e s c e n c e d e t e c t o r used t h r o u g h o u t
was
a Merlin
Kent).
This
the
(Model 10,023, PS A n a l y t i c a l L t d . ,
unit
i s shown i n F i g u r e
2.2,
and
study
02:pington,
features
a
f l u o r e s c e n c e measuring system which may be employed e i t h e r as
a s t a n d alone
computer
i n s t r u m e n t , o r as p a r t o f a f u l l y i n t e g r a t e d
controlled
analytical
system.
When
using
the
f l u o r e s c e n c e d e t e c t o r f o r t h e d e t e r m i n a t i o n o f mercury, h i g h
sensitivity
a
wide
w i t h r o u t i n e d e t e r m i n a t i o n s below 1 pg ml"^
linear
range
over
seven
orders
of
and
magnitude
are
available.
Finally,
an
spectrometer
L i b e r t y 200
AES
was
coupled
a l s o used. The
plasma-atomic
instrument
emission
employed was
( V a r i a n I n s t r u m e n t s ) and f u r t h e r d e t a i l s o f
ICP-
Microwave h e a t i n g s y s t e m
microwave h e a t i n g system employed was
(Prolabo,
Paris,
France).
This
unit
a Microdigest
employs
a
microwave beam, has f u l l power c o n t r o l i n 5% s t e p s and
of
a
i n s t r u m e n t s are g i v e n i n S e c t i o n 2.2.3.
2.1.3.3
The
inductively
the h e a t i n g p e r i o d from seconds t o h o u r s .
p r i m a r i l y designed
The
focused
control
system
f o r the d i g e s t i o n of i n d i v i d u a l
301
was
samples,
c o n t a i n e d i n d i s c r e t e tubes i n t h e microwave c a v i t y area. I n
t h e a p p l i c a t i o n s p r e s e n t e d i n t h i s t h e s i s , i t was o p e r a t e d
as
a c o n t i n u o u s f l o w h e a t i n g system, w i t h t h e h e a t i n g c o i l b e i n g
38
F i g u r e 2.2
Mercury f l u o r e s c e n c e
detector
Photo
Multiplier
Tubc(PMT)
Filler
0
Mv-'IClltA
Semite
S:Hiipk'
Out
Sheath
Gas O u t
Sheath
Gas
Sample
LcMS
AperiLiic
Iniroduciioii C l i u n n c v
39
&
Aperture
contained
i n a blank
tube
within
the
operated v i a a simple m u l t i f u n c t i o n
cavity
area.
keypad, on t h e
I t is
control
u n i t which houses t h e m i c r o p r o c e s s o r system and o t h e r c o n t r o l
electronics.
Figure
The
b a s i c system i s shown i n schematic
form i n
2.3.
2.1.3.4
Photolysis
The h i g h power UV
source
source used was
c o n s t r u c t e d i n house,
a diagram o f i t i s shown i n F i g u r e 2.4.
and
The diagram shows t h e
h o r i z o n t a l c r o s s - s e c t i o n t h r o u g h t h e i n s t r u m e n t . The UV
source
i s an a x i a l l y mounted 400 w a t t mercury d i s c h a r g e lamp,
with
the u n i t b e i n g c o o l e d by a h i g h f l o w r a t e f o r c e d a i r system.
2.1.3.5
P y r o l y s i s oven
The p y r o l y s i s oven employed was a model 10.550 (PS A n a l y t i c a l
L t d . , O r p i n g t o n , Kent) . The u n i t i s shown i n diagrammatic
form
i n F i g u r e 2.5. I t f e a t u r e s a v e r y r a p i d h e a t i n g c y c l e , p r e c i s e
c o n t r o l o f t e m p e r a t u r e , and i s f u l l y programmable. I t has been
engineered
tube
i n such a way
remaining
cool
t e m p e r a t u r e s o f 800
t h a t t h e ends o f t h e s i l i c a
to
the
^'C.
40
touch,
even
with
sample
sample
F i g u r e 2.3
Microwave h e a t i n g
system
MAIN UNIT
CONTAINING
POWER S U P P L Y
SAMPLE
TUBE
SHIELDING
WAVE
GUIDE
CONTROL UNIT
41
MAGNETRON
F i g u r e 2.4
Photolysis
source
3 0 0 mm
SAMPLE
TUBES
O
o
2 0 0 mm 1ong
25 mm I . D .
4 0 0 WATT U.V
SOURCE
o
SAMPLE
TUBES
42
3 0 0 mm
F i g u r e 2.5
Pyrolysis
oven
HEAT SINKS
HEATING ELEMENT
y\
v^s v \ v s
SILICA
SAMPLE
TUBE
PROGRAMMABLE HEATING
CONTROLLER
43
2.1.3.6
Mercury p r e - c o n c e n t r a t i o n
unit
The i n s t r u m e n t used was a Galahad (Model 10.500, PS A n a l y t i c a l
Ltd.,
Orpington,
Kent). This u n i t
f e a t u r e s the t r a p p i n g o f
mercury vapour on a g o l d - sand t r a p . T r a p p i n g may
f o r a p r o l o n g e d p e r i o d i f necessary,
take place
a f t e r which t h e mercury
i s r e l e a s e d from t h e amalgam formed by h e a t i n g . The
vapour i s t h e n swept from t h e t r a p
mercury
into
the d e t e c t o r i n a
stream o f c a r r i e r gas, u s u a l l y argon. The
l a y o u t i s shown i n
F i g u r e 2.6.
Other models o f mercury p r e - c o n c e n t r a t i n g u n i t s
are a v a i l a b l e from t h e same m a n u f a c t u r e r ,
computer
controlled
dedicated a n a l y t i c a l
valving
systems
requirements.
44
i n which
are
automatic
employed
for
F i g u r e 2.6
Mercury p r e - c o n c e n t r a t i o n
unit
S I L I C A WOOL PLUGS
^€ATER COIL
CON^ECTIONS
CO0L1M3 GAS OUTLET
SAMPLE tM-ET
SAMPLE OUTLET
COXING GAS INLET
0-RINC SEALS
COLO SATO
hCATING ELEMEhTT
45
2.2
Background and t h e o r y b e h i n d the c h o i c e o f d e t e c t i o n
systems
There are f o u r d e t e c t i o n systems which are commonly employed
in
conjunction
absorption
with
h y d r i d e g e n e r a t i o n . These
spectrometry
(AAS),
atomic
are
atomic
fluorescence
s p e c t r o m e t r y (AFS), i n d u c t i v e l y c o u p l e d plasma-atomic e m i s s i o n
spectrometry
(ICP-AES) and
inductively
c o u p l e d plasma-mass
spectrometry
(ICP-MS). Other t e c h n i q u e s have been used,
but
are now r a r e l y employed.
2.2.1
Atomic a b s o r p t i o n
spectrometry
T h i s t e c h n i q u e r e l i e s upon t h e a b s o r p t i o n o f e l e c t r o m a g n e t i c
energy, a t a wavelength s p e c i f i c t o t h e element concerned,
by
f r e e atoms o f t h a t element. Fraunhofer^^ n o t i c e d dark l i n e s i n
the
spectrum o b t a i n e d from s u n l i g h t ,
and
i n 1823,
measured
t h e i r wavelengths. He, and a number o f l a t e r astronomers were
o b s e r v i n g atomic a b s o r p t i o n l i n e s . A l t h o u g h t h e method
c o n s i d e r e d t h e o r e t i c a l l y p o s s i b l e f o r use i n
was
the laboratory,
i t was o f l i t t l e importance i n a n a l y t i c a l c h e m i s t r y due t o t h e
very
high
measurements.
O.OOlnm,
resolution
required
Monochromators
would
be
to
capable
required
to
make
of
isolate
a
quantitative
resolution
typical
of
atomic
a b s o r p t i o n l i n e s . These would be p r o h i b i t i v e l y expensive t o
produce,
even i f t h e i r p r o d u c t i o n were p o s s i b l e .
Thus t h e
a p p l i c a t i o n o f atomic a b s o r p t i o n s p e c t r o m e t r y f r o m a continuum
source was not f e a s i b l e .
46
In
1953, Walsh^^ achieved t h e b r e a k t h r o u g h
i n the form o f a
l i n e source, which enabled atomic a b s o r p t i o n s p e c t r o m e t r y t o
become t h e most w i d e l y used t e c h n i q u e f o r t h e d e t e c t i o n and
determination
of
metallic
and
metalloid
elements.
Walsh
r e a l i s e d t h a t by r e p l a c i n g t h e continuum source w i t h an atomic
s p e c t r a l source, t h e monochromator would o n l y be r e q u i r e d t o
f i l t e r o u t o t h e r l i n e s produced by t h e source
(e.g. f r o m t h e
lamp f i l l e r g a s ) , which would u s u a l l y be w e l l s e p a r a t e d
the
from
l i n e s o f i n t e r e s t . This p r o v i d e d t h e b a s i s f o r a p r a c t i c a l
atomic a b s o r p t i o n s p e c t r o m e t e r .
The l a y o u t o f a t y p i c a l i n s t r u m e n t i s shown i n F i g u r e 2.7. I t
consists
of
a
source,
electrodeless
discharge
monochromator
and
a
a
hollow
lamp
cathode
(EDL),
d e t e c t i o n system.
an
lamp
(HCL)
or
atom
cell,
a
The
source
emits
r a d i a t i o n c h a r a c t e r i s t i c o f t h e element i n q u e s t i o n , and t h i s
r a d i a t i o n i s d i r e c t e d i n t o t h e atom c e l l . T h i s may s i m p l y be
a
flame,
but
other
cells
have
been
developed
to
meet
p a r t i c u l a r r e q u i r e m e n t s . The purpose o f t h e atom c e l l i s t o
produce a c l o u d o f atoms i n t h e ground
ground
produce
energy
state.
These
s t a t e atoms t h e n absorb photons from t h e source,
a r e d u c t i o n i n t h e amount
o f energy
and
reaching the
d e t e c t o r when an a n a l y t e i s p r e s e n t . The l i g h t passes t h r o u g h
a monochromator, which n o r m a l l y has a r e s o l u t i o n i n t h e o r d e r
of
0.2
- 0.02
nm,
and on t o t h e d e t e c t o r . The d e t e c t o r i s
u s u a l l y a p h o t o m u l t i p l i e r tube, b u t t h e s u i t a b i l i t y o f s o l i d
s t a t e d e t e c t o r s i s s t a r t i n g t o be i n v e s t i g a t e d .
47
F i g u r e 2.7
The
optical
layout
of
a
typical
atomic
absorption spectrometer
DEUTERIUM
SOURCE
(FOR BACKGROUND
CORFECTIONI
HOLLOW CATHODE
LAMP
0
0
BURNER
SPRAY
CHAMBER
MONOCHROMATOR
O
PHOTOMULTIPLIER
TUBE
FUEL
OXIDANT
ELECTRONICS
UNIT (AMPLIFIERDISPLAY. CONTROL)
SAMPLE
INLET
48
Figures
2.8
cathode
and
lamp
respectively.
2.9
and
show
the
the
The
latter
construction of
electrodeless
type
of
lamp
the
hollow
discharge
lamp
offers
a
higher
i n t e n s i t y o f l i g h t o u t p u t , b u t are more complex t o d r i v e
and
construct,
the
and
t h e r e f o r e more
costly
to
utilise
in
i n s t r u m e n t . Since h i g h l i g h t o u t p u t i s not a r e q u i r e m e n t
atomic a b s o r p t i o n s p e c t r o m e t r y , t h e use
in
o f these lamps tends
t o be r e s t r i c t e d t o those elements f o r which h o l l o w cathode
lamps are u n a v a i l a b l e due t o poor
With
the
huge
improvements
in
performance.
the
production
of
optical
components which has o c c u r r e d s i n c e Walsh's o r i g i n a l work, t h e
use o f a continuum
the
work
possessing
has
very
source has been exploited^^'^^, b u t so f a r
been
high
restricted
resolution
to
research
laboratories
monochromators.
If
such
monochromators s h o u l d become a v a i l a b l e i n t h e f u t u r e a t v e r y
low c o s t , an i n s t r u m e n t o f f e r i n g m u l t i - e l e m e n t c a p a b i l i t y w i t h
background
correction
built
in
viable.
49
could
become
financially
F i g u r e 2.8
Diagram o f a h o l l o w c a t h o d e lamp
ANODE
ELECTRICAL +
—•
OUARTZ WINDOW
CONNECTIONS _
HOLLOW CATHODE
GLASS ENVELOPE CONTAINING Ne OR Ar
AT
1 - 5
50
TORR
Figure
2.9
Diagram o f a n e l e c t r o d e l e s s d i s c h a r g e lamp
C E R A M I C TUBE
O
QUARTZ
CO[L
4 Lur-ns R F - r-emolnder
t o o c L OS
BULB c o n t o 1 n 1 ng element, or- s o 1L
i n Ar oL
low
eorb-Ked
sci-cen
pi-essure
EARTHED BRASS TUBE
51
END
WINDOW
2.2.2
Atomic f l u o r e s c e n c e
Atomic
fluorescence
spectrometry
is a relative
newcomer t o t h e
list
of
t e c h n i q u e s a v a i l a b l e t o the a n a l y s t . As t h e name i m p l i e s , t h e
t e c h n i q u e r e l i e s upon l i g h t b e i n g absorbed and t h e n r e - e m i t t e d
by
the
atoms
of
concentrations,
the
interest
in
i n t e n s i t y of
the
the
analyte.
emitted
At
low
radiation
is
d i r e c t l y p r o p o r t i o n a l t o t h e c o n c e n t r a t i o n o f t h e a n a l y t e , and
to
the
radiant
power
of
the
source
at
the
analytical
wavelength. At h i g h c o n c e n t r a t i o n s o f a n a l y t e , s e l f a b s o r p t i o n
can
occur.
The
two modes o f f l u o r e s c e n c e o f i n t e r e s t are resonance
and
non-resonance. I n t h e former, t h e l i g h t i s e m i t t e d a t t h e same
w a v e l e n g t h as
t h a t used t o p r o v i d e e x c i t a t i o n ,
generally
more i n t e n s e
Figure
the
2.10
fluorescence
radiational
shows
the
processes"",
processes,
and
of
the
and
fluorescence
transitions
involved
with
lines
solid
dotted
lines
this is
processes.
in
both
representing
representing
non-
r a d i a t i o n a l ones. I n t h e n o n - r a d i a t i o n a l t r a n s i t i o n s , a s i n g l e
headed arrow i n d i c a t e s n o n - r a d i a t i o n a l d e a c t i v a t i o n , w h i l s t
a double headed arrow s i g n i f i e s a t h e r m a l a c t i v a t i o n
process.
When t h e e m i t t e d r a d i a t i o n i s o f a h i g h e r energy and t h e r e f o r e
a s h o r t e r wavelength,
t h e term A n t i - S t o k e s
Since t h e f l u o r e s c e n c e e m i s s i o n
the e x c i t a t i o n i n t e n s i t y ,
i s applied.
intensity i s proportional to
t h e need f o r a v e r y
i n t e n s i t y l i n e source i s apparent.
52
I t was
stable, high
t h i s f a c t o r which
F i g u r e 2.10
Transitions
involved
in
the
fluorescence
processes
71^
7K
7K
71s:
IK
INCREASING EhCRGY
NORMAL
DIRECT
RESONANCE
STEPWISE
Lire
Th«
solid
lines
indicoLo
pt~ocsss«s. On d o L L s d
dsecLlvoLion
ant I-SLoUss
rodiaLional
Imss. a smgla
end a d o u b l s hoadsd
I s sraplpysd
nCRMALLY
ASSISTED
RESONANCE
procsssos.
ThERMALLY
ASSISTED
ANTISTOKES
doLLod
h o a d s d ar-row
tinss
mdiceLss a
arrow, e LKornoI a c L l v o L l o n
v K s n t h e «aiiLL«d r a d i a t i o n
53
TVCRMALLY
ASSISTED
D I R E C T Llt-C
mdicato
GROLTO
STATE
non-rodisLiv«
non-radiaLional
procsss.
Tho Lorm
i s of a s h o r t s r w a v e l s n g t h .
delayed
t h e development o f t h e t e c h n i q u e . Continuum
do
provide
not
sufficient
r e g i o n o f t h e spectrum.
energy,
particularly
sources
i n the
UV
Xenon a r c lamps have been employed,
b u t problems o f s c a t t e r do occur. Moreover, t h e i n t e n s i t y over
the a b s o r p t i o n h a l f - w i d t h i s r e l a t i v e l y low when compared w i t h
l i n e sources, i n which a l l t h e r a d i a t e d energy i s c o n c e n t r a t e d
i n t h e few e m i s s i o n l i n e s o f
interest.
I n cases where s u i t a b l e vapour d i s c h a r g e
lamps e x i s t ,
they
have been employed w i t h success, b u t t h e range o f such lamps
is severely l i m i t e d .
The
c o n v e n t i o n a l h o l l o w cathode lamp as
absorption
spectrometers
do
not
provide
employed i n
atomic
sufficiently
high
i n t e n s i t y e m i s s i o n s , u n l e s s t h e y are p u l s e d . Boosted d i s c h a r g e
h o l l o w cathode lamps (BDHCL) have overcome t h e low
problem,
2.11.
and
an
emission
example o f such a lamp i s shown i n F i g u r e
I n these lamps, t h e i n t e n s i t y o f e m i s s i o n i s r a i s e d by
superimposing
a p o s i t i v e column d i s c h a r g e
across
the hollow
cathode discharge^^. T h i s p r o v i d e s a d d i t i o n a l e l e c t r o n s f o r t h e
h o l l o w cathode plasma, e n s u r i n g t h a t most o f t h e atoms are
excited,
thus i n c r e a s i n g r a d i a t e d output w i t h o u t i n c r e a s i n g
sputtering.
The
absence o f ground s t a t e atoms i n t h e
light
p a t h combined w i t h t h e low v o l t a g e - h i g h c u r r e n t d i s c h a r g e
obviates
self-absorption.
BDHCLs have overcome t h e low source i n t e n s i t y problem i n many
cases, and have a l l o w e d t h e development o f commercial
54
atomic
F i g u r e 2.11
Diagram of a b o o s t e d d i s c h a r g e
h o l l o w cathode lamp
HOLLOW CATHODE
EXCITED ATOMS
ANODE 1
>
" ^^^ r
RADlAT ION
BOOST
DISCHARGE
ANODE 2 ^ '
ELECTRON
EMITTER
f l u o r e s c e n c e i n s t r u m e n t s / such as t h e one used t h r o u g h o u t
this
study.
been
Other
high
intensity
sources
which
have
i n v e s t i g a t e d i n c l u d e microwave e x c i t e d e l e c t r o d e l e s s d i s c h a r g e
lamps, and
l a s e r s . The
former
are between 200
- 2000 times
more i n t e n s e t h a n h o l l o w cathode lamps, b u t have s u f f e r e d from
s t a b i l i t y problems. These have been reduced by t h e o p e r a t i o n
o f t h e lamps w i t h i n t h e r m o s t a t t e d microwave c a v i t i e s . Lasers
have a l s o been employed as sources,
i n p a r t i c u l a r the tunable
dye l a s e r . The o u t p u t from l a s e r s can be s u f f i c i e n t t o cause
saturation
fluorescence,
which
nullifies
the
effects
of
quenching and s e l f - a b s o r p t i o n .
The
design
importance
through
and
in
operation
atomic
quenching,
critical.
of
the
atom
fluorescence.
the
Since
selection of
W h i l s t n i t r o g e n may
cell
the
is
of
energy
vital
is
carrier
lost
gas
is
be employed as t h e c a r r i e r f o r
atomic a b s o r p t i o n d e t e r m i n a t i o n s , b e i n g d i a t o m i c i t i s u s u a l l y
avoided
for
fluorescence
work.
Fortunately,
n e g l i g i b l e quenching c r o s s - s e c t i o n a l area,
and
argon
has
a
i s generally
employed. Hydrogen a l s o has a low v a l u e , and so the use o f a
hydrogen d i f f u s i o n flame has been employed w i t h g r e a t e f f e c t .
The
preference
for circular
flames
f o r fluorescence
studies
non-dispersive
atomic
i s almost u n i v e r s a l .
The
basic
fluorescence
diagram,
layout
of
instrument
i t can
be
a
typical
i s shown i n F i g u r e
seen t h a t
the
2.12.
instrument
is
From
the
basically
s i m p l e i n d e s i g n , c o n s i s t i n g o f a source and a l e n s t o focus
56
F i g u r e 2,12
Diagram
of a
fluorescence
typical
non-dispersive
atomic
spectrometer
BOOSTED DISCHARGE
HOLLOW CATHODE
SIGNAL
LAMP
PROCESSING
AND
DISPLAY
FILTER
FLAME
ATOM
PHOTOMULTI P L I E R
TUBE
57
AMPLIFIER
t h e beam i n t o t h e atom c e l l . A t r i g h t angles t o t h i s assembly
is
a c o l l e c t o r lens, f i l t e r
and a l i g h t d e t e c t o r ( u s u a l l y a
p h o t o - m u l t i p l i e r t u b e ) . The o u t p u t from t h e d e t e c t o r i s f e d
through
an a m p l i f i e r and i n t o
design
of the o p t i c a l
a readout
assembly
d e v i c e . The a c t u a l
is critical,
i f optimum
performance i s t o be o b t a i n e d . Very s m a l l q u a n t i t i e s o f l i g h t
a r e i n v o l v e d , and these must be c o l l e c t e d as e f f i c i e n t l y as
possible,
with efforts
being
made t o a v o i d
degrading
the
signal v i a s c a t t e r etc. I n a dispersive instrument, the f i l t e r
is
s i m p l y r e p l a c e d by a monochromator.
Atomic f l u o r e s c e n c e
has a number o f advantages over
atomic
a b s o r p t i o n and atomic e m i s s i o n . As may be observed from F i g u r e
2.12,
of
t h e i n s t r u m e n t a t i o n i s l e s s complicated than i n e i t h e r
the other
particularly
techniques
techniques.
Good
i n t h e f a r UV
are r e l a t i v e l y
sensitivity
region,
where
i s obtained,
the other
two
i n s e n s i t i v e . The method i s h i g h l y
s e l e c t i v e due t o t h e employment o f a l i n e source, and has low
spectral
i n t e r f e r e n c e . The t e c h n i q u e
a l s o enjoys e x c e l l e n t
l i n e a r i t y over a t l e a s t s e v e r a l o r d e r s o f magnitude (seven i n
the case o f t h e mercury f l u o r e s c e n c e d e t e c t o r ) .
A p a r t from t h e disadvantages
r e g a r d i n g quenching, s c a t t e r i n g
and s e l f a b s o r p t i o n a t h i g h c o n c e n t r a t i o n s a l r e a d y mentioned,
the o n l y o t h e r major l i m i t a t i o n o f t h e t e c h n i q u e
i s reduced
s e n s i t i v i t y f o r elements w h i c h absorb and e m i t i n t h e v i s i b l e
r e g i o n o f t h e spectrum, when compared w i t h atomic
58
emission.
2.2.3
Atomic e m i s s i o n
spectrometry
A number o f atomic e m i s s i o n s p e c t r o m e t e r s have been employed
as
d e t e c t o r s f o l l o w i n g h y d r i d e g e n e r a t i o n . These i n c l u d e t h e
d i r e c t c u r r e n t plasma
(DCP), microwave induced plasma (MIP)
and i n d u c t i v e l y c o u p l e d plasma (ICP). W h i l s t t h e r e i s s t i l l
some work b e i n g c a r r i e d o u t u s i n g t h e DCP, most a t t e n t i o n i s
now c o n c e n t r a t e d on ICP. MIP has a number o f advantages
ICP, b u t as y e t has s t i l l
n o t been f u l l y
over
developed. The new
g e n e r a t i o n o f ICP i n s t r u m e n t s o p e r a t e a t h i g h e r f r e q u e n c i e s
{41MHz c . f . 27.12MHz), and a r e more t o l e r a n t t o h i g h e r l e v e l s
of
gases such
as hydrogen
which i s produced
i n the hydride
g e n e r a t i o n r e a c t i o n t h a n were t h e i r p r e d e c e s s o r s .
Atomic e m i s s i o n s p e c t r o s c o p y i s t h e measurement o f t h e l i g h t
e m i t t e d by atoms, which have been e x c i t e d by t h e i n p u t o f
energy,
as t h e y drop
back
to their
ground
states.
Atomic
e m i s s i o n may be observed a t t h e s i m p l e s t q u a l i t a t i v e l e v e l i n
the
" p l a t i n u m w i r e flame t e s t s " which w i l l be f a m i l i a r t o any
s t u d e n t o f c h e m i s t r y . The t e c h n i q u e i s a l s o commonly employed
i n flame photometers f o r t h e q u a n t i t a t i v e d e t e r m i n a t i o n o f t h e
alkaline
earth
solution
i s aspirated
light
elements.
I n these
into
i n s t r u m e n t s , t h e sample
t h e flame, and t h e i n c r e a s e i n
e m i s s i o n due t o t h e element o f i n t e r e s t measured by a
p h o t o c e l l , s p e c t r a l s e l e c t i o n b e i n g accomplished w i t h a s i m p l e
filter
between
t h e flame
and t h e d e t e c t o r .
Many
atomic
a b s o r p t i o n s p e c t r o m e t e r s a l s o have t h e f a c i l i t y f o r o p e r a t i o n
i n t h e e m i s s i o n mode, a g a i n u s i n g t h e flame as t h e e x c i t a t i o n
59
source.
A g r e a t advance was made i n t h e l a t e 1970's when two workers
operating
independently
produced
similar
designs
f o r an
i n d u c t i v e l y c o u p l e d plasma t o r c h . The main d i f f e r e n c e between
the
d e s i g n s was t h a t o f t h e o v e r a l l d i a m e t e r - t h e F a s s e l "
d e s i g n has an o u t s i d e d i a m e t e r o f 18mm, w h i l s t t h e G r e e n f i e l d
t o r c h has one o f 27mm. A t y p i c a l
d e s i g n o f an i n d u c t i v e l y
c o u p l e d plasma t o r c h i s shown i n F i g u r e 2.13. I t c o n s i s t s o f
three
c o n c e n t r i c quartz tubes, w i t h
a water
c o o l e d two o r
t h r e e t u r n r a d i o - f r e q u e n c y (RF) c o i l around one end. The i n n e r
tube c a r r i e s an a e r o s o l o f t h e sample i n a stream o f argon,
at a f l o w r a t e o f about 1 1 min"^. The m i d d l e tube may have an
a u x i l i a r y t a n g e n t i a l f l o w o f argon t h r o u g h i t , t h i s f l o w b e i n g
optional
i n a l l - a r g o n plasmas. The o u t e r t u b e has a n o t h e r
t a n g e n t i a l f l o w o f argon p a s s i n g t h r o u g h i t a t a f l o w r a t e o f
10 - 15 1 min"\
which p r o v i d e s argon f o r t h e plasma i t s e l f ,
whilst simultaneously providing cooling f o r the torch.
The plasma i s s t r u c k by a h i g h v o l t a g e d i s c h a r g e from a T e s l a
coil,
which i o n i z e s some o f t h e argon, and p r o v i d e s a source
o f e l e c t r o n s . These i o n s and e l e c t r o n s t h e n i n t e r a c t w i t h t h e
r a p i d l y changing magnetic
this
causes
circular
the ions
horizontal
f i e l d i n d u c e d by t h e RF c o i l , and
and e l e c t r o n s t o f l o w
path. C o l l i s i o n a l
energy
i n a closed
exchange
then
occurs between i o n s , e l e c t r o n s and t h e argon atoms, t o produce
a white hot f i r e b a l l .
The t a n g e n t i a l
f l o w o f argon
i n the
o u t e r tube p r o v i d e s t h e r m a l i s o l a t i o n between t h e f i r e b a l l and
60
F i g u r e 2.13
Diagram o f t h e i n d u c t i v e l y c o u p l e d p l a s m a t o r c h
TAIL
FLAME
PLASMA
R.F.
COIL
TANGENTIAL FLOW OF
COOLANT ARGON
TANGENTIAL
FLOW OF ARGON
AEROSOL OF ARGON AND
SAMPLE
61
the
t u b e . The f l o w o f argon and sample though t h e i n n e r tube
must be s u f f i c i e n t t o punch a h o l e t h r o u g h t h e f i r e b a l l , and
thus an annulus
i s produced. The a n a l y t e atoms r e s i d e i n t h e
area o f t h e f i r e b a l l
f o r about 2 m i l l i s e c o n d s , d u r i n g which
t h e y a r e s u b j e c t e d t o temperatures
and
temperatures
nitrous
are at least
oxide-acetylene
elements i n atomic
atomization
o f 6 - SOOO'^K. These t i m e s
twice
flames,
as
those
experienced i n
used
f o r refractory
absorption spectrometers.
i s more complete,
fewer
The consequent
chemical i n t e r f e r e n c e s
occur and i o n i z a t i o n i n t e r f e r e n c e s are s m a l l o r n o n - e x i s t e n t ^ ^ .
The
plasma
itself
is a brilliant
white,
non-transparent
doughnut shaped e n t i t y , topped by a f l a m e l i k e t a i l .
which
extends
a few m i l l i m e t r e s
shows a continuum
spectrum,
above t h e t o p o f t h e tube
produced by t h e r e - c o m b i n i n g o f
argon and o t h e r i o n s w i t h e l e c t r o n s . T h i s continuum
and
an o p t i c a l l y
above
t h e core,
t r a n s p a r e n t r e g i o n occurs
with
The core
spectral
diminishes
some 10 - 30 mm
measurements
made
usually
between 5 and 25mm above t h e c o i l .
The
RF
capable
energy
i s s u p p l i e d by a h i g h
power RF
generator,
o f p r o v i d i n g up t o about 2 kW o f power a t 27 MHz.
Higher f r e q u e n c i e s are now b e i n g employed i n some i n s t r u m e n t s ,
which
things,
gases
result
i n a more
s t a b l e plasma,
which
among
other
e x h i b i t s a h i g h e r t o l e r a n c e t o o r g a n i c s o l v e n t s and
such
instruments.
as hydrogen
These l a t e r
which
were p r o b l e m a t i c
i n earlier
generation instruments operate at
a p p r o x i m a t e l y 41 MHz.
62
As a source f o r atomic e m i s s i o n measurements, t h e i n d u c t i v e l y
coupled
plasma o f f e r s h i g h s t a b i l i t y ,
high
sensitivity
and
freedom from i n t e r f e r e n c e s , b u t t h e c a p i t a l and r u n n i n g c o s t s
are h i g h . A l l such instr\aments are computer c o n t r o l l e d ,
are
therefore well
able
t o operate
i n an
and
automated mode.
Optimum v i e w i n g h e i g h t s f o r a l l t h e elements are s t o r e d i n t h e
s o f t w a r e , and may
ICP-AES
offers
be a u t o m a t i c a l l y s e t by t h e i n s t r u m e n t .
a
wide
choice
of
spectral
l i n e a r i t y , and t h e p o s s i b i l i t y o f simultaneous
d e t e r m i n a t i o n s . This
fully
latter
f e a t u r e has
lines,
good
multi-element
r e c e n t l y been more
e x p l o i t e d by commercial m a n u f a c t u r e r s
in their
latest
g e n e r a t i o n o f i n s t r u m e n t s which employ s o l i d s t a t e d e t e c t o r s .
2.2.4
The
is
I n d u c t i v e l y c o u p l e d plasma - mass s p e c t r o m e t r y
i n d u c t i v e l y coupled
undoubtedly
instruments
for
one
of
trace
plasma - mass s p e c t r o m e t e r
the
most
element
sensitive
and
versatile
determinations
currently
a v a i l a b l e t o t h e a n a l y t i c a l chemist.
to
p e r f o r m simultaneous
(ICP-MS)
I t o f f e r s the
facility
d e t e r m i n a t i o n s on most elements
and
also i s o t o p i c determinations.
The
l a y o u t o f a t y p i c a l i n s t r u m e n t i s shown i n F i g u r e
The
ICP t o r c h d e s c r i b e d i n t h e p r e v i o u s s e c t i o n i s employed
63
2.14.
F i g u r e 2.14
B l o c k d i a g r a m of a t y p i c a l i n d u c t i v e l y c o u p l e d
plasma-mass
spectrometer
QUADRUPOLE MASS F I L T E R
ION L E N S
tn
_ /
ICR TORCH
I
SAMPLE AEROSOL
FROM SPRAY
CHAMBER
/
COOLANT
DETECTOR
VACUUM PUMPS
INTERFACE
AUXILLIARY
ARGON
GAS
CONTROL
SYSTEM
COMPUTER
I
1
64
ARGON
in
t h i s case as an i o n source, and i s h e l d i n t h e h o r i z o n t a l
p l a n e . The stream o f i o n s g e n e r a t e d by t h e plasma i s d i r e c t e d
towards t h e f i r s t o f two n i c k e l ( o r p l a t i n u m ) cones, which a r e
h e l d a x i a l l y w i t h t h e t o r c h . The f i r s t o f these cones i s t h e
sampler, t h e second t h e skimmer, and these a c t i n such a way
as t o produce a beam o f i o n s i n t o t h e quadrupole mass f i l t e r .
There i s a t h r e e stage p r e s s u r e r e d u c t i o n between t h e t o r c h
which o p e r a t e s a t a t m o s p h e r i c p r e s s u r e , t o t h e quadrupole mass
f i l t e r which o p e r a t e s a t a p r e s s u r e o f a p p r o x i m a t e l y 2 x 10'^
bar.
A f t e r p a s s i n g t h r o u g h t h e cones, t h e beam o f i o n s t h e n
passes t h r o u g h a c c e l e r a t i n g
mass f i l t e r .
in
electrodes before entering the
The l a y o u t o f a quadrupole mass f i l t e r
i s shown
F i g u r e 2.15. The f i l t e r c o n s i s t s o f f o u r p r e c i s i o n
ground
m e t a l r o d s , which a r e s y m m e t r i c a l l y a r r a n g e d and p a r a l l e l t o
the
a x i s o f t h e i o n beam. The opposing
rods a r e connected
e l e c t r i c a l l y t o one a n o t h e r , and each p a i r i s connected t o a
v a r i a b l e d i r e c t c u r r e n t (DC) s u p p l y . Each p a i r o f rods i s a l s o
connected t o a v a r i a b l e r a d i o f r e q u e n c y (RF) g e n e r a t o r i n such
a way t h a t each p a i r has a s i g n a l which i s 180** o u t o f phase
w i t h t h e o t h e r p a i r . Mass scanning i s a c h i e v e d by a l t e r i n g t h e
frequency
o f t h e RF
supply
whilst
holding
the applied
p o t e n t i a l c o n s t a n t , o r by a l t e r i n g t h e p o t e n t i a l s o f t h e two
sources w h i l s t k e e p i n g t h e r a t i o between t h e s e p o t e n t i a l s and
the
f r e q u e n c y c o n s t a n t . The e f f e c t on t h e i o n s i s t o cause
o s c i l l a t i o n s about t h e a x i s i n t h e f i e l d s produced,
the
resonant
and o n l y
i o n s i . e . those i o n s who's masses a r e o f t h e
c r i t i c a l v a l u e , pass t h r o u g h t h e f i l t e r t o t h e d e t e c t o r - nonresonant ions c o l l i d e w i t h t h e rods.
65
F i g u r e 2.15
L a y o u t o f a q u a d r u p o l e mass f i l t e r
OETECTOR
PATH OF NON
RESONANCE
ION
AXiS
- A C GENERATOR
0 C SUPPLY
FOCUSSED ION BEAM
66
The t e c h n i q u e does s u f f e r from a number o f i n t e r f e r e n c e s , one
o f w h i c h i s due t o o v e r l a p o f e l e m e n t s
This
process
i s known
as i s o b a r i c
having
t h e same mass.
interference,
and
with
r e f e r e n c e to t h i s p a r t i c u l a r study which has c o n c e n t r a t e d i n
particular
of t h i s
this
interference
renders the use
i n s t r u m e n t p r o b l e m a t i c . T h i s i s due t o t h e f a c t
naturally
masses
upon s e l e n i u m ,
occurring
correspond
selenium
i n some
has
cases
compounds p r o d u c e d i n t h e plasma,
s i x isotopes,
that
and
their
of
argon
and from w h i c h t h e y
could
to
t h e masses
not t h e r e f o r e be s e p a r a t e d .
In
one,
g e n e r a l however,
and
new
the technique
instruments
i s an e x t r e m e l y
are being
developed
improve d e t e c t i o n l i m i t s and s p e c i f i c i t y .
67
to
powerful
further
2.3
Reagents
A number o f r e a g e n t s
These
are l i s t e d
have been employed d u r i n g
below,
with
their
this
respective
study.
source
supply :
Sodium s e l e n i t e ( 9 9 % )
Aldrich,
Gillingham,
Dorset
Sodium s e l e n a t e (99%)
Aldrich,
Gillingham,
Dorset
Sodium B o r o h y d r i d e (98%)
Aldrich,
Gillingham,
Dorset
H y d r o c h l o r i c a c i d (37%)
Aldrich,
Gillingham,
Dorset
Hydrochloric
Dorset.
Merck, Poole,
Dorset.
Merck, Poole,
Dorset.
Merck, Poole,
Dorset.
acid
(Aristar)
Nitric
MercJc, Poole,
acid
(Aristar)
M e r c u r y AA s t a n d a r d
(lOOOppm)
68
of
J
On-line hydride
systems
69
3.1
Introduction
The
work
described
determination
in
by h y d r i d e
this
chapter
concentrates
g e n e r a t i o n o f t h e element
S e l e n i u m i s an e s s e n t i a l
t r a c e element
on
the
selenium.
f o r mammals, a v i a n s ,
c e r t a i n p l a n t s and b a c t e r i a , but i s a l s o h i g h l y t o x i c t o t h e s e
species.
It is a
peroxidase,
constituent
which
affords
o x i d a t i v e damage^\
of
the
protection
enzyme
to
glutathione
cells
against
I n t h e environment, s e l e n i u m may be found
i n t h e o r g a n i c form a s s e l e n o - a m i n o a c i d s and s e l e n o - e n z y m e s "
which
are
readily
dimethyldiselenide".
transformed
Selenium
window" o f any e s s e n t i a l
is
into
dimethylselenide
has the narrowest
required
toxic
"tolerance
element^'''^^. The " t o l e r a n c e
t h e d i f f e r e n c e between t h e minimum amount
and
window"
o f an element
f o r good h e a l t h , and t h e amount w h i c h w i l l e x h i b i t
effects.
A
selenium
d e f i c i e n c y i n man
may
result
in
cardiomyopathy^^. S i n c e t h i s window i s so n a r r o w i n t h e c a s e
of
selenium,
techniques
t h e need f o r a c c u r a t e
f o r the determination
and s e n s i t i v e
analytical
o f t h i s element a r e r e a d i l y
apparent.
S e l e n i u m i s a Group V I A m e t a l l o i d , w i t h
78.96 and h a s o x i d a t i o n
accounts
mainly
f o r only
as
an
states
o f +6,
an a t o m i c w e i g h t o f
+4,
+2,
0 and - 2 . I t
9 x 10"^ % o f t h e e a r t h s c r u s t ,
impurity
i n sulphur,
sulphide
and o c c u r s
and
sulphate
d e p o s i t s " . I t has i n d u s t r i a l use as a r e d colourant f o r g l a s s ,
and
owing t o t h e f a c t
t h a t t h e m e t a l - l i k e form i s v e r y
light
s e n s i t i v e , i t i s u s e d i n p h o t o c o p y i n g m a c h i n e s , and t o a s m a l l
70
extent
in
selenium
the
electronics
i n sea water
industry.
i s low,
The
concentration
i . e . 4 pg ml'^ i n s h a l l o w
i n deep w a t e r
as s e l e n i t e ,
and
of
water
and
60 pg ml'^
30 pg ml'^
120
pg ml"^ r e s p e c t i v e l y as s e l e n a t e " . S i n c e i t i s e s t i m a t e d
t h a t 8000 t o n n e s o f s e l e n i u m a r e i n t r o d u c e d i n t o t h e s e a
year",
these
localised
Selenium
'^Se
f i g u r e s may
has
to i n c r e a s e , p a r t i c u l a r l y
s i x n a t u r a l l y o c c u r r i n g i s o t o p e s , '^Se
(9.02%), "Se
trace
suffers
each
in
areas.
( 9 . 1 9 % ) . The
many
start
and
( 7 . 5 8 % ) , '''Se ( 2 3 . 5 1 % ) , '°Se
first
choice technique
elements
is
often
(0.89%),
(49.81%)
and
f o r the determination
ICP-MS,
but
this
"Se
of
technique
from a niomber o f i n t e r f e r e n c e s when s e l e n i u m h a s
to
be d e t e r m i n e d . I s o b a r i c i n t e r f e r e n c e s such as "Ar'*°Ar, ^^Ar^Ar,
"Ar*°Ar, "Ar*°Ar and ^°Ar"Cl a l l l i m i t t h e u s e f u l n e s s o f ICP-MS
i n selenium s t u d i e s . Fortunately, hydride generation
a method w h i c h a c h i e v e s v i r t u a l l y 100%
and
combined w i t h an
atomic
to determine selenium
fluorescence
is
absorption
down t o about
employed
as
improvement i n s e n s i t i v i t y
the
1-2
conversion
provides
efficiency
spectrometer
ng ml'^.
method
of
of approximately
When
i s able
atomic
detection,
an
three orders
of
magnitude i s p o s s i b l e .
In
inorganic
encountered
only the
+4
compounds,
selenium
is
most
frequently
i n e i t h e r t h e +4 o r +6 o x i d a t i o n s t a t e s . Of t h e s e ,
state
reacts
t o form t h e h y d r i d e ,
and
therefore
a r e d u c t i o n o f any +6 form p r e s e n t i n t h e sample i s n e c e s s a r y .
This property
of selenium
has
been r e g a r d e d
71
as a problem i n
many c a s e s , a d d i n g a f u r t h e r s t e p f o r t h e a n a l y s t t o c a r r y out
when t o t a l s e l e n i u m d e t e r m i n a t i o n s
problem may however be t u r n e d
regarding
sample may
reduction
selenium.
t o a d v a n t a g e when
the i n d i v i d u a l species
first
be a n a l y z e d
concentration
present
of
information
i s required.
for selenium(IV),
s t e p c a r r i e d out, before
The
a r e r e q u i r e d . The p e r c e i v e d
and t h e n
a n a l y s i n g again
selenium(VI)
The
may
a
for total
then
be
c a l c u l a t e d by s u b t r a c t i n g t h e former f i g u r e from t h e l a t t e r .
72
3.2
An
investigation
enhancement
due
s e l e n i u m by
3.2.1
A
into
to
the
nitric
apparent
acid
when
signal
determining
hydride generation
Background
number
of
enhancement
workers
when
generation
have
selenium
reported
has
been
an
apparent
determined
signal
by
hydride
The i n c r e a s e i n s i g n a l o n l y occured when n i t r i c
a c i d was p r e s e n t , as i s t h e case when n i t r i c a c i d i s added t o
l i q u i d samples a t t h e time o f c o l l e c t i o n t o p r e v e n t
c a t i o n s due
t o a d s o r p t i o n onto v e s s e l s u r f a c e s
(plate o u t ) .
There a r e a number o f r e p o r t s i n t h e l i t e r a t u r e
t h a t n i t r i c a c i d i s an i n t e r f e r e n t s u p p r e s s i n g
selenium
hydride
suggesting
the s i g n a l i n
However
no
r e f e r e n c e c o u l d be found r e p o r t i n g an enhancement e f f e c t ,
and
so t h i s was
The
systems^^'^^'^^'""" .
investigated further.
apparatus
Figure
generation
loss of
3.2.1
used f o r t h i s i n v e s t i g a t i o n i s i l l u s t r a t e d i n
I t consists of a hydride generator,
an
atomic
a b s o r p t i o n s p e c t r o m e t e r employing a q u a r t z f u r n a c e , w i t h d a t a
collection
and
processing
by
computer
using
Touchstone®
software.
It
was
first
necessary
enhancement
effect
conditions.
This
was
to
could
evaluate
be
whether
observed
accomplished
by
the
under
first
reported
laboratory
preparing
s t a n d a r d c o n t a i n i n g 25 iig 1"^ o f selenium as sodium s e l e n i t e
73
a
F i g u r e 3.2.1
Diagram o f t h e b a s i c h y d r i d e
furnace atomic
generation-quartz
absorption detection
system
RADIATION
HEATED
FROM
DETECTOR.
ELECRONICS
HOLLOW
QUARTZ
CATHODE
TUBE
AND
READOUT
LAMP
HYDRIDE
GENERATOR
BLANK
REDUCTANT
74
SAMPLE
i n 3 mol 1"^ h y d r o c h l o r i c a c i d . The s i g n a l o b t a i n e d f o r t h i s
s t a n d a r d was measured ( i n r e p l i c a t e ) and compared w i t h a 3 mol
1"^ h y d r o c h l o r i c a c i d b l a n k . Two 25 ml a l i q u o t s o f t h e s t a n d a r d
were t h e n t a k e n . To one o f these 25 / i l o f c o n c e n t r a t e d
acid
was
added,
hydrochloric
and
acid.
t o the other
These
25
modified
/xl o f
nitric
concentrated
standards
were
then
e v a l u a t e d a g a i n s t t h e 3 mol I ' - h y d r o c h l o r i c a c i d b l a n k . The
solution
containing the n i t r i c
absorbance
reading
over
10%
a c i d was
higher
found
than
t o give
the
an
solution
c o n t a i n i n g no n i t r i c a c i d . T h i s o b v i o u s l y e s t a b l i s h e d t h a t an
enhancement
undertaken
d i d occur,
i n an
and
attempt
so
a
t o more
systematic
fully
study
was
understand
the
mechanisms i n v o l v e d .
3.2.2
Experimental
A s e r i e s o f s o l u t i o n s were p r e p a r e d by t a k i n g 25 /xl a l i q u o t s
of
a 130 mg 1"^ s o l u t i o n o f s e l e n i u m
as sodium s e l e n i t e and
adding them t o 100 ml v o l u m e t r i c f l a s k s . To each o f these was
added
26.7
ml o f h y d r o c h l o r i c a c i d , t o g i v e a f i n a l
s o l u t i o n s t r e n g t h o f 3 mol 1"^. A n a l a r n i t r i c
added i n t h e range 10-500 / i l t o these
making up
t o volume w i t h
a c i d was
solutions,
d e i o n i s e d water.
acid
then
prior to
They were
then
analyzed, a g a i n u s i n g t h e h y d r i d e g e n e r a t i o n - q u a r t z f u r n a c e
atomic a b s o r p t i o n technique, against a standard prepared i n a
s i m i l a r method, b u t c o n t a i n i n g no n i t r i c a c i d . D e t e r m i n a t i o n s
75
were c a r r i e d o u t by t h e b r a c k e t i n g method, t h a t i s s t a n d a r d ,
sample
1, s t a n d a r d ,
sample
2, s t a n d a r d
e t c . The
reading
o b t a i n e d f o r t h e sample i n each case was t h e n compared w i t h
the
mean v a l u e
f o r t h e standards
compensates f o r i n s t r u m e n t a l d r i f t .
observed,
be
either
side. This
method
Again, enhancements were
a l t h o u g h t h e i n c r e a s e s i n s i g n a l d i d n o t appear t o
directly
proportional
t o the quantity
of n i t r i c
acid
present.
A f u r t h e r s e r i e s o f experiments
sample s o l u t i o n s were p r e p a r e d
first
was performed
i n which t h e
i n d i f f e r e n t o r d e r s . For t h e
s e r i e s of s o l u t i o n s , the a l i q u o t of concentrated stock
was p l a c e d
i n the flask,
f o l l o w e d by t h e n i t r i c
h y d r o c h l o r i c a c i d and f i n a l l y
acid, the
t h e water. I n t h e next
series,
the
h y d r o c h l o r i c a c i d was added b e f o r e t h e n i t r i c a c i d , and i n
the
f i n a l s e r i e s , t h e n i t r i c a c i d a d d i t i o n was made a f t e r t h e
water.
These s o l u t i o n s were t h e n d e t e r m i n e d
as p r e v i o u s l y ,
a g a i n u s i n g t h e b r a c k e t i n g t e c h n i q u e . The r e s u l t s a r e shown i n
Figure
3.2.2.
As
may
be
preparation
has a g r e a t
enhancement
observed.
observed,
effect
the order
o f sample
upon t h e degree o f s i g n a l
Since t h e r e was a p o s s i b i l i t y o f m o l e c u l a r s p e c i e s c a u s i n g t h e
enhancement,
background
these
experiments
correction.
Determinations
were
also
were
Similar
made
repeated
results
using
were
two o t h e r
systems, namely i n d u c t i v e l y coupled plasma - atomic
employing
obtained.
detection
emission
s p e c t r o m e t r y and atomic f l u o r e s c e n c e s p e c t r o m e t r y . A g a i n
76
F i g u r e 3.2.2
Diagram showing t h e e f f e c t of s o l u t i o n
preparation
on o b s e r v e d enhancement
78
u
r
c
&0
Q
c:
0
CO
Q
40
3a h
2a
la
SB
lea
Uoltime
^
^ e 4e5 0
15a 2 e a 2 5 a s e a s s e
HC1-H20-HN03
of
B
ni-tr*ic
a.oid
HC1-HN03-H20
500
i n uL
""O
HN03-HC1-H20
similar
results
performed
were
obtained.
i n which
other
Further
aspects
of
experiments
were
t h e system
were
i n v e s t i g a t e d , i n c l u d i n g t h e g a s - l i q u i d s e p a r a t o r , reagents and
pump t u b i n g . Using
reagents
from
other
s u p p l i e r s made no
d i f f e r e n c e t o t h e observed enhancements. A U-tube t y p e gasl i q u i d s e p a r a t o r was f a b r i c a t e d from Q u i c k - f i t glassware i n house,
and produced
virtually
identical
results
t o those
p r e v i o u s l y o b t a i n e d . The e f f e c t s o f t h e pump t u b i n g were t h e n
examined.
Up u n t i l t h i s stage o f t h e s t u d y , o n l y one t y p e o f pump t u b i n g
had been s u p p l i e d f o r use w i t h t h e h y d r i d e g e n e r a t o r
system.
T h i s was t h e s t a n d a r d t u b i n g s u p p l i e d by t h e m a n u f a c t u r e r o f
the hydride
material,
system,
a pink
and was o f a s i l i c o n e
transparent
sample and a c i d b l a n k ,
type
being
rubber
supplied
type of
f o r the
and a b l u e t r a n s p a r e n t type f o r t h e
r e d u c t a n t . T h i s t y p e o f t u b i n g had been s u c c e s s f u l l y employed
f o r t h e d e t e r m i n a t i o n o f selenium
elements
f o r many y e a r s .
and o t h e r h y d r i d e
However, because
f r a g i l e n a t u r e , i t was subsequently
tubing,
which
advantage
that
i s physically
i t provided
of i t ' s rather
r e p l a c e d by 'Santoprene'
more
a
forming
robust.
more
constant
had t h e
delivery
t o the gas-liquid
repeated
u s i n g t h i s t u b i n g , and no enhancement e f f e c t s were
sample
The experiments
of
reagents
noted,
separator.
This
were
w i t h a s e c t i o n o f t h e o l d t u b i n g p l a c e d between t h e
inlet
and t h e pump,
t h e enhancements
observed.
78
were
again
3,2.3
Further
e x a m i n a t i o n o f t h e pump txibing
An more d e t a i l e d e x a m i n a t i o n o f t h e s u r f a c e o f b o t h t y p e s o f
pump t u b i n g was t h e n conducted. An i n i t i a l e x a m i n a t i o n o f t h e
s u r f a c e o f t h e t u b i n g was made by o b t a i n i n g i n f r a r e d s p e c t r a ,
to
see i f any t r a c e
observed.
This
was
carried
r e f l e c t a n c e attachment
spectrometer.
o f carbon-selenium
out using
bonds
a
could
be
total
internal
on a P e r k i n Elmer F o u r i e r
Transform
No such bonds were d e t e c t a b l e , b u t s i n c e t h e
s e l e n i u m c o n c e n t r a t i o n i n t h e t u b i n g would be e x t r e m e l y low i f
p r e s e n t a t a l l , t h i s t e c h n i q u e would be u n l i k e l y t o be capable
of i t ' s d e t e c t i o n .
Samples o f b o t h types o f t u b i n g were examined u s i n g a scanning
e l e c t r o n microscope.
The photomicrographs
a r e shown i n F i g u r e
3.2.3. The s u r f a c e o f t h e i n s i d e o f t h e e a r l y tube t y p e can
c l e a r l y be seen t o have what appears t o be a honeycomb o f
h o l e s i n i t ' s s u r f a c e , a l t h o u g h from these p i c t u r e s , i t i s n o t
p o s s i b l e t o be c e r t a i n whether these a r e h o l e s i n , o r mounds
on, t h e s u r f a c e . A n a l y s i s o f t h e t u b i n g was a l s o c a r r i e d o u t
u s i n g t h e X-ray back s c a t t e r i n g attachment on t h i s i n s t r u m e n t .
The
r e s u l t s f r o m t h i s study were i n c o n c l u s i v e - no s e l e n i u m
was
found i n e i t h e r sample o f t i i b i n g ,
a l t h o u g h t h i s was n o t
s u r p r i s i n g s i n c e t h e l i m i t o f d e t e c t i o n on t h i s i n s t r u m e n t i s
i n t h e o r d e r o f 1 % . The main d i f f e r e n c e observed between t h e
s p e c t r a produced f o r t h e two t u b i n g s was t h e presence o f h i g h
c o n c e n t r a t i o n s o f aluminium i n t h e Santoprene t u b i n g . I t i s
79
F i g u r e 3.2.3
Photomicrographs of the b o r e s of the piin^
tubing
Upper - t h e o l d e r p i n k
tubing
Lower - new
tubing
Santoprene
80
assumed t h a t alumina i s used as a f i l l e r i n t h e f o r m u l a t i o n o f
t h i s t u b i n g , and t h i s i d e a i s r e i n f o r c e d by e x a m i n a t i o n o f t h e
p h o t o m i c r o g r a p h o f t h i s t u b i n g . The s u r f a c e i s e x t r e m e l y rough
with
jagged
peaks
and t r o u g h s
i n the surface
structure.
W h i l s t t h i s m a t e r i a l may p r o v i d e a s u r f a c e a t which r e a c t i o n s
may occur,
i t c l e a r l y does n o t have h o l e s i n i t i n t o
which
s e l e n i u m compounds may be t r a p p e d .
Since
selenium
d i e t h y l d i t h i o c a r b a m a t e [Se (SzCNEtj) J
used i n s i l i c o n e
may be
rubber as a v u l c a n i z i n g agent^^ and o t h e r
s e l e n i u m compounds a r e used as c o l o u r a n t s e s p e c i a l l y i n t h e
p r o d u c t i o n o f red coloured g l a s s " , the p o s s i b i l i t y of selenium
b e i n g p r e s e n t i n t h e m a t r i x o f t h e t u b i n g i t s e l f c o u l d n o t be
r u l e d o u t . E n q u i r i e s t o t h e m a n u f a c t u r e r s were u n s u c c e s s f u l
due t o commercial c o n f i d e n t i a l i t y .
An a t t e m p t was made t o d i g e s t t h e s i l i c o n e r u b b e r i n a m i x t u r e
of
nitric
resulting
and h y d r o f l u o r i c a c i d s , i n a PTFE c o n t a i n e r . The
liquid
was a n a l y s e d
f o r selenium,
but again the
r e s u l t s were i n c o n c l u s i v e . Since s e l e n i u m h e x a f l u o r i d e i s a
gaseous compound, i t i s p o s s i b l e t h a t any s e l e n i u m p r e s e n t was
c o n v e r t e d i n t o t h i s form, and was s u b s e q u e n t l y
lost.
I t would appear t h e r e f o r e t h a t two p o s s i b i l i t i e s c o u l d account
for
t h e enhancement. The f i r s t
contributes
t o the overall
i s that
selenium
selenium i n t o t h e a n a l y t e s o l u t i o n ,
t h e former
level
releasing
t h i s o n l y o c c u r i n g when
n i t r i c acid i s present i n trace q u a n t i t i e s .
81
by
tubing
T h i s t h e o r y does
not e x p l a i n why t h e enhancements are d i f f e r e n t , depending upon
the
o r d e r i n which t h e s o l u t i o n s are
prepared.
A second p o s s i b i l i t y concerns t h e apparent h o l e s i n t h e s u r f a c e
of t h e t u b i n g . I f selenium i s a t t r a c t e d i n t o these h o l e s , t h e n
an e q u i l i b r i u m may be s e t up, i n which t h e r e i s an i n t e r c h a n g e
between selenium i n t h e s o l u t i o n p a s s i n g t h r o u g h t h e t u b i n g ,
and
t h a t h e l d a t t h e s u r f a c e . When t h e s t a n d a r d s o l u t i o n i s
passed t h r o u g h t h e t u b e , a c e r t a i n percentage w i l l be r e t a i n e d
on o r i n t h e w a l l o f t h e t u b e . T h i s w i l l
value
being
selenium
o b t a i n e d by
complex
structure,
and
may
is
t h e d e t e c t i o n system.
formed,
be
r e s u l t i n a given
i t is
likely
to
If a
be
a
too great t o enter the holes
nitrolarge
i n the
t u b i n g . I n such c i r c u m s t a n c e s , more selenium w i l l a r r i v e a t t h e
detection
exactly
system
the
and
will
same t o t a l
result
selenium
i n a higher reading, f o r
c o n c e n t r a t i o n . This
could
account f o r t h e observed e f f e c t s r e g a r d i n g t h e o r d e r i n which
the
samples
were p r e p a r e d .
The
largest
enhancement
effect
o c c u r r e d when t h e a l i q u o t o f c o n c e n t r a t e d n i t r i c a c i d was added
d i r e c t l y t o t h e selenium s t o c k s o l u t i o n . I f t h e f o r m a t i o n o f
a complex were t o occur, i t i s l i k e l y t o do so more e f f i c i e n t l y
at
h i g h c o n c e n t r a t i o n s o f r e a c t a n t s . As
dilutions
occurred,
e.g. by adding t h e n i t r i c a c i d a f t e r t h e h y d r o c h l o r i c a c i d , t h e
e f f e c t s d i m i n i s h . C l e a r l y , f u r t h e r work would be necessary t o
prove t h i s h y p o t h e s i s , a l t h o u g h t h i s i s o u t s i d e t h e scope o f
t h i s work. However, t h e s t u d y does demonstrate t h e need f o r
caution
when
selecting
82
appropriate
tubing,
especially
f o r use
i n on-line
c o n n e c t i o n s may have t o be made.
83
systems where
a number
of
3.3
Conditioning
From t h e e a r l i e s t
into
selenium
o f s e l e n i u m h y d r i d e g e n e r a t i o n systems
experiments
c a r r i e d out d u r i n g t h i s
h y d r i d e g e n e r a t i o n , i t was
response d r i f t e d
with
time
i n an
apparent
that
upward d i r e c t i o n
s t a n d a r d sampled a t t i m e zero would g i v e a l o w e r
study
the
i.e. a
absorbance
r e a d i n g t h a n t h e same s t a n d a r d r u n a g a i n say 30 minutes
later.
These e a r l y experiments
atomic
a l l employed q u a r t z
absorption spectroscopy
furnace
(QFAAS) as t h e d e t e c t i o n system, w i t h
s o l u t i o n s t r e n g t h s i n t h e 10 - 30 ng ml'^ range.
I t was
noted
t h a t these apparent i n c r e a s e s i n s e n s i t i v i t y c o n t i n u e d f o r an
hour o r so, a f t e r which t i m e t h e system became more
I n s t r u m e n t d r i f t was
practice
to
set
a t f i r s t suspected,
the
instruments
settled.
and so i t became t h e
running
at
least
an
hour
b e f o r e any measurements were made, b u t t h i s d i d not overcome
the
increase
in
monochromator i n
sensitivity.
It
was
noted
that
the
t h e atomic a b s o r p t i o n s p e c t r o m e t e r used d i d
r e q u i r e f r e q u e n t adjustment as i t warmed p r o g r e s s i v e l y d u r i n g
use,
but
again
this
did
not
account
for
the
change
in
sensitivity.
During
conversations
with
other
users
of
the
hydride
g e n e r a t i o n equipment, i t became apparent t h a t t h i s e f f e c t
well
known, a l t h o u g h
cases,
users
seemingly
condition
the
little
system
documented.
by
running
I n most
selenium
s o l u t i o n s t h r o u g h t h e equipment a number o f t i m e s b e f o r e
84
was
any
measurements a r e made. I n a d d i t i o n , f o r h i g h e s t accuracy, t h e
procedure
of bracketing
3.2.2). The r e a d i n g s
should
be
employed
f o r t h e standards
(see s e c t i o n
either
side o f the
sample a r e averaged, and t h e r e a d i n g f o r t h e sample c a l c u l a t e d
a g a i n s t t h i s mean v a l u e . T h i s i s a v e r y e f f e c t i v e
for
overcoming d r i f t ,
even f a i r l y s h o r t t e r m d r i f t .
however slow t h e r a t e o f sample t h r o u g h p u t ,
technique
I t does
and s i n c e most
workers use some k i n d o f computer system f o r d a t a c a p t u r e and
calculation
of results,
b r a c k e t i n g becomes something
problem t o f i t i n t o these
systems. The Touchstone® s o f t w a r e
employed i n these l a b o r a t o r i e s does a l l o w f o r new
to
be
run p e r i o d i c a l l y ,
of a
and
will
back
standards
calculate the
c o n c e n t r a t i o n s o b t a i n e d from t h e e a r l i e r r e s u l t s , b u t i t does
not
do
so
relative
t o time.
The u s e r
c o n c e n t r a t i o n s e i t h e r based on t h e f i r s t
therefore
obtains
s e t o f standards,
when t h e response was low o r from t h e l a t e r s e t o f s t a n d a r d s ,
when t h e response has i n c r e a s e d . The p r o b l e m w i t h t h i s i s t h a t
t h e i n c r e a s e i n s e n s i t i v i t y occurs
t h e exact
rate o f increase
o v e r a p e r i o d , and s i n c e
i s unknown, i t i s i m p o s s i b l e t o
c a l c u l a t e r e s u l t s o b t a i n e d i n t h e way d e s c r i b e d .
When u s i n g t h e QFAAS system, t h e d e t e c t i o n l i m i t s o b t a i n e d a r e
g e n e r a l l y i n t h e low ng ml"^ r e g i o n , and t h e v a r i a t i o n s due t o
t h e above e f f e c t
accurate
work.
a r e n o t t o o s e r i o u s , except
Once t h e atomic
fluorescence
f o r t h e most
system became
a v a i l a b l e here t h i s d r i f t became more p r o b l e m a t i c , due t o t h e
much g r e a t e r s e n s i t i v i t y o f t h e i n s t r u m e n t a t i o n . D u r i n g work
i n t h e BCR program t o measure selenium
85
i n n a t u r a l waters f o r
example, t h i s i n c r e a s e i n s e n s i t i v i t y became a r e a l
problem,
even a f t e r c o n d i t i o n i n g o f t h e equipment had been c a r r i e d out
i n t h e way p r e v i o u s l y employed f o r use w i t h QFAAS. Thus i t was
d e c i d e d t o i n v e s t i g a t e t h e process
i n more d e t a i l .
A 250 ml s t a n d a r d s o l u t i o n c o n t a i n i n g 1 ng ml'^ o f s e l e n i u m as
sodiiam s e l e n i t e i n 3 mol 1'^ h y d r o c h l o r i c a c i d was
A 1.3
prepared.
% s o l u t i o n o f sodium b o r o h y d r i d e i n 0.1 mol 1"^ sodium
h y d r o x i d e was
f r e s h l y p r e p a r e d , as was a 3 mol 1'^ s o l u t i o n o f
h y d r o c h l o r i c a c i d . A c o n t i n u o u s f l o w h y d r i d e g e n e r a t i o n system
was
s e t up as d e s c r i b e d i n Chapter 2, w i t h t h e apparatus
not
h a v i n g been employed f o r selenium d e t e r m i n a t i o n s f o r a t l e a s t
f o u r weeks p r i o r
t o t h e experiment.
T h i s was
coupled
t o an
atomic f l u o r e s c e n c e d e t e c t i o n system. The d e t e c t i o n system was
s w i t c h e d on and a l l o w e d t o warm up f o r two hours p r i o r t o use,
and t h e h y d r i d e g e n e r a t o r was a l l o w e d t o o p e r a t e pumping b l a n k
solutions
f o r one
hour p r i o r
selenium s t a n d a r d was
t o readings being taken.
The
t h e n i n t r o d u c e d i n t h e normal way,
and
t h e r e s u l t s from making one d e t e r m i n a t i o n e v e r y two
minutes
or so were r e c o r d e d t o produce t h e p l o t shown i n F i g u r e 3.3.1.
The i n s t r u m e n t c o n d i t i o n s are shown i n Table
3.3.1.
The commonly suggested e x p l a n a t i o n f o r t h e observed e f f e c t i s
that
active
selenium
sites
hydride
on
t h e s u r f a c e o f t h e glassware
molecules
and
that
this
results
capture
in
a
d i m i n u t i o n o f the analyte reaching the d e t e c t o r . Gradually,
these a c t i v e s i t e s are f i l l e d up, and t h u s an i n c r e a s e i n t h e
86
F i g u r e 3.3.1
Diagram o f the s i g n a l d r i f t a g a i n s t
55
2B
Time
3B
Cm i n s )
87
time
Table
3.3.1
Instrument
operating
conditions
Reductant f l o w r a t e
4 ml min'^
Sample f l o w r a t e
8 ml min"^
Blank f l o w r a t e
8 ml min"^
Argon purge f l o w r a t e
200 ml min'^
Instrument
100
gain
Data c o l l e c t i o n mode
X
10
Peak h e i g h t
88
numbers o f selenium atoms r e a c h i n g t h e d e t e c t o r o c c u r s . Since
t h e c o n c e n t r a t i o n o f t h e selenium
i n these s o l u t i o n s i s v e r y
low, t h e f i l l i n g o f t h e a c t i v e s i t e s i s slow.
for
T h i s accounts
t h e o b s e r v a t i o n t h a t t h e e f f e c t i s l e s s o f a problem when
employing QFAAS - t h e h i g h e r d e t e c t i o n l i m i t s o f t h i s system
mean t h a t
higher
concentrations
o f analyte
a r e used,
and
t h e r e f o r e t h e s i t e s a r e f i l l e d more q u i c k l y .
Thus i t i s recommended from t h i s work t h a t s e v e r a l i n j e c t i o n s
are made u s i n g a h i g h c o n c e n t r a t i o n o f sodium s e l e n i t e
ml"^)
a t t h e commencement o f work. T h i s s h o u l d
a c t i v e s i t e s , and i f t h e system i s t h e n l e f t
( 1 pg
saturate the
f o r a period of
f i f t e e n minutes before running the standards, e q u i l i b r i u m w i l l
be r e - e s t a b l i s h e d .
89
3.4
Development o f a microwave r e d u c t i o n
system
As d i s c u s s e d above ( S e c t i o n 2.1), c e r t a i n elements must be i n
the a p p r o p r i a t e o x i d a t i o n s t a t e i n order t o f a c i l i t a t e the
f o r m a t i o n o f t h e h y d r i d e . I n t h e case o f selenium, t h i s i s t h e
+4
state.
generation
When
determining
i t i s therefore
total
selenium
necessary
by
t o perform
hydride
a
pre-
r e d u c t i o n s t e p i n o r d e r t o reduce any s e l e n i u m ( V I ) which may
be p r e s e n t t o s e l e n i u m ( I V ) . A number o f systems may be used t o
achieve t h i s r e d u c t i o n . D'Ulivo"'^° and others^^ a r e advocates
of hydrogen bromide r e d u c t i o n , b u t by f a r t h e most commonly
employed
technique
temperature
uses
i n an open
hydrochloric acid
topped
a t an e l e v a t e d
container.
The r e d u c t i o n
r e a c t i o n proceeds as f o l l o w s :
HSeO^"
+
3H' +
2C1' =
H2Se03
+
Clztaqj
+
H2O
The r e a c t i o n i s g e n e r a l l y c a r r i e d out a t about 70°C, a l t h o u g h
o t h e r t e m p e r a t u r e s have been r e p o r t e d , w i t h g r e a t e r o r l e s s e r
t i m e s cjuoted f o r t h e r e a c t i o n " " ^ ^ . A n a l y t e l o s s e s have been
r e p o r t e d when t h e s o l u t i o n has been b o i l e d , and these l o s s e s
have been a t t r i b u t e d t o v o l a t i l e selenium c h l o r i d e s and 0x0c h l o r i d e s such as SeO.Clz and Se02.2HCl^^ p a r t i a l r e d u c t i o n t o
metallic
selenium^^ o r a d s o r p t i o n o n t o t h e v e s s e l s u r f a c e " .
Using t h e r a d i o t r a c e r ''^Se, Piwonka e t . al.^"^ found t h a t when
using
5M h y d r o c h l o r i c a c i d
and b o i l i n g
t h e m i x t u r e f o r 25
minutes o r more, a l t h o u g h a l l o f t h e r a d i o - t r a c e r remained i n
the
solution,
t h e p r o p o r t i o n converted
90
into
the hydride
decreased, i n d i c a t i n g t h a t some change i n o x i d a t i o n s t a t e had
occurred,
probably
t o Se(0).
Again employing r a d i o t r a c e r s , Krivan e t . a l . " e s t a b l i s h e d t h a t
back o x i d a t i o n o c c u r r e d
when t h e reduced s o l u t i o n were
left
b e f o r e a n a l y s i s , and t h i s c o u l d e x p l a i n some o f t h e apparent
losses
which had been r e p o r t e d . A f t e r one week, 94% o f t h e
Se(IV) produced had been o x i d i s e d back t o Se{VI) , and was t h u s
r e n d e r e d u n d e t e c t a b l e by h y d r i d e g e n e r a t i o n
(without a f u r t h e r
r e d u c t i o n s t e p ) . The back o x i d a t i o n may be e x p l a i n e d
by t h e
reaction :
HzSeOa
+
CI2 +
Over a f u r t h e r p e r i o d
H2O ^
HzSeO^
o f f o u r weeks,
+
2HC1
t h e 6% r e m a i n i n g as
s e l e n i t e d i d n o t change, and t h i s may t h e r e f o r e be c o n s i d e r e d
t o be t h e e q u i l i b r i u m c o n c e n t r a t i o n . T h i s back o x i d a t i o n may
be o b v i a t e d t o an e x t e n t by t h e use o f an open topped
in
which
t o heat
the a c i d i f i e d
sample, t h u s
vessel
allowing the
c h l o r i n e g e n e r a t e d i n t h e r e d u c t i o n r e a c t i o n t o escape.
When t h e d e t e r m i n a t i o n o f t o t a l s e l e n i u m i s r e q u i r e d , t h e p r e r e d u c t i o n s t e p may be c o n s i d e r e d t o be a nuisance, s i n c e i t
involves considerable
also provides
e f f o r t on t h e p a r t o f t h e chemist, and
o p p o r t u n i t i e s f o r contamination
When s p e c i a t i o n d e t e r m i n a t i o n s
91
t o occur.
o f selenium a r e r e q u i r e d , t h e
i n a b i l i t y of selenium(VI)
t o form t h e h y d r i d e may be used t o
advantage. Since i n t h e i n o r g a n i c form, s e l e n i u m e x i s t s o n l y
i n t h e o x i d a t i o n s t a t e s o f +4, +6 and 0 ( e l e m e n t a l s e l e n i u m ) ,
by
first
analysing
the
sample
before
reduction,
c o n c e n t r a t i o n o f selenium{IV) i s determined.
then
subjected
t o a r e d u c t i o n step,
the
I f t h e sample i s
and t h e n
re-analyzed,
t o t a l i n o r g a n i c selenium w i l l be d e t e r m i n e d . The c o n c e n t r a t i o n
of
s e l e n i u m ( V I ) may t h e r e f o r e be o b t a i n e d by s u b t r a c t i n g t h e
selenium(IV)
With
figure
the current
from
the t o t a l
interest
selenium
i n selenium
concentration.
s p e c i a t i o n , i t was
d e c i d e d t o i n v e s t i g a t e t h e p o s s i b i l i t y o f an automated o n - l i n e
speciation
system,
capable
in
the
first
d e t e r m i n i n g s e l e n i u m ( I V ) and s e l e n i u m ( V I )
instance
of
(via the t o t a l ) but
w i t h t h e p o t e n t i a l t o be adapted f o r use w i t h o t h e r elements
w i t h r e d u c i b l e species such as a r s e n i c and t i n .
3.4.1
The
Design
basic
flow
criteria
injection-hydride generation
system
is
d e s c r i b e d i n s e c t i o n 2.1.2, and t h i s formed t h e b a s i s f o r t h e
work. I t was r e c o g n i s e d t h a t t h e d e t e r m i n a t i o n o f s e l e n i u m { I V )
c o u l d be c a r r i e d o u t o n - l i n e by s i m p l y segmenting t h e incoming
sample stream. W h i l s t some peak s p r e a d i n g c o u l d occur, t h i s i s
not a huge p r o b l e m when d e t e r m i n i n g
selenium
using
hydride
g e n e r a t i o n , s i n c e measurements a r e made when t h e system i s i n
equilibrium.
selenium
I n addition,
hydride
generated
by
measuring
could
92
be
peak
areas, a l l
ascribed
t o the
a p p r o p r i a t e sample. I f p a r t o f t h e sample c o u l d be reduced onl i n e and s u b s e q u e n t l y
selenium
analyzed,
concentration could
then a f i g u r e f o r t h e t o t a l
also
be o b t a i n e d .
The main
problem became one o f h e a t i n g t h e sample i n o r d e r t o achieve
the d e s i r e d r e d u c t i o n s t e p .
I t was t h e g e n e r a l p r a c t i c e i n .these l a b o r a t o r i e s t o reduce a
sample by adding
an equal volume o f h y d r o c h l o r i c a c i d , and
t h e n h e a t i n g t h e m i x t u r e i n a beaker i n a water b a t h a t 70 °C
f o r 30 m i n u t e s . The sample was t h e n c o o l e d , any l o s s e s caused
by e v a p o r a t i o n made up w i t h water,
performed.
during
Since
heating,
b e f o r e t h e a n a l y s i s was
some a c i d would a l s o u n d o u b t e d l y
this
process
was n o t i d e a l ,
be l o s t
since
i t
is
i m p o r t a n t t o m a t r i x match samples and standards as c l o s e l y as
p o s s i b l e i f g r e a t e s t accuracy i s t o be o b t a i n e d . I n d e s i g n i n g
t h e o n - l i n e system, t h e assumption was made t h a t t h e r e d u c t i o n
t o o k i n t h e o r d e r o f 30 minutes.
time halved
Assuming t h a t t h e r e a c t i o n
f o r each 10 °C r i s e i n t e m p e r a t u r e ,
t h e n i t was
supposed t h a t i t would be necessary t o c a r r y out t h e r e d u c t i o n
in
t h e gas phase i n o r d e r
t o achieve
acceptable
reduction
times t o s u i t an o n - l i n e system.
A number o f h e a t i n g systems were c o n s i d e r e d
baths, " g r a p h i t e baths"
including o i l
and so on, b u t t h e obvious
f o r such work was microwave h e a t i n g .
93
choice
3.4,2
P r i n c i p l e s o f microwave
heating
There a r e t h r e e ways i n which heat may be t r a n s f e r r e d f r o m a
source
t o a body,
these
being
c o n d u c t i o n , c o n v e c t i o n and
r a d i a t i o n . W h i l s t almost a l l commonly employed h e a t i n g systems
are s p e c i f i e d as r e l y i n g on one o f these h e a t i n g methods, heat
i s i n f a c t t r a n s f e r r e d from t h e heat source t o t h e body b e i n g
heated by a t l e a s t two, and more g e n e r a l l y a l l t h r e e systems.
Using t h e example o f h e a t i n g a beaker o f w a t e r on a h o t p l a t e
for
instance,
most
heat
is
transferred
directly
via
c o n d u c t i o n . However, s i n c e a l l b o d i e s w i t h t e m p e r a t u r e s above
a b s o l u t e zero e m i t i n f r a
red radiation,
some heat
transfer
w i l l o c c u r t h r o u g h r a d i a t i o n f r o m t h e h o t p l a t e b e i n g absorbed
by
t h e beaker
and c o n t e n t s . There w i l l
a l s o be c o n v e c t i o n
c u r r e n t s i n t h e a i r s u r r o u n d i n g t h e t o p o f t h e h o t p l a t e and
the
beaker, and these w i l l c o n t r i b u t e t o t h e o v e r a l l heat g a i n
of
t h e w a t e r and beaker,
within
the licjuid
i n t h e beaker,
t h e r e w i l l a l s o be c o n v e c t i o n c u r r e n t s a t work, t r a n s f e r r i n g
the
heat
throughout
t h e body
of the l i q u i d .
From
this
seemingly s i m p l e example, i t can be seen t h a t a l l t h r e e heat
t r a n s f e r systems make a c o n t r i b u t i o n t o t h e o v e r a l l h e a t i n g
process.
Microwaves a r e a form o f e l e c t r o m a g n e t i c r a d i a t i o n , and have
a f r e q u e n c y range from 300 MHz t o 300 GHz. T h i s r a d i a t i o n i s
n o n - i o n i z i n g , and causes m o l e c u l a r m o t i o n by m i g r a t i o n o f i o n s
and r o t a t i o n o f d i p o l e s . I t does n o t however, cause any change
in
the molecular
structure
of the material
94
being
heated.
M a t e r i a l s range i n t h e i r a b i l i t y t o absorb microwave
radiation
f r o m t h o s e which a r e r e f l e c t i v e , and do n o t t h e r e f o r e absorb
any energy, t h r o u g h those which absorb some o f t h e energy, t o
those which a r e t o t a l l y
t r a n s p a r e n t , and a g a i n do n o t absorb
any o f t h e energy. Only a b s o r p t i v e m a t e r i a l s a r e heated by
microwave energy. R e f l e c t i v e m a t e r i a l s such as m e t a l s f i n d use
in
the c o n s t r u c t i o n
of the i n s i d e
o f t h e microwave
oven,
w h i l s t t r a n s p a r e n t m a t e r i a l s a r e used as c o n t a i n e r s f o r t h e
m a t e r i a l s which r e q u i r e h e a t i n g .
A b s o r p t i v e samples which a r e s u b j e c t e d t o microwave
will
heat
up
partly
i n accordance
with
their
energy
dissipation
factor,
t a n 6, which i s t h e d i e l e c t r i c l o s s o f t h e m a t e r i a l
divided
by
the d i e l e c t r i c
dielectric
dissipate
loss
is a
measure
t h e i n p u t microwave
w h i l s t the d i e l e c t r i c
ability
constant of
of
a
energy
that
material.
sample's
ability
The
to
i n t h e f o r m o f heat,
c o n s t a n t i s a measure o f t h e sample's
t o impede t h e microwave
energy as i t passes t h r o u g h
the sample^^ . Thus when microwave energy e n t e r s a m a t e r i a l , t h e
rate
at
which
dissipation
the
energy
i s absorbed
f a c t o r of that m a t e r i a l .
have a t a n 6 o f i n f i n i t y ,
value
of
tan
6
of
and
zero.
depends
upon
the
Transparent m a t e r i a l s
reflective
Materials
m a t e r i a l s have a
which
have
a
high
d i s s i p a t i o n f a c t o r provide a r e s u l t a n t lower p e n e t r a t i o n of
microwave
energy
at
a
given
frequency.
One
way
of
c h a r a c t e r i z i n g p e n e t r a t i o n i s by c o n s i d e r i n g t h e h a l f - p o w e r
d e p t h . T h i s measure i s t h e d e p t h below
material
a t which
t h e power d e n s i t y
95
the surface of the
i s half
that
a t the
surface.
The
half-power
depth
inverse
of
varies
the
with
frequency,
approximately
the
square
root
f r e q u e n c y , and
i s therefore specified at a given
by
of
that
frequency.
As s t a t e d above, t h e two mechanisms by which microwave energy
i s t r a n s f e r r e d t o a m a t e r i a l are i o n i c c o n d u c t i o n and
dipole
r o t a t i o n , and i n most p r a c t i c a l a p p l i c a t i o n s , these processes
occur s i m u l t a n e o u s l y .
the
applied
I o n i c c o n d u c t i o n occurs as a r e s u l t o f
electromagnetic
m i g r a t i o n of d i s s o l v e d
field,
and
is
the
conductive
i o n s , r e s u l t i n g i n h e a t i n g due
t o the
e l e c t r i c a l r e s i s t a n c e o f t h e sample t o induced c u r r e n t
The
absorption
o f energy due
t h e s i z e , charge and
subject
flow.
t o i o n i c m i g r a t i o n depends upon
c o n d u c t i v i t y of d i s s o l v e d
i o n s , and
is
t o i o n i n t e r a c t i o n w i t h s o l v e n t m o l e c u l e s . Thus t h e
f a c t o r s which a f f e c t i o n i c c o n d u c t i o n are i o n
concentration,
i o n m o b i l i t y and t e m p e r a t u r e . I n an aqueous s o l u t i o n o f sodium
c h l o r i d e , t h e r e i s an a p p r o x i m a t e l y f o u r f o l d i n c r e a s e
i n tan
6 between 0 molar ( i . e . pure w a t e r ) and a 0.5
molar s o l u t i o n .
The
due
other
rotation.
partial
cause
of
When an
alignment
microwave
electric
of
heating
field
molecules
is
which
is
applied,
have
a
to
dipole
there
is
permanent
induced d i p o l e moment. I n f a c t , t h e average t i m e spent by
molecule i n the a l i g n e d
the
state i s only s l i g h t l y greater
t i m e i t spends i n a n o n - a l i g n e d p o s i t i o n , and
electric
field
starts
to
collapse,
thermal
or
the
than
when t h e
disorder
r e s t o r e d . During the p e r i o d of alignment, a small
a
is
amount of
energy i s t a k e n up by t h e m o l e c u l e , and when r e l a x a t i o n o f t h e
96
f i e l d o c c u r s , t h i s energy
i s absorbed by t h e system as h e a t .
The e f f i c i e n c y o f h e a t i n g due t o d i p o l e r o t a t i o n i s dependant
upon t h e sample's t e m p e r a t u r e and v i s c o s i t y , which a f f e c t t h e
d i e l e c t r i c r e l a x a t i o n t i m e o f t h e sample i n q u e s t i o n . A t 2.45
GHz,
one o f t h e f o u r
frequencies a l l o c a t e d
f o r microwave
h e a t i n g , a l i g n m e n t and randomising o f t h e molecules occurs 4.9
x 10^ t i m e s p e r second.
The r e l a t i v e c o n t r i b u t i o n s made by d i p o l e r o t a t i o n and i o n i c
c o n d u c t i o n i n t h e h e a t i n g o f a sample v a r y w i t h
temperature,
and w i t h t h e c o m p o s i t i o n o f t h e sample. I n i c e , t h e molecules
a r e r i g i d l y h e l d i n a c r y s t a l l a t t i c e , and t h e r e f o r e m o l e c u l a r
mobility i s inhibited.
The d i e l e c t r i c d i s s i p a t i o n f a c t o r i s
t h e r e f o r e c o r r e s p o n d i n g l y low - 2.7 x 10
a t 2.45 GHz, as
opposed t o t h e v a l u e o f 12.2 f o r water a t 27 °C.
3.4.3
The b a s i c o n - l i n e microwave s y s t e m
Once i t had been e s t a b l i s h e d t h a t h e a t i n g by microwave d i d
produce t h e r e q u i r e d r e d u c t i o n o f s e l e n i u m { V I )
on a b a t c h
a n a l y s i s , a b a s i c o n - l i n e system was d e v i s e d . T h i s i s shown i n
F i g u r e 3.4.1.
The a c i d i f i e d sample i s i n t r o d u c e d v i a a p e r i s t a l t i c pump t o
t h e h e a t i n g c o i l c o n t a i n e d i n a microwave c a v i t y . From here,
97
F i g u r e 3.4.1
Diagram
o£
the
basic
on-line
microwave
r e d u c t i o n system
>
PBSSTALTC
REACTOICOL
ARGON
<
OUARTZ T - n f l E
0/\S-UOUD S5>ARAT0R
rtXNG VALVE
PBBSTALT1C
fiBXCTANTI
98
W
the
sample
travels
unheated, and
on
through
a
reaction coil
thence t o a c o o l i n g c o i l
which
contained
i n an
is
ice
b a t h . A f t e r c o o l i n g , t h e sample e n t e r s t h e m i x i n g v a l v e , where
it
meets a stream
o f sodium b o r o h y d r i d e
solution,
which i s
pumped u s i n g a second p e r i s t a l t i c pump. The
reacting mixture
then
from
passes
to
a
gas-liquid
hydrogen s e l e n i d e produced
separator,
i s . flushed into
q u a r t z tube i n an atomic a b s o r p t i o n
The
e a r l y problems encountered
a
whence
flame
the
heated
spectrometer.
i n v o l v e d the
assumption r e g a r d i n g t h e temperature
and
aforementioned
residence
time i n
o r d e r f o r t h e r e d u c t i o n r e a c t i o n t o t a k e p l a c e . Attempts
were
made t o produce
from
a
bumping, caused by
heating
coil.
The
flow
system which
uneven b o i l i n g
first
d i d not
suffer
of
the reactants i n the
heating c o i l
employed i s shown i n
F i g u r e 3.4.2.a, b u t d i d s u f f e r from v i o l e n t bumping. A number
of
a l t e r n a t i v e s were e v a l u a t e d ,
shown as 3.4.2.b. I t was
diminished
using
this
c u l m i n a t i n g w i t h the
found t h a t bumping was
coil.
In this
design,
design
considerably
the
reactant
s o l u t i o n passes t h r o u g h a mass of minute g l a s s beads, which
cause d i s p e r s i o n o f t h e l i q u i d and thus p r o v i d e a v e r y l a r g e
s u r f a c e area.
effect
boiling
by
As
the
process.
the l i q u i d b o i l s ,
glass
beads,
t h e r e i s a l s o a damping
resulting
in a
much
U n f o r t u n a t e l y , even w i t h t h e l e s s
b o i l i n g produced by
t h i s heating c e l l ,
smoother
violent
t h e huge changes i n
volume produced by f i r s t b o i l i n g t h e l i q u i d , and then c o o l i n g
i t back t o about room temperature,
baseline.
99
r e s u l t e d i n a very unstable
F i g u r e 3.4.2
Initial
heating c o i l designs
GLASS WIDOL PLUG
GLASS BEADS
GLASS F R I T
FIGURE
FIGURE
3.4.2o
100
3.4.2b
I t was
observed
t h a t r a p i d f l u c t u a t i o n s occurred i n the flow
r a t e o f t h e a n a l y t e i n t o t h e g a s - l i q u i d s e p a r a t o r , and i t was
r e a l i s e d t h a t these would cause t h e b a s e - l i n e i n s t a b i l i t y .
An
a t t e m p t t o dampen out these f l u c t u a t i o n s u s i n g p u l s e damping
t u b i n g was made. T h i s c o m m e r c i a l l y a v a i l a b l e p r o d u c t c o n s i s t s
o f a narrow bore tube, which has more l a t e r a l e l a s t i c i t y b u i l t
i n t o t h e tube m a t e r i a l t h a n i s normal. The
r e s u l t should
be
t h a t as t h e p r e s s u r e i n c r e a s e s due t o a p u l s e , t h e tube w a l l
dilates
t o absorb
t h e i n c r e a s e . As
reverse takes place,
t h e p u l s e subsides,
so damping o c c u r s . A t t e m p t s
the
t o employ
t h i s t y p e o f p r o d u c t on t h i s system were t o t a l l y u n s u c c e s s f u l
- i t is felt
t h a t t h e y are almost
c e r t a i n l y more s u i t e d t o
l i q u i d o n l y systems i n which t h e p r e s s u r e i n c r e a s e s would
be
greater.
Pressure check v a l v e s are c o m m e r c i a l l y a v a i l a b l e , b u t are v e r y
expensive,
and
so
available materials,
one
was
designed
and
constructed
from
t h e d e s i g n o f which i s shown i n F i g u r e
3.4.3. As can be seen, i t i s s i m p l y a n o n - r e t u r n v a l v e , t h e
body f a b r i c a t e d f r o m P.T.F.E., as was t h e diaphragm. The
major
problem o f t h e v a l v e i n v o l v e d t h e s p r i n g . C l e a r l y , t h i s needed
t o be f a b r i c a t e d f r o m an i n e r t m a t e r i a l , and a l t h o u g h a number
d i f f e r e n t p l a s t i c m a t e r i a l s were t r i e d ,
none proved a b l e t o
w i t h s t a n d t h e h o s t i l e chemical environment
required t o operate.
101
i n which t h e y were
F i g u r e 3.4.3
Diagram o f a n o n - r e t u r n v a l v e
DIAPHRAGM
INLET
PTFE BODY
OUTLET
SPRING
102
since
the
system
still
suffered
f l u c t u a t i n g b a s e l i n e , i t was
from
decided
an
unacceptably
t o i n v e s t i g a t e whether
t h e system c o u l d be o p e r a t e d a t a lower t e m p e r a t u r e , below t h e
boiling
point
of
the
analyte
c o n s i s t e n t r e d u c t i o n s . I t was
mixture,
and
still
give
t h e o r i s e d t h a t even i f a l l t h e
s e l e n i u m ( V I ) p r e s e n t was n o t reduced, i f a c o n s t a n t p r o p o r t i o n
was reduced, t h e n t h e b a s i s o f a r e d u c t i o n system c o u l d e x i s t .
It
was
thus discovered
that
" c o n v e n t i o n a l wisdom" had
been
wrong r e g a r d i n g t h e k i n e t i c s of t h e r e d u c t i o n r e a c t i o n , and
later
work
(see
seStion
a c c u r a t e i d e a o f how
3.7)
r e s u l t e d i n p r o v i d i n g a more
long the r e d u c t i o n a c t u a l l y takes.
By c a r e f u l o p t i m i s a t i o n o f t h e system parameters, i t was
found
t h a t t o t a l r e d u c t i o n c o u l d be achieved w i t h o u t b o i l i n g , and on
a t i m e s c a l e which enabled an o n - l i n e system t o be developed.
First,
the
effect
of
differing
hydrochloric
acid
c o n c e n t r a t i o n s were i n v e s t i g a t e d s i n c e , f o r economic reasons,
it
is
desirable
consistent
with
to
full
use
the
reduction. A
selenium as sodium s e l e n a t e was
acidified
to
produce
lowest
equal
acid
25
prepared,
volumes
of
concentration
fig 1"^
solution
of
a l i q u o t s taken
and
sample
with
acid
s t r e n g t h s o f 2,4,6,8 and 10 mol 1"^ . The r e s u l t i n g absorbances
a r e shown i n Table 3.4.1. As may
be observed, no i n c r e a s e i n
absorbance o c c u r r e d a t c o n c e n t r a t i o n s above 6 mol 1~\
t h i s a c i d s t r e n g t h was
and
employed f o r t h e r e s t of the work.
103
so
T a b l e 3.4.1
Absorbance r e a d i n g s f o r i n c r e s i n g
concentrations of h y d r o c h l o r i c a c i d
Acid concentration
(mol 1
Absorbance
2
0.005
4
0.011
6
0.160
8
0.160
10
0.159
104
s i n c e t h e o u t p u t o f t h e microwave u n i t was
steps,
i t was
decided
a d j u s t a b l e i n 5%
t o s e t t h e f l o w r a t e o f t h e sample a t
t h e maximum p e r m i t t e d u s i n g t h e p a r t i c u l a r pump and
tubing
c o m b i n a t i o n employed (9.3ml min'M , and t o v a r y t h e power i n p u t
to
the
sample
cell.
The
r e d u c t i o n e f f i c i e n c y was
power
output
to
achieve
maximum
t h e n o p t i m i z e d . T h i s was a c h i e v e d
at
a power s e t t i n g o f 35%, w i t h a d i m i n u t i o n o c c u r r i n g above t h i s
f i g u r e . T h i s r e d u c t i o n i n e f f i c i e n c y may
o x i d a t i o n of the selenium{IV)
be due
t o t h e back
by f r e e c h l o r i n e .
A number o f d i f f e r e n t d e s i g n s o f h e a t i n g , r e a c t i o n and c o o l i n g
coils
were
provided
evaluated,
maximum
before
conversion
arriving
at
efficiency.
a
system
which
coils
were
The
f a b r i c a t e d i n house from b o r o - s i l i c a t e g l a s s , t o t h e s t r u c t u r e
shown i n F i g u r e 3.4.4
Once
these
evaluated
range 10
and s i z e s shown i n T a b l e 3.4.2.
parameters
using
sodium
had
been
selenate
optimized,
standard
the
system
was
solutions i n
the
- 50 /xg 1"^. Reductions were c a r r i e d o u t u s i n g
t h e o n - l i n e microwave system and
both
t h e c o n v e n t i o n a l method o f
h e a t i n g a l i c p a o t s o f t h e s o l u t i o n i n 6 mol
1"^ t o 70 °C f o r 30
m i n u t e s on a h o t p l a t e . The r e s u l t s are shown i n F i g u r e 3,4.5.
The
r e s u l t s i n d i c a t e t h a t t h e response o f t h e microwave f l o w
i n j e c t i o n system was l i n e a r up t o a c o n c e n t r a t i o n of 30 /ig 1"^,
w h i c h r e p r e s e n t s an absorbance o f 0.3,
becomes n o n - l i n e a r due
above which t h e
to self absorption.
105
plot
F i g u r e 3.4.4
Diagram showing t h e c o n s t r u c t i o n d e t a i l s o f t h e
h e a t i n g , r e a c t i o n and c o o l i n g
A.
13
1.06
coils
T a b l e 3.4.2
Table
of
the
dimensions
of
the
heating,
N
turns
r e a c t i o n and c o o l i n g c o i l s
A mm
B mm
C nmi
D min
HEATING COIL
200
13
20
20
REACTION COIL
180
0
20
55
13
COOLING COIL
200
10
20
100
25
107
5 -
F i g u r e 3.4.5
C o m p a r i s o n between microwave f l o w
and
conventionally digested
injection
standards
15.0 n
12.5
A
MICROWAVE
F.I
CO
<
10.0 H
<
lU
DC
<
7.5 -
<
5.0 H
LU
CONVENTIONAL DIGEST
2.5
A
0.0
50
CONCENTRATION
108
(ug
1"^)
3.5
Incorporation
o f microwave
heating
into
a
fully
automated h y d r i d e g e n e r a t i o n s y s t e m
Since t h e e f f i c a c y o f t h e t e c h n i q u e t o p r o v i d e t h e r e d u c t i o n
of
selenium(VI)
basic
t o selenium(IV)
had been
system was developed t o p r o v i d e
speciation
system. The d e s i g n f o r t h i s
established, the
a totally
automated
system i s shown i n
F i g u r e 3.5.1. I n t h i s system, a d u a l - c h a n n e l p e r i s t a l t i c pump
i n t r o d u c e s b o t h a sample stream and h y d r o c h l o r i c a c i d stream
t o a s w i t c h i n g v a l v e which i s under computer c o n t r o l . I n t h e
p o s i t i o n shown i n t h e diagram, t h e sample t r a v e l s t h r o u g h t h e
microwave
route,
and i s d i r e c t e d
by
a
second
computer
c o n t r o l l e d s w i t c h i n g v a l v e t h r o u g h t h e h y d r i d e g e n e r a t o r stage
and on t o t h e d e t e c t o r . H y d r o c h l o r i c a c i d i s meanwhile pumped
from
the reservoir,
bypasses
t h e microwave
unit,
and i s
d i r e c t e d t o waste by t h e second s w i t c h i n g v a l v e .
When s w i t c h i n g v a l v e
1 i s rotated
through
90°, t h e sample
passes t o waste, w h i l s t t h e h y d r o c h l o r i c a c i d stream passes
i n t o t h e microwave u n i t . The second s w i t c h i n g v a l v e c o n t r o l s
which o f t h e two streams passes on t o t h e h y d r i d e g e n e r a t i o n
s t e p , and s u b s e q u e n t l y t o t h e d e t e c t i o n system. Should sample
volumes be l i m i t e d , t h e a u t o s a m p l e r may sample from a second
hydrochloric
introduced
acid
reservoir,
t o t h e system.
microwave u n i t
has e i t h e r
once
t h e sample
I t i s obviously
a blank acid
stream p a s s i n g t h r o u g h i t a t a l l
109
times.
vital
has
been
that the
s t r e a m o r a sample
Figure-3.5.1
Computer
reduction
AUTOSAMPLER
c o n t r o l l e d automated microwave
selenium speciation
DUAL C H A M C L
SWITCHING
PERISTALTIC PUMP
VALVE
HQ
RESERVOR
MICROWAVE
MXWG VALVE
system
REACTION
COOLNG
COL
COL
SWrTCHNG
VALVE
•!
PERISTALTIC
DETECTOR
QUARTZ T - T l B E
WASTE
REDUCTANT
GAS-LIOUD
SEPARATOR
OATAUC
OOKIHOLlfC
110
The
switching
profiles
throughput
arrangements,
are
shown
in
timings
Figure
and
3.5.2.
resultant
Using
this
r a t e s o f f i f t e e n samples per hour were
signal
system,
achieved.
T h i s f i g u r e c o u l d almost c e r t a i n l y be improved by upon f u r t h e r
optimization
design
of
purge gas
r a t e s and
modifications to
o f t h e m i x i n g v a l v e . These s h o u l d
the
reduce t h e s i g n a l
decay t i m e s , and t h u s i n c r e a s e - t h e speed o f t h e system.
To v a l i d a t e t h e system, a c e r t i f i e d r e f e r e n c e m a t e r i a l , NIST
1643c Trace Elements i n Water was
obtained
agreement
are
given
was
i n Table
obtained
f i g u r e o f 12.7
3.5.1. As
between
c o n c e n t r a t i o n {12.3 fig I ' M
certified
analyzed,
the
and
can
be
the
seen,
total
i n t h e samples analyzed,
± 0.7
^g 1'^ . I t was
results
good
selenium
and
further
the
found
t h a t t h e presence o f s m a l l c o n c e n t r a t i o n s o f n i t r i c a c i d had
no e f f e c t on t h e accuracy o f t h e r e s u l t s , p r o v i d e d t h a t m a t r i x
m a t c h i n g o f sample and
standards
s h o u l d be normal p r a c t i c e .
Ill
was
c a r r i e d o u t , as i s , o r
F i g u r e 3.5.2
V a l v e s w i t c h i n g and t i m i n g
diagram
SAMPLE #1
T O T A L Se
Se(IV)
TIME
SWITCHING
VALVE #1
SWITCHING
VALVE #2
(mins)
SAMPLE # 2
Table
3.5.1
A n a l y s i s of C e r t i f i e d Reference
Material
NIST 1643c - d e t e r m i n a t i o n o f s e l e n i u m
Analysis
no
Se(IV)
pg
1"^
S e ( V I ) ]iq
T o t . Se
Vig 1-^
1
9.8
2.5
12.3
2
9.8
2.5
12.3
3
9.8
2.6
12.4
4
10.0
2.6
12.6
5
9.9
2.4
12.3
Mean
9.8
2.5
12.3
113
3.5.1
When
Data
selenium
handling
i s determined
using
flow
injection-hydride
g e n e r a t i o n , steady s t a t e c o n d i t i o n s a r e u s u a l l y sought i n t h e
gas-liquid
s e p a r a t o r , and a f l a t
topped, p l a t e a u shaped peak
r e s u l t s . T h i s p e a k shape l e n d s i t s e l f
t o quantification using
p e a k h e i g h t m e a s u r e m e n t s . However t h e T o u c h s t o n e S o f t w a r e
Analytical
L t d . , Orpington,
Kent)
used
with
the
(PS
system
e m p l o y e d a l s o e n a b l e d peak a r e a m e a s u r e m e n t s t o be made. S i n c e
i n p r a c t i c e n o t a l l p e a k s w e r e i d e a l i n shape, a c o m p a r i s o n o f
the r e s u l t s obtained from i d e n t i c a l
s a m p l e s was
carried out,
u s i n g b o t h p e a k a r e a a n d p e a k h e i g h t modes. B o t h m e t h o d s o f
quantification
gave
almost
p e a k a r e a mode was a d o p t e d
The
s o f t w a r e uses
a
performs
a
box-car
analogue
to digital
Turbo Pascal,
identical
f o r the rest of t h i s
system
of
integration
1.
This
handled,
the f u l l
continuous
on
c o n v e r t e r . The
and t h e e s s e n t i a l
Appendix
shows
the
from
and
the
i s written in
are i l l u s t r a t e d i n
the data
c o m p l e x due
operating instructions etc.
114
the
study.
readings
i n which
b e i n g more
However,
measurement,
programme
elements
t h e method
programme
i n c l u s i o n o f hardware
results.
is
t o the
3.6
Seleniiim s p e c i a t i o n
The
automated
studies
system d e s c r i b e d
i n the previous
s e c t i o n was
u s e d t o e v a l u a t e t h e i n o r g a n i c s e l e n i u m s p e c i a t i o n i n a number
o f aqueous s a m p l e s . The r e s u l t s compared f a v o u r a b l y w i t h
obtained
using
system r e l i e d
other
techniques.
However
t h e basic
those
on-line
upon t h e d i f f e r e n c e method o f c a l c u l a t i n g t h e
r e s u l t s . A number o f c r i t i c i s m s o f t h i s a p p r o a c h c a n b e made.
The
prime
that
reason
i f an e r r o r
f o r not applying a d i f f e r e n c e c a l c u l a t i o n i s
i s made i n t h e d e t e r m i n a t i o n o f one o f t h e
species, t h i s r e s u l t a u t o m a t i c a l l y a f f e c t s the r e s u l t obtained
via
the calculation
f o r the other
moiety.
Thus
further
d e v e l o p m e n t o f t h e s y s t e m was i n v e s t i g a t e d i n o r d e r t o s e e i f
it
could
be m o d i f i e d
or altered
t o determine
each
species
directly.
In
order
t o achieve
this,
i t would
clearly
be n e c e s s a r y t o
s e p a r a t e t h e s p e c i e s p r i o r t o a n a l y s i s , a n d t h e o b v i o u s way i n
which
t o attempt
Performance L i q u i d
employing
this
separation
Chromatography
was b y t h e u s e o f H i g h
(HPLC).
The a d v a n t a g e o f
HPLC f o r t h e s e p a r a t i o n i s t h a t t h e s p e c i e s
present
a r e a n a l y z e d d i r e c t l y w i t h o u t d e r i v i t i z a t i o n . The m a i n p r o b l e m
however,
i n adapting
t h e system
described
i n the previous
s e c t i o n t o i n c o r p o r a t e HPLC s e p a r a t i o n , became one o f d e t e c t o r
sensitivity.
min"\
A typical
elution
rate
f o r HPLC i s 1 - 1.5 m l
t h e sample i n t r o d u c t i o n r a t e i n t h e system d e s c r i b e d i n
C h a p t e r 3.5 i s 8 m l m i n " \
overcome.
I n addition,
a n d t h i s m i s m a t c h a l s o h a d t o be
the previous
115
system
used
a pre-
a c i d i f i e d s a m p l e w i t h a n a c i d c o n c e n t r a t i o n o f 6 m o l 1"^ - s u c h
an
acid
concentration
could
n o t be
employed
with
an i o n
exchange column, s i n c e i t w o u l d d e s t r o y t h e column.
Therefore
p o s t - c o l u m n a c i d i f i c a t i o n w o u l d be r e q u i r e d , f u r t h e r
diluting
t h e e l u e n t . I t became o b v i o u s
sensitive
to
be
used
as
t h a t AAS w o u l d be i n s u f f i c i e n t l y
the detector,
and
the
atomic
f l u o r e s c e n c e d e t e c t o r w o u l d h a v e t o be e m p l o y e d . A s y s t e m was
t h e r e f o r e s e t up as shown i n F i g u r e
As
may
be
observed
from
3.6.1.
t h e diagram,
the f i r s t
block
of
i n s t r u m e n t a t i o n i s a s i m p l e HPLC s e t up i n c l u d i n g t h e a b i l i t y
to
switch
between
different
eluents.
A
post-column
mixing
v a l v e was f i t t e d a n d 7 m o l 1"^ h y d r o c h l o r i c a c i d was i n t r o d u c e d
to
t h e a n a l y t e s t r e a m a t t h e r a t e o f 7 m l min"*, t h u s a c h i e v i n g
t h e d e s i r e d f l o w r a t e and a c i d c o n c e n t r a t i o n f o r t h e microwave
reduction
that
step.
employed
liquid
The r e m a i n d e r o f t h e s y s t e m was s i m i l a r t o
i n earlier
separator.
emanating
from
automated
The s e l e n i u m
the separator
system,
hydride
t o t h e gas-
and hydrogen
was p a s s e d
d r y i n g tube b e f o r e e n t e r i n g t h e atomic
up
through
mixture
a membrane
fluorescence detector.
D r y i n g o f t h e g a s e s a r e n e c e s s a r y due t o t h e f a c t t h a t
vapour quenches
fluorescence.
The
f i r s t experiments
see
i f separation
containing
sodium
c a r r i e d o u t were t o e v a l u a t e e l u e n t s , t o
could
be
5 n g ml'^ o f s e l e n i u m
selenate
water
was
prepared,
achieved.
as b o t h
and
performance o f t h e eluents. I n i t i a l l y ,
116
A
mixed
sodium
used
standard
s e l e n i t e and
t o determine
the
low concentrations of
F i g u r e 3.6.1
Diagram o f t h e automated
HPLC-microwave
r e d u c t i o n - h y d r i d e generation-AFS
speciation
selenium
system
BEHSON B A X - 1 0
MilOU
I
EXCBAIIGE COLUMN
a.UENT \
ELUENT 2
^—^
nixirc
VALVE
PeRISrALTlC
PERISTALTIC
o
o
MIXING
VALVE
REDUCTANTI
OOOLirC C O I L
REACTICW C O I L
MICROWAVE
DRIER TVJ8E
AWIC
R-UORESCEMIE
CAS-LIQUID
SEPARATOR
DEfECTOR
I
COrWTER
DATA L i r e
Selenite
Time/s
Typical
chromatogram
selenite
and
of
5
ng
selenate
117
g
selenium
as
I
phthalate
(100 mmol I'M
were employed, and w h i l s t e x c e l l e n t
s e p a r a t i o n was a c h i e v e d ,
c r y s t a l s o f p h t h a l i c a c i d formed a t
the
post-column
mixing
valve
when t h e a c i d
stream
met
the
c o l u m n e l u e n t . These c r y s t a l s e v e n t u a l l y l e d t o a b l o c k i n g o f
t h e t u b i n g , r e n d e r i n g p h t h a l a t e u n s u i t a b l e f o r use. Potassium
sulphate
was
then
tried,
s e p a r a t i o n when a s t e p
and
this
provided
base-line
c o n c e n t r a t i o n g r a d i e n t was
i . e . 25 mmol 1"^ a t pH 5 f o r t h e f i r s t
employed,
200 s e c o n d s f o l l o w e d b y
100 mmol 1"* a t a s i m i l a r pH f o r t h e r e m a i n d e r o f t h e r u n . A
number o f d i f f e r e n t pHs w e r e e v a l u a t e d b e t w e e n pH 5 a n d pH 7,
w i t h no d i s c e r n a b l e d i f f e r e n c e s b e i n g d e t e c t e d i n e i t h e r p e a k
shape o r p e a k a r e a . The c o n d i t i o n s p e r t a i n i n g t o t h e m i c r o w a v e
o p e r a t i o n w e r e as d e s c r i b e d p r e v i o u s l y i n C h a p t e r 3.5.
Once t h e s y s t e m h a d b e e n shown t o o p e r a t e
identical
efficiently,
five
i n j e c t i o n s o f t h e m i x e d s t a n d a r d w e r e made and t h e
p e a k a r e a s m e a s u r e d . These r e s u l t s a r e shown i n T a b l e 3 . 6 . 1 .
As c a n be s e e n , t h e p r e c i s i o n o f f e r e d b y t h e s y s t e m i s i n t h e
order
of
1.5
and
2.0%
RSD
for
selenite
and
selenate
r e s p e c t i v e l y . The l i m i t s o f d e t e c t i o n w e r e t h e n d e t e r m i n e d f o r
selenium(IV)
and s e l e n i u m ( V I )
a n d w e r e f o u n d t o be 0.2 a n d 0.3
ng ml'^ r e s p e c t i v e l y .
In
order
t o evaluate
Material,
analyzed.
and
2.8
content
NXST
SRM
a
real
1643c
sample, a
-
Trace
C e r t i f i e d Reference
Elements
i n Water
I t was f o u n d t o c o n t a i n 10.6 n g ml"^ o f
ng
ml"^ o f
selenium(VI) ,
o f 13.4 n g m l * ^
This
giving
a
was
selenium(IV)
total
selenium
i s i n good agreement w i t h t h e
118
Table
3.6.1
A n a l y s i s o£ a mixed s e l e n i u m
standard
(5 ng ml-^ S e ( I V ) and S e ( V I ) )
Peak
LOD
areas
Analysis
Se(IV)
1
761
249
2
776
238
3
770
249
4
761
247
5
788
241
Mean
771.2 S
244.8 S
Sn-l
11.34 S
(3 X s,.i)
0.22 n g ml**
119
Se{VI)
5.02
S
0.31 n g ml'^
certified
is
value
currently
selenium
of
12.7
± 0.7
ng ml'*. No
reference material
a v a i l a b l e w i t h c e r t i f i e d values
species.
The
values
for
individual
o b t a i n e d agree c l o s e l y w i t h the
v a l u e s p r e v i o u s l y o b t a i n e d w i t h t h e e a r l i e r automated system.
It
was
rise
found
t h a t higher concentrations of selenium(VI)
t o an e l e v a t e d b a s e - l i n e , b u t t h i s c o u l d be
gave
overcome
d i l u t i o n o f more c o n c e n t r a t e d s e l e n i u m ( V I ) s o l u t i o n s . The
o p e r a t i n g c o n d i t i o n s employed f o r t h e t o t a l system are
by
full
shown
i n T a b l e 3.6.2.
The
s y s t e m was
t h e n employed t o e v a l u a t e t h e c o n c e n t r a t i o n s o f
t h e s e l e n i u m s p e c i e s i n samples o f s e l e n i u m s o l u t i o n s s u p p l i e d
to
this
laboratory
for
a n a l y s i s programme. The
agreement w i t h those
analysis
under
the
BCR
selenium
c o n c e n t r a t i o n s o b t a i n e d were i n g o o d
obtained
by
conventional
reduction
subsequent a n a l y s i s v i a h y d r i d e g e n e r a t i o n coupled
fluorescence detection.
120
and
t o atomic
Optimized
T a b l e 3.6.2
con5)lete
operating conditions for the
system
HPLC
Sample l o o p
1 ml
Sample pH
7
M o b i l e phase 1
25
mmol 1'^ KjSO^,
pH5
M o b i l e phase 2
100 mmol 1"' K2SO4,
pH5
M o b i l e phase s w i t c h i n g
200
seconds
Eluent
2.0
ml min"^
flow
rate
Microwave
20%
Power
Hydride
Reductant
HCl
flow
flow
continuous
generation
4 ml min'^
rate
8 ml min"^
rate
Argon purge f l o w
rate
350 ml min"^
Argon d r i e r f l o w
rate
1000
ml min"^
Detector
Primary
Boost
lamp c u r r e n t
25 mA
lamp c u r r e n t
25 mA
Gain
1000
121
X 10
3.7
An
i n v e s t i g a t i o n i n t o the k i n e t i c s o f t h e r e d u c t i o n
selenium
( V I ) t o selenixim ( I V )
As d i s c u s s e d i n S e c t i o n 3 . 1 , t h e r e a p p e a r s t o be
reports
in
the
literature
selenium(VI) i s reduced
also
of
reflected
regarding
the
conflicting
rate
at
which
t o selenium(IV) . This disagreement
i n t h e recommended
c o n d i t i o n s f o r which
is
the
r e d u c t i o n s h o u l d be c a r r i e d o u t , t h e d e b a t e b e i n g f u e l l e d
an a p p a r e n t
loss o f selenium d u r i n g the r e d u c t i o n process.
As p r e v i o u s l y s t a t e d ,
t h e most commonly
employed method f o r
the r e d u c t i o n o f selenium(VI) i s t o heat
in
by
t h e sample t o 70 "^C
t h e p r e s e n c e o f 6M h y d r o c h l o r i c a c i d
"Conventional
wisdom"
suggests
that
f o r a given period.
this
period i s usually
t h i r t y m i n u t e s . A number o f w o r k e r s h a v e r e p o r t e d t h a t h e a t i n g
above
this
compounds.
temperature
a
loss
of volatile
selenium
I t has b e e n r e p o r t e d b y K r i v a n e t . a l . ^® f o l l o w i n g
work
using
were
i n fact
produced,
causes
radiotracers,
due
by t h e
that
apparent
losses
t o t h e back o x i d a t i o n
of
selenium
o f the selenium (IV)
action of free chlorine.
T h e i r work
showed
t h a t i f t h e s e l e n i u m ( I V ) s o l u t i o n produced by t h e h y d r o c h l o r i c
a c i d r e d u c t i o n was l e f t
was
converted
back
to
f o r one week, 94% o f t h e s e l e n i u m ( I V )
selenium(VI).
This
figure
remained
c o n s t a n t o v e r t h e n e x t f o u r weeks, and so may be r e g a r d e d
the
equilibrium
c o n c e n t r a t i o n . Since
h y d r i d e depends u p o n t h e s e l e n i u m
as
the generation of the
being
i n the selenium(IV)
oxidation state, i t i s clear that the a n a l y t i c a l determination
should proceed
immediately
upon c o m p l e t i o n o f t h e r e d u c t i o n
122
step.
I t i s also
easy
to
understand
the
early
confusion
p r o d u c e d by t h i s back o x i d a t i o n .
During
the
development
of
the
microwave
reduction
system
( S e c t i o n 3 . 4 ) , i t had been assumed t h a t i f t h e r e d u c t i o n s t e p
took
thirty
minutes
at
70
""C,
then
in
order
to
achieve
r e d u c t i o n i n an o n - l i n e p r o c e s s , t h e r e d u c t i o n r e a c t i o n would
have t o be c a r r i e d out a t a h i g h e r t e m p e r a t u r e .
n e c e s s a r y due
to the l i m i t e d
time
that
T h i s would be
t h e a n a l y t e would
be
i n t h e s y s t e m . I t was a n t i c i p a t e d t h a t t h e r e a c t i o n would have
t o be
carried
out
i n the
S e c t i o n 3.4.1, t h i s was
gas
phase.
As
d i s c u s s e d above
not i n f a c t found t o be the c a s e ,
in
and
so t h e k i n e t i c s o f t h e r e d u c t i o n were s t u d i e d i n more d e t a i l .
3.7.1
The
Experimental
d e t e r m i n a t i o n o f t r a c e l e v e l s o f s e l e n i u m has g e n e r a l l y
relied
an
upon t h e t e c h n i q u e
element
specific
of hydride
detector,
generation,
since
i t was
coupled
to
introduced
by
Holak. L a t t e r l y , t h e u s e of a q u a r t z f u r n a c e - a t o m i c
spectrometer
(QFAAS)
as
a
detection
system
absorption
has
become
i n c r e a s i n g l y common, s i n c e i t p r o v i d e s good d e t e c t i o n l i m i t s ,
good r e p r o d u c i b i l i t y and employs equipment commonly a v a i l a b l e
i n most l a b o r a t o r i e s . U s i n g t h i s t e c h n i q u e , w i t h a c o n t i n u o u s
flow hydride
a typical
generator
a n a l y s i s may
give a response
similar
take
to t h a t
shown i n F i g u r e
i n the order
of
as shown i n F i g u r e 3.7.1. As
123
2.1,
2 minutes,
c a n be
seen,
and
Figure
3.7.1
Typical analysis trace obtained
generation
by
hydride
o p e r a t i n g under e q u i l i b r i u m
conditions
u
(J
•Ui
u
ifi
a:
o
r)
>j
TIMI-;
124
(Kcconds)
tou
t h e r e i s an i n i t i a l d e l a y p e r i o d o f t y p i c a l l y 10-15
during
which
time
the
sample
reaches
the
seconds,
switching
valve,
i m m e d i a t e l y p r i o r t o sample i n t r o d u c t i o n i n t o t h e g a s - l i q u i d
separator.
T h i s i s f o l l o w e d by a r i s e time o f t y p i c a l l y
seconds i n which the d e t e c t o r
output heads towards a
state.
around
An
analysis period
of
45-60 seconds
15-20
steady
follows,
d u r i n g w h i c h time the measurement i s made. At t h e end o f
period,
the
switching valve ceases
to supply
this
a n a l y t e to
the
g a s - l i q u i d s e p a r a t o r , and t h e s i g n a l r e t u r n s t o the b a s e l i n e
as
the
gas-liquid
a n a l y t e . The
method r e l i e s
separator
is
cleared
of
any
remaining
d e c a y time i s t y p i c a l l y 30-45 s e c o n d s . Thus
upon an
e q u i l i b r i u m being
reached
i n the
l i q u i d s e p a r a t o r , and as a consequence o f t h i s , i t has
difficult
these
The
to f o l l o w anything
inbuilt
use
orders
of atomic
proved
but t h e s l o w e s t r e a c t i o n s due
fluorescence
detectors,
o f magnitude more s e n s i t i v e t h a n
to
o f an
analyte.
which are s e v e r a l
QFAAS, o f f e r a r a p i d
I t i s also possible
to a l t e r the c o n d i t i o n s used f o r the hydride
generation,
under computer c o n t r o l , use v e r y s h o r t sample i n j e c t i o n
these
gas-
delays.
response to the presence
Under
the
conditions
the
resulting
peaks
and
times.
resemble
flow
i n j e c t i o n p e a k s as shown i n F i g u r e 3.7.2, and t h e e q u i l i b r i u m
c o n d i t i o n s i n t h e gas l i q u i d s e p a r a t o r a r e not u p s e t
v e r y s h o r t p e r i o d i n w h i c h t h e sample i s b e i n g
over the
introduced.
As
can be s e e n i n F i g u r e 3.7.2, the c o m p l e t e a n a l y s i s i s o v e r i n
less
lasts
than
for
45
2
seconds.
seconds,
Since
a
the
sample i n j e c t i o n
"snapshot"
125
of
the
stage
r e a c t i o n may
only
be
Figure
3.7.2
Trace o b t a i n e d employing h y d r i d e
coupled to atomic fluorescence
w i t h a one
second i n j e c t i o n
generation
detection
time
to
u
'CO
u
'CO
•cC
C
•' I Ml-:
( crcoful::)
126
00
o b t a i n e d . I f t h i s i s r e p e a t e d a t one minute i n t e r v a l s , i t i s
t h e n p o s s i b l e t o b u i l d up a p i c t u r e o f t h e complete
reaction
process.
3.7.1.1
The
Instrumentation
apparatus
generator
used
{model
f o r thi-s
study
consisted of a hydride
10.004 from PS A n a l y t i c a l L t d . , Sevenoaks,
Kent) f i t t e d w i t h a Perma-pure d r i e r tube (PS A n a l y t i c a l L t d ) ,
and used i n c o n j u n c t i o n w i t h an atomic f l u o r e s c e n c e d e t e c t o r
(Excalibur,PS A n a l y t i c a l
computer
Ltd.).
control
using
Ltd.).
The i n s t r u m e n t s were under
Touchstone™
software(PS
Analytical
F u r t h e r d e t a i l s o f t h e i n s t r u m e n t a t i o n are g i v e n i n
Chapter 2.
3.7.1.2
Reagents
The r e a g e n t s used f o r t h i s s t u d y were sodium b o r o h y d r i d e 98%
( A l d r i c h ) , sodium s e l e n a t e 99% ( A l d r i c h ) , sodium s e l e n i t e 99%
( A l d r i c h ) and h y d r o c h l o r i c a c i d
(Analar-BDH).
Water was 18M
ohm from a M i l l i - Q system.
100
pg ml"^ s t o c k
solutions
s e l e n i t e were p r e p a r e d .
ml'^
were
prepared
solutions
1.3% m/v
daily
o f sodium
s e l e n a t e and sodium
Working s t r e n g t h s o l u t i o n s o f 50ng
from
these.
Sodium
borohydride
i n 0.1 mol 1"^ NaOH were a l s o
127
prepared
daily.
6 mol
1"^ h y d r o c h l o r i c a c i d s o l u t i o n s were p r e p a r e d
as
required.
3.7.1.3
Method
A beaker containing
200ml o f
p l a c e d i n a w a t e r bath,
(50, 55,
60,
achieved,
every
whilst
seconds,
calculated
out
in
maintaining
and
then
and
operated
e q u i l i b r i u m had
at
continuous
operated
the
s t o r e d by
triplicate
to
resulting
sample
temperature
solution
from t h e
. The
used, and
The
beaker
was
carried
software
felt
was
that
this
g r e a t e r p r e c i s i o n t h a n peak h e i g h t , b e a r i n g
mind t h e v e r y s h o r t s a m p l i n g t i m e s
was
concentrations
computer. T h i s p r o c e s s
each
been
agitation.
selenium
i n t h e peak a r e a mode, s i n c e i t was
would p r o v i d e
was
and h e a t e d t o t h e r e q u i r e d t e m p e r a t u r e
70 °C) . Once t h e r m a l
s y s t e m was
60
1'^ h y d r o c h l o r i c a c i d
a s m a l l a l i q u o t (100 p i ) o f t h e s t o c k
introduced
hydride
65 and
6 mol
the sharp
in
peaks
obtained.
3.7.2
Results
The
results
the
detector
plotted
of the t r i p l i c a t e
output
(related
e x p e r i m e n t s were averaged,
and
t o Selenium(IV) c o n c e n t r a t i o n )
a g a i n s t t i m e f o r e a c h t e m p e r a t u r e as shown i n F i g u r e
3.7.3. I t c a n be seen t h a t t h e c u r v e s o b t a i n e d a r e e x p o n e n t i a l
128
F i g u r e 3.7.3
Plot of fluorescence detector output
time f o r v a r i o u s
against
terr^^eratures
OH
;H
Time
o
o
C
•JG
( n i « s >
GH
C
129
C
OS
c
VH
C
i n c h a r a c t e r , and so were r e p l o t t e d u s i n g I n ( o u t p u t maximum o u t p u t ) a g a i n s t t i m e as shown i n F i g u r e 3.7.4. The r e s u l t i n g
straight
order
lines
indicate
i n character.
that
I t can
the reaction i s pseudo-first
be
seen
from
the
Arrhenius
equation,
k=A exp(-EyRT)
where k = r a t e
constant, A = pre-exponential f a c t o r ,
=
a c t i v a t i o n energy, r = gas c o n s t a n t and T = temperature
(K)
t h a t t h e s l o p e o f these l i n e s may be c a l c u l a t e d t o o b t a i n t h e
r a t e c o n s t a n t . A p l o t o f l n ( k ) a g a i n s t 1/T
a l s o produced a
s t r a i g h t l i n e as shown i n F i g u r e 3.7.5, t h e s l o p e o f which i s
equal
t o E;^/R. The
result
obtained
gave
a
value
f o r the
a c t i v a t i o n energy f o r t h e r e a c t i o n o f 90.39kJ mol"' u s i n g 6M
hydrochloric acid.
3.7.3
Discussion
The a c t i v a t i o n energy c a l c u l a t e d i n t h i s s t u d y i s h i g h e r t h a n
that
r e p o r t e d by
Bye
& Lund^^ b u t lower
than
the
figure
r e p o r t e d by P e t t e r s o n & 01in"'° (90.4kJ mol"^ c . f . 83kJ mol"^ and
126
kJ
mol"^
respectively).
However,
c o n d i t i o n s w i t h r e g a r d t h e chemical
the
experimental
composition of the t e s t
s o l u t i o n s were n o t i d e n t i c a l t o those used by Bye and Lund.
They were l o o k i n g a t s o l u t i o n s c o n t a i n i n g p e r c h l o r i c a c i d i n
130
F i g u r e 3.7.4
P l o t o f I n (output maximum - output)
against
time
Tine
Cmins)
--^>-- Ga c
131
— I — fes c
7B
C
F i g u r e 3.7.5
Plot of In(k) against
1/T
0.0 n
0.5 -
o.
o
-1.0
-
CO
S
-1.5 -
-2.0
-2.5
0.0029
0.00295
0.00300
1/T
132
0.00305
0.0031
addition to hydrochloric
duplicate
conditions
a c i d , i n an a t t e m p t t o more c l o s e l y
applicable
to
"real"
samples.
a d d i t i o n , t h e i r v a l u e f o r the a c t i v a t i o n energy was
u s i n g 4 mol
1"^ h y d r o c h l o r i c
used i n t h i s s t u d y . The
fluorescence
points
a p p l i c a t i o n o f the more r a p i d atomic
Olin
explained
energy
as
a
Lund w i t h r e g a r d s the
that
t h e r e was
flask
to
function
of
water
exact m a t r i x
bath
and
the
for
the
uncertainties
in
used by
used. They
of
Bye
indicated
introduction
contents
r e a c h i n g a c o n s t a n t t e m p e r a t u r e , and
value
conditions
a t i m e l a g between the
the
calculations.
t h e i r higher
t e m p e r a t u r e , s i n c e t h e y r e p e a t e d the
and
1'^
system i n t h i s work a l s o enabled many more data
and
activation
obtained
a c i d , as opposed t o the 6 mol
t o be o b t a i n e d , on which t o base the
Petterson
In
of
that
a l s o t h a t the
the
flask
contents
o f the f l a s k d i d not reach the t e m p e r a t u r e o f t h e b a t h . I t i s
difficult
to believe
these
factors
likely
that
would
the
o b t a i n e d by Bye
t h a t Bye
affect
lower
and
four points
the
accuracy
Lund d i d not
their results,
figure
Lund was
From t h e i r p u b l i c a t i o n ,
and
due
for
the
realise
and
that
i t i s more
activation
energy
t o the l a c k o f data
points.
i t appears t h a t
they obtained
only
at each t e m p e r a t u r e - c l e a r l y t h i s would a f f e c t
of
the
produced. Since the
on these g r a d i e n t s ,
plots,
and
hence the
slopes
of
lines
c a l c u l a t i o n o f a c t i v a t i o n energy r e l i e s
an obvious e x p l a n a t i o n o f the lower f i g u r e
becomes a p p a r e n t . I n t h i s study, the a c t u a l t e m p e r a t u r e o f
the
t e s t s o l u t i o n s were measured. P e t t e r s o n and O l i n a l s o i n d i c a t e
that f u l l
reduction
t o o k t h i r t y minutes a t 65 ''C.
133
As
can
be
seen from
Figure
temperature,
full
3.7.3, our
results
r e d u c t i o n was
indicate
achieved
that
at
this
w i t h i n t e n minutes
under our e x p e r i m e n t a l c o n d i t i o n s .
From t h e p l o t s o f o u t p u t a g a i n s t time i n F i g u r e 3.7.3, i t can
be seen t h a t t h e r e a c t i o n goes t o c o m p l e t i o n a f t e r 6 minutes
a t 70**C. Prolonged
are
h e a t i n g f o r - p e r i o d s o f 30 minutes o r more
t h e r e f o r e unnecessary. - S h o r t e r
heating
times
may
also
reduce t h e p o t e n t i a l f o r c o n t a m i n a t i o n when u s i n g open topped
c o n t a i n e r s . As r e p o r t e d by K r i v a n e t . a l . " ,
the s o l u t i o n
may
be b o i l e d under r e f l u x , b u t i t i s o f t e n more p r a c t i c a b l e when
dealing
w i t h l a r g e numbers o f
samples
t o have
containers
s i t t i n g i n a water b a t h , r a t h e r t h a n h a v i n g a l a r g e a r r a y o f
r e f l u x i n g systems i n o p e r a t i o n .
3.8
Photolysis
Since t h e development o f t h e HPLC-microwave-HG-AFS system
proved s u c c e s s f u l f o r t h e s p e c i a t i o n o f selenium i n i t ' s
inorganic
forms,
organo-selenium
compounds
were
also
i n v e s t i g a t e d , t o see whether t h e y were rendered d e t e c t a b l e by
microwave t r e a t m e n t . I n o r d e r t o achieve t h i s d e t e c t a b i l i t y ,
the t r e a t m e n t would need t o c l e a v e t h e selenium-carbon bonds,
and
convert
any
selenium
released t o the
selenium(IV)
o x i d a t i o n s t a t e . I n s p i t e o f a number o f e f f o r t s i n t h i s
direction,
they
remained
stubbornly
unaffected
and
undetectable.
134
Organo-selenium compounds are e x t r e m e l y r e s i s t a n t t o a c i d i c
degradation''^"", and as a r e s u l t , severe regimes have had t o
be employed f o r t h e i r d e t e r m i n a t i o n i n e n v i r o n m e n t a l
and
b i o l o g i c a l samples e.g. t h e use o f a m i x t u r e o f n i t r i c ,
s u l p h u r i c and p e r c h l o r i c acids'*. Newer t e c h n i q u e s u s i n g GC-MS
a f t e r d e r i v i t i z a t i o n t o form the t r i m e t h y l s i l y l a t e d compound
have been reported'^ f o r the a n a l y s i s o f s e l e n o - m e t h i o n i n e .
However, these t e c h n i q u e s are p r i m a r i l y employed when a number
of
organo-selenium
compounds
have
to
be
quantified
i n d i v i d u a l l y . I n environmental a n a l y s i s , i t i s f r e q u e n t l y only
required
t o s p e c i a t e on
the b a s i s
of
organo-selenium
compounds, s e l e n i u m { I V ) and s e l e n i u m ( V I ) , and so a method o f
r e l e a s i n g t h e selenium from organo-selenium compounds t o
render i t d e t e c t a b l e v i a h y d r i d e g e n e r a t i o n i s o f g r e a t
p o t e n t i a l use.
Since microwave energy had no e f f e c t i n an a c i d i c environment,
i t was d e c i d e d t o i n v e s t i g a t e whether p h o t o l y s i s would r e l e a s e
the selenium d i r e c t l y by b r e a k i n g t h e carbon-selenium bond.
This t e c h n i q u e has been employed i n s i m i l a r circumstances f o r
t h e d e t e r m i n a t i o n o f arsenic'^'" and tin''^.
3.8.1
Experimental
A stock s o l u t i o n o f seleno-methionine
{Aldrich, Gillingham,
Dorset) was prepared, c o n t a i n i n g a p p r o x i m a t e l y 100 mg 1'^ i n
3 mol 1"^ h y d r o c h l o r i c a c i d . From t h i s s o l u t i o n , a f u r t h e r
d i l u t i o n was made t o g i v e a s o l u t i o n o f 100 pg ml"S and t h i s
was d i v i d e d between t h e f o u r q u a r t z tubes o f t h e UV exposure
u n i t d e s c r i b e d i n S e c t i o n 2.1.3.6. The tubes were t h e n p l a c e d
i n t h e exposure u n i t , and exposed f o r 1,10, 30 and 60 m i n u t e s .
The samples were removed from t h e exposure u n i t a t t h e
a p p r o p r i a t e t i m e s , and
analyzed
using standard
hydride
g e n e r a t i o n t e c h n i q u e s (see S e c t i o n 3.3). The r e s u l t s are shown
135
g r a p h i c a l l y i n F i g u r e 3.8.1, and a s c a n be o b s e r v e d , show t h a t
t h e maximum c o n v e r s i o n
taken
place
repeated
after
using
a
from o r g a n i c t o i n o r g a n i c s e l e n i u m had
60 m i n u t e s .
more
The
dilute
experiments
solution
were
then
(25 ]ig ml"M , and
e x p o s u r e s made a t 5, 10, 20, 30, 40, 50, 60 and 70 m i n u t e s .
These s a m p l e s were a g a i n a n a l y z e d ,
to o b t a i n
and
recovery
yields.
and t h e r e s u l t s c a l c u l a t e d
These a r e shown i n F i g u r e
show t h a t 100% r e c o v e r y
i s reached
after
60 m i n u t e s .
S i n c e bromide/bromate i s employed t o c o n v e r t
species
to the inorganic
e x h i b i t e d by s e l e n i u m
form,
organo-mercury
and i n v i e w o f t h e a f f i n i t y
f o r bromine, t h e s e r e a g e n t s
to t h e seleno-methionine s o l u t i o n s i n v a r y i n g
and t h e e x p o s u r e s r e p e a t e d
the
photolytic
strengths
left
3.8.2,
were added
concentrations,
i n t h e hope t h a t t h e y may c a t a l y s e
reaction.
The
bromide/bromate
solution
were v a r i e d from 0.5 t o 10%, t h e s o l u t i o n s
f o r 30 m i n u t e s b e f o r e
being
12% h y d r o x y l a m i n e h y d r o c h l o r i d e .
decolourised
with
being
50 p i o f
I n a l l c a s e s , no improvement
i n r e a c t i o n r a t e was o b s e r v e d .
3.8.2
The
Discussion
exact
mechanism
involved
i n t h e breakdown
m e t h i o n i n e by UV r a d i a t i o n i n t h e s t r o n g l y a c i d i c
employed h e r e
released
seleno-
conditions
i s n o t known. I t i s p r o b a b l e t h a t t h e s e l e n i u m
from
selenium(IV)
of
the seleno-methionine
oxidation
state,
goes
directly
since the s o l u t i o n
to the
conditions
do n o t appear t o be l i k e l y t o g i v e r i s e t o f u r t h e r o x i d a t i o n .
However, t h e p o s s i b i l i t y
selenium(VI)
oxidation
of
of the r e a c t i o n proceeding to give
( w i t h f r e e c h l o r i n e p l a y i n g a p a r t a s i n t h e back
selenium(IV)
by
S e c t i o n 3.4) and t h i s b e i n g
to
the heating
effects
conventional
reduction -see
r e d u c e d back t o s e l e n i u m ( I V ) due
of
t h e lamp
and
the high
acid
c o n c e n t r a t i o n cannot be r u l e d o u t on t h e b a s i s o f t h e r e s u l t s
obtained.
Further investigations are obviously
136
required to
Figure
3.8.1
Plot of selenium
concentration
detected
a g a i n s t UV exposure time f o r s e l e n o methionine
7 S .B
GB .B
2S.B
h
IB
ZB
3B
Time
137
IB
(mi«s)
(>B
Figure
3.8.2
P l o t of selenium
concentration
detected
a g a i n s t UV exposure time f o r s e l e n o m e t h i o n i n e to i l l u s t r a t e r e a c t i o n r a t e
VB
Time
138
Cr»i«s)
elucidate
which o f
these
potential
mechanisms are
in
fact
followed.
W h i l s t t h e t i m e t a k e n t o achieve cleavage o f t h e carbonselenium bond may a t f i r s t appear t o be t o o l o n g f o r t h i s
t e c h n i q u e t o have any a p p l i c a t i o n i n an o n - l i n e system, i t
s h o u l d be remembered t h a t i n these experiments, l a r g e sample
volumes were used, and these were p l a c e d a t some d i s t a n c e from
t h e UV source. I f a c a p i l l a r y - c a r r y i n g t h e sample s o l u t i o n
were p l a c e d c l o s e r t o t h e lamp, a much s h o r t e r p e r i o d f o r t h e
r e a c t i o n s h o u l d be r e a l i s e d . The d i s t a n c e between sample and
lamp i s c r i t i c a l
since the i n t e n s i t y of the
radiation
d i m i n i s h e s w i t h t h e square o f t h e d i s t a n c e . Thus, t h e
p o s s i b i l i t y o f i n c o r p o r a t i n g a UV exposure stage w i t h i n a
system s i m i l a r t o t h a t d e s c r i b e d i n S e c t i o n 3.6 c o u l d p r o v i d e
a " u n i v e r s a l selenium s p e c i a t i o n system", capable o f p r o v i d i n g
d e t e r m i n a t i o n s f o r s e l e n i u m { I V ) , s e l e n i u m ( V I ) and organoselenium compounds.
3.9
Summary
The s t u d i e s c a r r i e d out on selenium have y i e l d e d a number o f
interesting results.
The apparent i n c r e a s e i n s i g n a l when d e t e r m i n i n g selenium i n
t h e presence o f t r a c e s o f n i t r i c a c i d v i a h y d r i d e g e n e r a t i o n
was found t o be a r e a l e f f e c t and found t o be a s s o c i a t e d w i t h
t h e t y p e o f pump t u b i n g employed. The
exact mechanism
r e s p o n s i b l e f o r t h e e f f e c t was not determined, a l t h o u g h two
p o s s i b l e e x p l a n a t i o n s were suggested.
The work c a r r i e d out on t h e c o n d i t i o n i n g o f the h y d r i d e
g e n e r a t i o n systems i n d i c a t e d t h a t a t t r a c e and u l t r a t r a c e
139
l e v e l s , c o n d i t i o n i n g c o u l d t a k e some c o n s i d e r a b l e t i m e i f t h e
system i s not p r e - t r e a t e d by i n i t i a l sample i n j e c t i o n s o f a
r e l a t i v e l y high concentration of selenium(IV). I f conditioning
i s not c a r r i e d o u t , t h e n d e t e r m i n a t i o n s s h o u l d be c a r r i e d out
u s i n g t h e " b r a c k e t i n g method".
The use o f an o n - l i n e microwave p r e - r e d u c t i o n s t e p was found
t o be f e a s i b l e , and t h i s l e d t o t h e development o f an
automated system f o r selenium s p e c i a t i o n d e t e r m i n a t i o n s u s i n g
h y d r i d e g e n e r a t i o n and t h e n ^ q u a r t z f u r n a c e - a t o m i c a b s o r p t i o n
spectroscopy
as a d e t e c t i o n system. T h i s development was
f u r t h e r r e f i n e d t o achieve d i r e c t d e t e r m i n a t i o n s o f selenium
species by s e p a r a t i o n u s i n g HPLC, and employing t h e more
s e n s i t i v e atomic f l u o r e s c e n c e d e t e c t o r t o compensate f o r
sample d i l u t i o n .
The i n v e s t i g a t i o n i n t o t h e k i n e t i c s o f the
selenium{VI)
r e d u c t i o n process i n d i c a t e d t h a t under our c o n d i t i o n s , f u l l
r e d u c t i o n t o o k p l a c e w i t h i n s i x minutes a t 70**C, thus
o b v i a t i n g t h e need f o r l o n g e r h e a t i n g , and t h e r e b y r e d u c i n g
t h e dangers o f c o n t a m i n a t i o n .
The use o f a p h o t o l y s i s stage t o f r e e selenium from organoselenium compounds has been e s t a b l i s h e d t o be e f f e c t i v e . The
p o s s i b i l i t y o f i n c o r p o r a t i n g such a stage i n t o a comprehensive
selenium s p e c i a t i o n system i s an obvious f u t u r e development.
140
'er 4
AppUcatiom q
on-line pyrotysm
141
Pyrolysis
4.1
Introduction
Pyrolysis
may
organic
be
d e f i n e d as
compounds"
materials
have
concentrated
"the t h e r m a l d e c o m p o s i t i o n
although
also
for
been - i n c l u d e d .
exclusively
on
mercury
of
this
study
inorganic
The
study
has
as
the
metal
also
under
investigation.
Mercury i s an unusual element,
b e i n g t h e o n l y m e t a l which i s
l i q u i d a t room t e m p e r a t u r e . I t forms amalgams w i t h most o t h e r
metals
(except i r o n and p l a t i n u m ) and these t a k e t h e form o f
solids
or
liquids,
depending on
the p r o p o r t i o n of
mercury
p r e s e n t . T h i s p r o p e r t y i s employed w i d e l y i n d e n t i s t r y ,
a l s o p r o v i d e s t h e b a s i s f o r a simple and e x t r e m e l y
detector
f o r traces
o f mercury.
The
change
in
and
sensitive
electrical
p r o p e r t i e s o f a g o l d w i r e due t o amalgamation are employed t o
provide
quantitative
measurements
f o r the
element.
After
exposure and measurement, t h e w i r e i s s i m p l y heated t o d r i v e
o f f t h e mercury and r e n d e r i t s u i t a b l e f o r t h e next sample.
Both t h e m e t a l and i t ' s compounds are h i g h l y t o x i c , and t h e r e
are
very
studies
exists
many r e p o r t s i n t h e
on
flora
and
literature
fauna^°. Since
from b o t h n a t u r a l and
of
toxicological
environmental
anthropogenic
sources,
mercury
and
in
view o f t h e severe e f f e c t s t r a c e q u a n t i t i e s o f t h e element
may
have on b o t h i n d u s t r i a l processes and on human h e a l t h , much
142
work
i s currently
biogenic
Mercury
being
undertaken
to fully
elucidate the
pathways o f t h e species i n v o l v e d .
contamination
i n c i d e n t s such
as
t h e much
quoted
Minamata Bay d i s a s t e r ^ ^ have a l e r t e d s c i e n t i s t s t o some o f t h e
problems
concerning
mercury,
but i n various
areas
of the
w o r l d , mercury i s s t i l l w i d e l y used f o r r e c l a i m i n g g o l d from
muds and g r a v e l s ,
rivers
and hence
and s u b s e q u e n t l y
large quantities
reach
t h e sea. There
escape
into
i s current
concern i n t h e U n i t e d S t a t e s over h i g h l e v e l s o f mercury found
i n f i s h and t h e mammals which f e e d on them i n t h e Everglades.
These h i g h l e v e l s a r e thought
gold
t o come from mercury used by
p r o s p e c t o r s i n t h e Amazon and i t ' s t r i b u t a r i e s ,
which
i s washed by ocean c u r r e n t s around t h e bay o f Mexico, b e f o r e
b e i n g d e p o s i t e d i n t h e Everglades
area^^.
Concern i s a l s o growing r e g a r d i n g t h e use o f mercury amalgams
i n d e n t i s t r y . A number o f s t u d i e s have i l l u s t r a t e d t h e h a r m f u l
e f f e c t s which
may a r i s e
with
amalgams®^'^^, and concern
mercury
from h a v i n g d e n t a l c a v i t i e s
i s growing
filled
for
the
h e a l t h o f d e n t i s t s and t h e i r s t a f f who have been s u b j e c t e d t o
c o n t i n u o u s l y h i g h l e v e l s o f mercury i n t h e i r work. Mercury,
once i n g e s t e d , c o n c e n t r a t e s
through
i n t h e kidneys
t h e b r a i n membrane b a r r i e r .
b l o c k s v a r i o u s nerve f u n c t i o n s .
and a l s o passes
Once i n t h e b r a i n , i t
Mercury i s a l s o a b l e t o pass
t h r o u g h t h e p l a c e n t a i n t o t h e f o e t u s " , and development o f t h e
b r a i n and nervous system may be i m p a i r e d . There i s i n c r e a s i n g
evidence o f a l i n k between mercury and A l z h e i m e r s disease"'®"',
143
and a l s o o f i t ' s d e t r i m e n t a l e f f e c t s upon t h e immune system^ .
There i s c l e a r l y a need f o r methods f o r t h e d e t e r m i n a t i o n o f
mercury which a r e b o t h
s e l e c t i v e and v e r y s e n s i t i v e .
Early
t e c h n i q u e s were based on c o l o u r i m e t r i c methods, b u t s u f f e r e d
from a l a c k o f s e n s i t i v i t y ,
and a l s o problems o f l o s s e s and
c o n t a m i n a t i o n due t o t h e l o n g p r e p a r a t i o n steps i n v o l v e d . I n
1963, t h e use o f c o l d
spectrometry
vapour g e n e r a t i o n - atomic
was reported^^, and s i n c e t h e n , t h i s has become
t h e most w i d e l y employed t e c h n i q u e
mercury
i n biological
fluorescence
generation
absorption
samples^".
The l a t e r
d e t e c t i o n instruments
has
improved
still
d e t e c t i o n , and t h i s t e c h n i q u e
f o r the determination of
introduction of
coupled
t o cold
vapour
further
the l i m i t s
of
i s f a s t g a i n i n g acceptance as
a s i m p l e , s e n s i t i v e and r e l i a b l e method f o r t h e d e t e r m i n a t i o n
of
mercury.
throughout
4.2
For
A
mercury
fluorescence
detector
was
used
t h i s s t u d y and t h i s i s d e s c r i b e d i n Chapter 2.
Experimental
a l l the investigations i n t h i s
employed
section,
was c o n f i g u r e d as shown i n F i g u r e
observed from t h e diagram, t h e apparatus
t h e system
4.1. As may be
c o n s i s t e d o f a gas
m i x i n g v a l v e a t t h e i n l e t o f t h e p y r o l y s i s oven. The c a r r i e r
gas
employed
addition
was argon,
o f oxygen
and t h e m i x i n g
when
required.
valve
The
allowed the
gases
from
the
p y r o l y s i s oven t h e n passed i n t o a p r e - c o n c e n t r a t i o n u n i t ,
144
Figure 4.1
Basic p y r o l y s i s system
OXYGEN
INLET
123,4
Cn
GOLD TRAP
TQ CLEAN
ARGON
ARGON
INLET
PYROLYSiS
OVEN
PRE-
MERCURY
CONCENTRATION
FLUORESCENCE
UNIT
DETECTOR
which
t r a p p e d t h e mercury
liberated
f r o m t h e sample
i n the
o v e n on a g o l d sand t r a p . The o u t l e t o f t h e p r e - c o n c e n t r a t i o n
u n i t was c o n n e c t e d d i r e c t l y
Mercury
has a vapour
t o the fluorescence detector.
p r e s s u r e o f 0.0016 mbar a t 22*" C w h i c h
c o r r e s p o n d s t o a c o n c e n t r a t i o n o f a p p r o x i m a t e l y 14 mg m'^.
vapour
therefore
provides a
source
The
o f contamination which
a f f e c t s e v e r y t h i n g i n t h e vicinity^°. A s s u m i n g t h a t t h e a n a l y s t
has
amalgam f i l l i n g s ,
c o n t a c t between t h e sample and e x h a l e d
b r e a t h must be a v o i d e d , s i n c e t h i s c o n t a i n s e a s i l y
mercury
with
vapour.
detectable
Rigorous c l e a n i n g o f a l l equipment
the analysis
of this
element
is vital.
concerned
This
cleaning
e x t e n d s t o t h e s i l i c a sample t u b e s used i n t h e p y r o l y s i s
T h i s was a c c o m p l i s h e d
by s u b j e c t i n g
t h e t u b e s t o an
oven.
initial
h e a t i n g a t 800° C f o r 30 m i n u t e s , a f t e r w h i c h t h e y w e r e s t o r e d
in
a 30% s o l u t i o n o f A r i s t a r n i t r i c
were i n t r o d u c e d ,
a c i d . B e f o r e t h e samples
t h e t u b e s were d r i e d
externally with
paper
t o w e l l i n g , t h e n a r g o n w h i c h h a d b e e n p u r i f i e d u s i n g an i n - l i n e
g o l d t r a p was u s e d t o d r y t h e i n s i d e o f t h e t u b e . The t u b e was
then
subjected to a
purified
to
t e n minutes
a
was
level
element
at
800** C,
with
a r g o n f l o w i n g t h r o u g h i t a f t e r w h i c h i t was a l l o w e d
c o o l b e f o r e t h e s a m p l e was l o a d e d i n t o
regime
to
further
found t o reduce
which
t h e background
d i d not a f f e c t
cleaning
l e v e l s o f mercury
the determination
i n any o f t h e s a m p l e t y p e s l i s t e d
146
i t . This
below.
of the
Using t h e system o u t l i n e d
above,
a number o f
investigations
were c a r r i e d o u t . These w e r e :
a) t h e d e t e r m i n a t i o n o f m e r c u r y i n s e d i m e n t s
b) an a t t e m p t
t o determine mercury i n s i l v e r
c ) an a t t e m p t
t o use t h e t e c h n i q u e
of
mercury i n h a i r
d) an a t t e m p t
4.2.1
f o r the determination
and-finally
t o determine mercury i n o i l
The d e t e r m i n a t i o n o f mercury i n
Since
the
more
nitrate
commonly
employed
sediments
methods
of
determining
m e r c u r y i n s e d i m e n t s i n v o l v e much sample p r e p a r a t i o n , w i t h t h e
c o n s e q u e n t i a l r i s k s o f l o s s e s and c o n t a m i n a t i o n , any
which
i s able
to
reduce
these
steps
i s of
interest.
w o r k e r s h a v e r e p o r t e d t h e use o f c a p i l l a r y gas
(GC)
f o r organo- mercury
studies^S
technique
Some
chromatography
b u t t h e s e methods
still
r e q u i r e c o n s i d e r a b l e sample p r e - t r e a t m e n t . A s i m p l e and r o b u s t
technique
f o r determining
therefore
desirable.
Using
total
the apparatus o u t l i n e d
samples
of
sediment
mercury
above
{approximately
in a
i n Chapter
30
mg)
sediment
5.2,
were
is
weighed
carefully
p o s i t i o n e d i n t h e h e a t i n g zone o f t h e s i l i c a sample t u b e s u s e d
in
the p y r o l y s i s
oven.
147
After
r u n n i n g a number o f d i f f e r e n t
found
that
produced
h e a t i n g the
the
temperatures
results
s e d i m e n t s t o 550° C
shown i n T a b l e
4.1.
i t was
for five
Heating
minutes
at
lower
resulted i n inconsistent results, whilst heating
at h i g h e r temperatures
heating
h e a t i n g regimes,
had
no b e n e f i c i a l e f f e c t ,
f o r l o n g e r p e r i o d s . The
results
neither did
show good
agreement
b e t w e e n t h e c e r t i f i e d v a l u e s and t h o s e o b t a i n e d e x p e r i m e n t a l l y
( 64.2
ng g"' c . f . 63 ± 12 ng g"^ i n t h e c a s e o f NIST
4.2.2
The
atterr5>ted
determination
of
mercury i n
1646).
silver
nitrate
Silver
nitrate
is
used
extensively
in
the
photographic
i n d u s t r y as a p r e c u r s o r i n t h e p r e p a r a t i o n o f p h o t o s e n s i t i v e
emulsions
in
both
films
and
contamination of s i l v e r n i t r a t e
f i l m manufacturers
analysis
silver
reported, but
nitrate
papers.
Mercury
i s a serious problem f o r the
e v e n a t l o w ng g"^. V a r i o u s m e t h o d s f o r t h e
o f mercury i n s i l v e r
have been
printing
into
most
nitrate
a t t h e nanogram
i n v o l v e the
a halide salt,
conversion
f o l l o w e d by
level
of
cold
the
vapour
m e t h o d s a p p l i e d t o t h e f i l t r a t e ^ ^ . As d i s c u s s e d above, p r o b l e m s
from
mercury
importance
losses
of
contamination
when a n a l y s i n g f o r t h i s
apparatus
was
silver nitrate
element,
are
and
of
paramount
thus
efforts
s e t up as d e s c r i b e d p r e v i o u s l y , and
samples
w e r e made t o e m p l o y
The
and
pyrolysis.
( a p p r o x i m a t e l y 30-40 mg)
148
were l o a d e d
into
T a b l e 4.1
D e t e r m i n a t i o n o f mercury i n c e r t i f i e d
reference
sediments
Analysis
NIST 8406
NIST 1646
Estuarine
sediment
Tenessee R i v e r
Sediment
1
67.2
58.1
2
61.8
59.3
3
66.0
58.1
4
64 .2
61.2
5
61.4
59.4
Mean
64.2
59.2
Certified
value
63 ± 12 ng g"^
149
(60) ng
g-1
the
silica
sample t u b e s . V a r i o u s
heating
c y c l e s were
tried,
from v e r y g e n t l e h e a t i n g a t 100** C up t o more i n t e n s e h e a t i n g
at
500*
C.
Absolutely
Various
no
sample was
heating
mercury
was
times
detected
were
at
also
any
employed.
temperature.
A
t h e n s p i k e d w i t h m e r c u r y vapour by d r a w i n g v a r i o u s
volumes o f t h e vapour above a r e s e r v o i r o f the
element
a gas s y r i n g e , and t h e n i n j e c t i n g t h i s i n t o the s i l v e r
into
nitrate
sample. A g a i n no m e r c u r y w a s - d e t e c t e d .
The i n s t r u m e n t a t i o n
was
checked
correctly,
the
and
found
c o n c l u s i o n was
to
be
and
and
r e a c h e d t h a t a t low t e m p e r a t u r e s ,
of m e r c u r y f o r s i l v e r
r e l e a s e d . At
operating
was
the
so
affinity
so g r e a t t h a t t h e mercury was
higher temperatures,
the s i l v e r n i t r a t e
not
sublimes
presumably the mercury i s t e m p o r a r i l y r e l e a s e d , only
to
recombine w i t h t h e s i l v e r n i t r a t e as soon as i t c o n d e n s e s . I t
was
t h e r e f o r e concluded
this particular
4.2.3
The
t h a t t h e t e c h n i q u e was u n s u i t a b l e f o r
problem.
The a t t e n p t e d d e t e r m i n a t i o n o f mercury i n hiiman h a i r
determination
o f m e r c u r y i n human h a i r may
screening technique
be
used
i f mercury exposure i s suspected.
m e r c u r y l e v e l s i n hair^° a r e around 1 yg g'\
a n a l y t i c a l procedures
a r e r e q u i r e d . The
and
so
Normal
sensitive
c o n v e n t i o n a l methods
employed f o r t h i s a n a l y s i s i n v o l v e t h e d i g e s t i o n of t h e
sample, and
A
number
s u b s e q u e n t a n a l y s i s by c o l d vapour
of
experiments
were
150
as a
performed
hair
generation.
along
the
lines
i l l u s t r a t e d p r e v i o u s l y , b u t w i t h sample w e i g h t s o f l e s s t h a n
10 mg. These l o w w e i g h t s o f sample were due t o t h e f a c t
that
t h e m a t e r i a l has e x t r e m e l y l o w d e n s i t y , a n d sample v o l u m e i s
l i m i t e d b y t h e s i z e o f t h e h e a t i n g zone a n d t h e b o r e
silica
of
sample t u b e s . H e a t i n g
300 °C i n a r g o n
alone
o f t h e sample t o a
of the
temperature
r e s u l t e d i n t h e p r o d u c t i o n o f smoke
p a r t i c l e s which f l o o d e d t h e fluorescence d e t e c t o r . Experiments
were
performed
to
investigate
released a t temperatures
samples
from
60
t h i s d i d occur,
whether
mercury
would
below t h e c h a r r i n g p o i n t by h e a t i n g
t o 150 °C i n 10 **C i n c r e m e n t s ,
r e s u l t s were v e r y
and w h i l s t
inconsistent.
The a d d i t i o n o f v a r y i n g amounts o f o x y g e n t o t h e a r g o n
gas was t h e n made so t h a t
the organic matter
was
i n t o c a r b o n d i o x i d e . S l o w l y i n c r e a s i n g t h e sample
at
a rate
be
o f 10 °C min"^
was
carrier
converted
temperature
required t o avoid
a n y smoke
p r o d u c t i o n , b u t a g a i n t h e r e s u l t s were i n c o n s i s t e n t , o f t e n b y
a t l e a s t one o r d e r o f m a g n i t u d e .
In spite of the fact
a
number
of
t h a t these experiments
occasions,
obtained.
This
may
be
equipment,
particularly
no
consistency
due
when
to
such
w e r e r e p e a t e d on
of
limitations
small
results
in
weights
were
weighing
are being
employed, d i f f i c u l t i e s i n h a n d l i n g t h e m a t e r i a l o r s i m p l y t h e
fact
that
such
small
sample
weights
are not
r e p r e s e n t a t i v e o f t h e main b u l k o f m a t e r i a l .
151
sufficiently
4.2.4
The atten5)ted d e t e r m i n a t i o n o f mercury i n o i l
A s i m i l a r t e c h n i q u e was a p p l i e d t o o i l s a m p l e s , t o t h a t
above.
In this
case,
t h e use
of
a
silica
crucible
s h a p e d and c o n s t r u c t e d t o f i t i n t o t h e s i l i c a
employed
A
lack
i n the
reproducibility of
o b t a i n e d , and t h e same c o n c l u s i o n s
i . e . that
problems
sample t u b e
and
sufficiently
4 .3
the
results
was
the very
that
the
represent
small
size
was
r e a c h e d as t o t h e p o s s i b l e
t h e l o w d e n s i t y o f t h e sample
i n weighing
precision,
samples
of
such
results
in
with sufficient
samples
may
now
t h e main b u l k o f t h e sample.
Mercury d e t e r m i n a t i o n s en5>loying gas chromatography
c o u p l e d to the p y r o l y s i s system w i t h
fluorescence
The
spoon,
t o c o n t a i n t h e o i l samples.
similar
causes
given
use
of
atomic
detection
capillary
gas
chromatography
as
a
means
of
s e p a r a t i n g v o l a t i l e o r g a n o - m e r c u r y compounds has a l r e a d y b e e n
r e f e r r e d to^°. T h i s i s commonly f o l l o w e d b y an e l e c t r o n c a p t u r e
detector
when
compounds.
A
process
the
halogenated
d e r i v i t i z a t i o n step
non-halogenated
considerable
since
determining
care
to
sensitivity
i s therefore required
compounds,
ensure
of
that
the
152
organo-mercury
no
and
this
requires
contamination
detector
is
to
so
occurs,
high.
Contamination o f solvents i soften a p a r t i c u l a r
Using
t h e apparatus
pyrolize
the
problem.
shown i n F i g u r e 4.2, i t was p o s s i b l e t o
organo-mercury
compounds
directly
s e p a r a t i o n b y t h e column, and t h u s d e t e c t t h e mercury
after
directly
u s i n g a t o m i c f l u o r e s c e n c e . The GC was a Pye U n i c a m m o d e l 104,
fitted
with
diameter
(S.G.E.,
internal
polar
an u l t r a b o r e
diameter
Australia).
fused s i l i c a ,
BP-1 s t a t i o n a r y
phase
0.53mm i n t e r n a l
diameter
employed.
coliimn,
This
inlet
adaptor
o f 0.53mm
The c o l u m n
was 3m x
coated w i t h
silica
employed
A 350mm x
post-column
as a
0.53mm
a 5 micron non-
(S.G.E., A u s t r a l i a ) .
fused
internal
was a l s o
resistively
heated
t r a n s f e r l i n e a n d h e l d a t 150 °C v i a a V a r i a c t r a n s f o r m e r , f e d
t h e e l u e n t s f r o m t h e column d i r e c t l y
i n t o the heated
zone o f
t h e p y r o l y s i s o v e n , w h i c h was h e l d a t 800 ^'C.
A number o f e x p e r i m e n t s were p e r f o r m e d t o d e t e r m i n e t h e e f f e c t
o f c o l u m n t e m p e r a t u r e , c a r r i e r gas f l o w r a t e ,
sample gas f l o w
r a t e a n d s h i e l d g a s f l o w r a t e , a n d t h e s e a r e shown i n F i g u r e s
4.3
-
4.6.
Using
the conditions
shown
i n Table
e x p e r i m e n t s were p e r f o r m e d t o d e t e r m i n e t h e response
mercury
c h l o r i d e and d i e t h y l mercury,
F i g u r e 4.7.
153
4.2,
of ethyl
a n d t h e s e a r e shown i n
F i g u r e 4.2
Coupled G C - p y r o l y s i s - A F S s y s t e m
SAMPLE
INLET
HELIUM
CARRIER GAS
G C OVEN
L NE
MERCURY
PYROLYSIS
FLUORESCENCE
OVEN
DETECTC3R
ARGON SAMPLE GAS
INLET
ARGON SHEATH
GAS INLET
154
F i g u r e 4.3
E f f e c t o f column
temperature
f.VHHHH
(
r.lHHHH
VH
Co 1Limii
X
±.emper*Ei.i:u.r'e
D i R t l xIfi l1
MCK-t^ur-*^
G C - F I D chromatogram of I yl of a standard muture o( 10 p&/ml of D E M . N W C
and ENlC as mercury in n-hexane.
^1 L
155
Figure
4.4
E f f e c t of c a r r i e r gas
flow r a t e
G3UBUU
r.HMHHH
Cair-r-ier-
zs
3B
cfa.s F l o w
r-a-te
156
F i g u r e 4.5
E f f e c t of s a n ^ l e gas flow
rate
G'lHBHB
eaBBBB
SSBBBB
•JHHHHH
1MHUBH
3!SBBBH
SBB
3BB
-X
Dic3t.l.Ml
157
M
F i g u r e 4.6
E f f e c t o f s h i e l d gas flow
rate
Sr.HHBB
HMUUU
-40BBBH
• BB
158
9BB
Table
4.2
Instrument
conditions f o r GC-pyrolysis-
atomic f l u o r e s c e n c e
Gas c h r o m a t o g r a p h
oven
100 °c
temperature
Column t y p e
BP-5
.600
X
0.53
( i d ) mm
5% p h e n y l d i m e t h y l s i l o x a n e .
Carrier
gas
Transfer l i n e
Helium
temperature
(7.5 m l
min'M
150 **C
P y r o l y s i s oven t e m p e r a t u r e
800 °C
Sheath
gas
Argon
(600 m l
min'M
Sample gas
Argon
(250 m l
min'M
159
F i g u r e 4.7
Response o f e t h y l mercury and d i m e t h y l mercury
leaaaee
UUMUUU
cuuuua
iHMHea
oa
-IB
Coiice«-tic*a.*t. i o n
^
•0
DEM
160
EMC
4.4
Summary
The u s e o f p y r o l y s i s as a m e t h o d o f d e t e r m i n i n g t o t a l m e r c u r y
i n s e d i m e n t s a m p l e s was f o u n d t o w o r k s a t i s f a c t o r i l y w i t h good
agreement
being
obtained
with
sample
concentrations o f
a p p r o x i m a t e l y 60 ng g"^. When a p p l i e d t o l o w e r d e n s i t y o r g a n i c
r i c h samples, t h e method d i d n o t prove
reliable.
I t is
strongly
suspected
that
this
is
due
to
the
u n r e p r e s e n t a t i v e n e s s o f t h e sample s i z e , r a t h e r t h a n a d e f e c t
i n t h e equipment o r t h e t e c h n i q u e .
The u s e o f t h e c o u p l e d t e c h n i q u e G C - P y r o l y s i s - A F p r o d u c e d some
encouraging
r e s u l t s , and w a r r a n t s
161
further investigation.
er 5
162
5
Development o f an automated pH a d j u s t i n g
5.1
In
system
Introduction
many
sample
analytical
is critical
determinations,
i foptimal
e m p l o y e d i s t o be m a i n t a i n e d .
been
mentioned
selenium
many
species
other
control
performance
o f t h e pH o f a
o f the technique
One e x a m p l e o f t h i s h a s a l r e a d y
i n C h a p t e r - 3,
i . e . the determination
of
b y HPLC-microwave r e d u c t i o n - H G - A F S , h o w e v e r
examples
do
exist.
I n many
cases,
samples a r e
a c i d i f i e d upon c o l l e c t i o n i n o r d e r t o s t a b i l i z e t h e sample and
prevent
" p l a t e o u t " , t h e l o s s o f c a t i o n s f r o m t h e s o l u t i o n due
t o t h e i r a d s o r p t i o n onto t h e surface o f glass containers'^. I n
such
cases,
i t i s o f t e n necessary
t o adjust
t h e pH o f t h e
sample j u s t p r i o r t o a n a l y s i s . T h i s p r o c e s s i s u s u a l l y c a r r i e d
out manually,
and
which i s extremely tedious f o r the p r a c t i t i o n e r ,
c o s t l y i n terms o f labour.
When t r a c e a n d u l t r a - t r a c e
determinations
are required, the
pH a d j u s t m e n t
also r e q u i r e s t h a t t h e reagents
contaminants
t o t h e sample.
At
such
u s e d do n o t a d d
levels,
p r o b l e m a t i c s i n c e many commonly u s e d r e a g e n t s
working
a t these
u n d e s i r a b l e f o r obvious
levels,
dilution
be
I n addition,
o f t h e sample i s
r e a s o n s . The n e e d f o r an a u t o m a t e d pH
a d j u s t i n g system, which addresses t h e a f o r e m e n t i o n e d
is therefore
may
are unavailable
i n t h e p u r i t y necessary t o avoid contamination.
when
this
apparent.
163
problems
Previous
work by Jackson e t a l ^ ^ d e s c r i b e s
a system
f o r the
n e u t r a l i z a t i o n o f samples o b t a i n e d f r o m t h e d i g e s t i o n o f f o o d
by s u l p h u r i c a c i d . I n t h i s p r o c e s s ,
was n e u t r a l i z e d
reagent
t h e s t r o n g l y a c i d i c sample
u s i n g g a s e o u s ammonia. S i n c e
t h e use o f t h i s
a v o i d s a f f e c t i n g t h e volume o f t h e sample s o l u t i o n by
dilution,
a n d a l s o a v o i d s c o n t a m i n a t i o n o f t h e s a m p l e , i t was
selected
as t h e r e a g e n t
employed
batch
f o r -this
work.
processing, - using
The o r i g i n a l w o r k
electromechanical
timers
e t c . T h i s a p p r o a c h c l e a r l y has l i m i t a t i o n s , a n d so t h e d e s i g n
o f an a u t o m a t e d o n - l i n e
s y s t e m was
initiated.
5.1. Theory
I n a sample o f pure w a t e r ,
falls
t o zero,
due
i t ' s electrical conductivity
t o s e l f - i o n i z a t i o n . The p r o c e s s
r e p r e s e n t e d by t h e equations
2H2O (1)
^
Using
(1)
^
may
be
:
H3O* (aq)
o r more
H2O
never
+
OH-
(aq) "
simply
H* (aq)
+
OH" (aq)
the l a t t e r equation, the e q u i l i b r i u m constant
may be
defined :
K,
=
[H*] tOH-]
[H2O]
Since o n l y a few m o l e c u l e s
o f water a c t u a l l y i o n i s e ,
be r e g a r d e d
at constant
as a c o n s t a n t
164
temperature.
[HjO] may
The i o n i c
product
o f water
K„
K^, i s
=
At a temperature
Kc
X
constant
o f 25''C,
=
[H *] [OH']
K^^ = 1 x 10"^^ mol^ dm"^ a n d s i n c e t h e
c o n c e n t r a t i o n s o f h y d r o g e n and h y d r o x y l i o n s a r e e q u a l ,
their
c o n c e n t r a t i o n s a r e 1 x lO"' m o l d i t i " \
In
any aqueous s o l u t i o n ,
is
determined
and
hydroxyl
and
t h e degree o f a c i d i t y o r a l k a l i n i t y
by t h e r e l a t i v e c o n c e n t r a t i o n s o f t h e hydrogen
ions.
If
[H^]
)
i f
[OH"]
[0H-] t h e s o l u t i o n w i l l
)
[H^]
be
the solution w i l l
acidic
be
alkaline
A n e u t r a l s o l u t i o n contains equal concentrations o f both
[H*]
The
=
[0H-] =
10 -''mol
pH o f a s o l u t i o n i s d e f i n e d as - l o g i o [ H * ] , a n d e f f e c t i v e l y
removes t h e n e e d f o r awkward n e g a t i v e
Measurement
of
pH
may
be
carried
indices.
out using
electrode i n conjunction with a standard reference
The
ions
c o n n e c t i o n b e t w e e n t h e e.m.f o f t h e c e l l
g i v e n by t h e Nernst
Eccii
=
a
hydrogen
electrode.
(E^^^n) a n d pH i s
equation :
E^^, +
( 2 . 3 R T / F ) l o g [H Maq) ]
I n o r d e r t o measure pH, t w o h a l f c e l l s may be c o n s i d e r e d , t h e
165
left-hand cell
electrode
t y p i c a l l y being
a saturated calomel
and has a p o t e n t i a l
Ejc^i,,
reference
whilst the right-hand
e l e c t r o d e i s a h y d r o g e n e l e c t r o d e who's p o t e n t i a l i s g i v e n b y
the equation
:
E(HVH2)
At
=
- 59.16mV
25°C, t h e p o t e n t i a l o f t h e c e l l
E
=
-59.16mV
x
x
t h u s becomes :
pH
-
Thus i t may be seen t h a t f o r - a n i n c r e a s e
voltage
from
the electrode
will
pH
i n pH o f 1 u n i t , t h e
d e c r e a s e b y 59.16 mV.
This
change i s w h a t i s m e a s u r e d b y "pH m e t e r s " w h i c h a r e i n f a c t
high
input
defined
impedance
as b e i n g
voltmeters.
at 0 volts,
By
convention,
pH
7 is
and t h e r e f o r e a c i d i c s o l u t i o n s
g i v e a p o s i t i v e o u t p u t , and a l k a l i n e s o l u t i o n s a n e g a t i v e one.
In
practice,
place
glass
o f t h e glass
o r membrane
electrodes
e l e c t r o d e , due t o g r e a t e r ease o f use, a n d
combination
electrodes
combination
electrode i s i l l u s t r a t e d
5.2
a r e employed i n
a r e now g e n e r a l l y e m p l o y e d . One s u c h
i n Figure
5.1.1.
Development o f t h e s y s t e m
The o v e r a l l d e s i g n c r i t e r i a i d e n t i f i e d f o r t h e s y s t e m i n v o l v e d
t h e t r a n s f e r o f t h e s a m p l e f r o m an a u t o s a m p l e r t o a v e s s e l i n
which treatment
w o u l d o c c u r , i . e . a d j u s t m e n t o f t h e sample's
166
F i g u r e 5.1.1
T y p i c a l combination pH e l e c t r o d e
FILLING
HOLE
AgIAgCl
ELECTRODE
AglAgCl
0.1M HC1
ELECTRODE
SOLUTION
3 M KCl
GLASS
CERAMIC-LI QUID
MEMBRANE
JUNCTI ON
167
pH
to
a
pre-determined
value.
The
sample
would
r e t u r n e d t o a f r e s h c o n t a i n e r p o s i t i o n e d on t h e
or passed d i r e c t l y
carry
out
vessel
the
and
determination.
connecting
cycle, before
An
t o the a n a l y t i c a l
overview
t u b i n g can
the process i s
of
this
At
then
autosampler,
i n s t r i i m e n t w h i c h was
this
be
stage,
step
reaction
subjected to a
cleaning
repeated.
process
i s shown as
a
f l o w diagram
i s t o reset the
system at
s w i t c h on.
solutions
controls.
then
The
of
known
d e s i r e d pH
pH
via
the
buffer
in
first
The
s t e p i s t o s e t up t h e pH e l e c t r o d e i n t h e n o r m a l way
buffer
to
the
F i g u r e 5 . 2 . 1 . From t h e d i a g r a m , i t can be s e e n t h a t t h e
process
be
next
using
and
two
slope
o f t h e t r e a t e d sample s o l u t i o n s i s
set.
From t h i s
point,
the
sample i n t r o d u c t i o n ,
removal
of
t u b i n g and
the
system e n t e r s
the adjustment
sample
from
the
a loop which consists
o f i t ' s pH and
system.
of
subsequent
Following
this,
the
e n t r y v a l v e s a r e washed t o e r a d i c a t e a l l t r a c e s o f
t h e s a m p l e , and t h e r e a c t i o n v e s s e l and e x i t s y s t e m a r e washed
o u t t w i c e . A t t h e end o f t h e s e c o n d wash c y c l e , t h e s y s t e m i s
reset,
event
and
that
the
next
sample
i s introduced.
In
the
unlikely
each sample r e q u i r e d a d j u s t i n g t o a d i f f e r e n t
t h e s e t t i n g o f t h e pH
c o u l d be
168
included i n the control
pH,
loop.
F i g u r e 5.2.1
Process overview
S W n C I I ON &
SEI
RESET
UP pll E E E C I R O D K
SEI
D E S I K E ! ) pH
INIRODUCESAMI'LI
A D . I U S I ph
Ri:sEr
PUMP OUI
SAMPLE
WASH X 2
169
5.2.1
Design o f the v a l v i n g
From t h e s p e c i f i c a t i o n
design
identified
above, i t was p o s s i b l e t o
t h e v a l v i n g a r r a n g e m e n t s f o r t h e f l o w o f s a m p l e , wash
s o l u t i o n and gases, and t h e s e
a r e shown i n F i g u r e 5.2.2. The
v a l v e s w i t c h i n g c o n d i t i o n s f o r e a c h p r o c e s s were
these
evaluated,and
a r e shown i n T a b l e 5 . 2 . I . - F i g u r e 5.2.2 shows t h e s y s t e m
i n t h e r e s e t c o n d i t i o n - a l l - v a l v e s a r e as shown and t h e pump
is
o f f . An
increment
s i g n a l i s sent
e n a b l e s e l e c t i o n o f a sample. A f t e r
t o t h e autosampler t o
a preset
delay
t h e pump
and v a l v e 2 a r e s w i t c h e d on a n d s a m p l e i n t r o d u c t i o n commences.
Valve
2
is
pressurised
a
vent,
during
i s sent
prevents
filling.
upon a s i g n a l f r o m
signal
and
Sample
the level
vessel
introduction
becoming
terminates
s e n s o r , w h e r e u p o n an
t o the autosampler
s o l u t i o n , adjustment
the
increment
f o r i t to select a
wash
o f t h e pH a n d w a s h i n g o f t h e e n t r y t u b i n g
commences. T h i s i s f a c i l i t a t e d b y v a l v e 1 s w i t c h i n g o n , so as
t o p a s s t h e wash s o l u t i o n t o w a s t e , a n d v a l v e 3 a l s o s w i t c h i n g
on,
t o allow
ammonia
laden
argon
to
enter
the reaction
v e s s e l . N e u t r a l i s a t i o n o f t h e s a m p l e s o l u t i o n now o c c u r s . When
t h e d e s i r e d pH o f t h e sample h a s b e e n a c h i e v e d
by
t h e pH
probe,
valve
3
switches
as
determined
o f f t o prevent
further
ammonia f r o m r e a c h i n g t h e s a m p l e . V a l v e s 4,5 and 6 s w i t c h o n ,
valves
5
solution,
and
6 to select
and v a l v e
V a l v e 2, t h e v e n t ,
4 t o allow
argon
taken
by
the treated
t o enter
the vessel.
i s now c l o s e d a n d t h u s t h e v e s s e l
p r e s s u r i s e d by t h e argon,
solution.
the route
and t h i s
becomes
e f f e c t i v e l y pumps o u t t h e
The s o l u t i o n may be d i r e c t e d t o a new c o n t a i n e r on
170
F i g u r e 5.2.2
Valving
diagram
PERISTALTIC
PUMP
VENT
WASTE
V2
REACTION VESSEL
CONTROL
ROTAMETERS
AMMONIA
RESERVOIR
WASTE
OUTLET
ARGON SUPPLY
171
T a b l e 5.2.1
VALVE 1
RESET
SAMPLE
INTRO
A D J U S T pH
—
—
—
EJECT
SAMPLE
+
WASH
INTRO
—
WASH
CYCLE
—
EJECT
WASH
—
WASH
INTRO
—
WASH
CYCLE
—
EJECT
WASH
—
I n i t i a l v a l v e s w i t c h i n g regime
VALVE 2
—
+
VALVE 3
—
+
—
—
+
—
+
+
—
—
—
—
—
—
—
VALVE 5
VALVE 6
—
—
—
—
—
—
—
+
+
+
—
—
—
—
—
—
—
+
+
VALVE 4
+
+
+
—
—
—
—
—
+
+
+
—
—
S signifies sample - W signifies wash
17
ALTTOSAMPLER
S
s
w
w
w
w
w
w
w
w
PUMP
—
+
—
+
+
—
—
+
—
—
the
original
or d i r e c t l y
Washing
autosampler,
t o another
autosampler a l t o g e t h e r
i n t o a measuring system.
o f the reaction vessel
now
commences
- a l l valves
s w i t c h o f f , w i t h t h e e x c e p t i o n o f v a l v e 2, a n d t h u s t h e v e s s e l
is
filled
that
4
and
w i t h wash s o l u t i o n .
the vessel
switched
thus
is full,
Once t h e l e v e l
sensor
t h e pump i s s w i t c h e d
on t o a l l o w a r g o n t o b u b b l e t h r o u g h
provide
agitation.
After
a preset
detects
o f f and v a l v e
the solution
wash p e r i o d , t h e
wash s o l u t i o n i s e j e c t e d t o w a s t e b y s w i t c h i n g on v a l v e 5 a n d
valve
before
2 o f f . This
whole
wash
procedure
t h e system advances t o t h e r e s e t
i s then
repeated,
position.
With t h e o p e r a t i o n o f t h e v a l v e system e s t a b l i s h e d , t h e design
of
the
t h e e l e c t r o n i c s was i n i t i a t e d .
design
of the different
A l l t h e components used i n
sections
are readily a v a i l a b l e
f r o m most s u p p l i e r s .
5.2.2
The
Development o f t h e e l e c t r o n i c s s y s t e m
e l e c t r o n i c s employed
sections
and t h e o v e r v i e w
i n this
system has t h r e e
i s shown
i n Figure
distinct
5.2.3..
The
p r i m a r i l y a n a l o g u e s e c t i o n w h i c h c a r r i e s o u t t h e measurement
and
comparisons o f values,
command,
a n d an i n t e r f a c e
a digital
which
valves.
173
s e c t i o n f o r c o n t r o l and
provides
switching of the
F i g u r e 5.2.3
E l e c t r o n i cs o v e r v i e w
ANALOGUE SECTION
AMPLIFY.
TIMERS,
DISPLAY
CONTROL SECTION
INTERFACE
VALVES. PUMP S
AUTOSAMPLER
174
5.2.2.1
An
The
overview
with
the
analogue system
o f t h e a n a l o g u e s y s t e m i s shown i n F i g u r e
full
circuit
d i a g r a m shown i n F i g u r e
5.2.5. As
be s e e n , t h e o u t p u t s i g n a l f r o m t h e p r o b e f i r s t
operational
high
amplifier.
i n p u t impedance
loading
the
probe.
This
device,
a CA
(10^^ ohms) w h i c h
The
amplifier
5.2.4,
can
e n t e r s a CMOS
3140E, has
a
i s required to
very
avoid
i s c o n f i g u r e d i n the
non-
i n v e r t i n g mode, w h i c h adds t o t h e o v e r a l l i n p u t impedance o f
the
input
circuit.
p r o d u c t i o n o f pH
to
Due
to
probes,
manufacturing
some a d j u s t m e n t s
variations
in
the
must be a v a i l a b l e
c o m p e n s a t e , and t h e s e a r e p r o v i d e d b y t h e s l o p e and b u f f e r
controls.
Since
pH
is
adjustment
i s a l s o r e q u i r e d t o compensate f o r v a r i a t i o n s
s o l u t i o n temperature.
between
the
output
a m p l i f i e r A I C 3 . The
an
analogue
crystal
p a c k a g e . The
w h i c h has
from
temperature
These a d j u s t m e n t s
of
the
output
to digital
readout,
also
these
input
amplifier
from t h i s
and
the
s e c t i o n i s measured v i a
converter
and
devices
being
displayed
on
incorporated
a
liquid
in
and
when t h e
p r o b e r e a c h e s t h e p r e - s e t d e s i r e d pH
equivalent
to
one
value
voltage
level,
the
c o m p a r a t o r AIC4 w i l l go h i g h , and t h i s s i g n a l
w i l l be s e n t t o t h e l o g i c c i r c u i t r y . S i n c e t h e s y s t e m has
designed
in
final
p r o c e s s e d s i g n a l i s a l s o compared w i t h a
output from the
an
are a l l i n c o r p o r a t e d
been p r e - s e t by t h e o p e r a t o r ,
t h e pH
dependant,
neutralize
acidified
t o t h e d e s i r e d pH
will
from the probe, u n t i l adjustment
achieved.
175
solutions,
the
always exceed the
been
voltage
voltage
o f t h e s o l u t i o n ' s pH has
been
F i g u r e 5.2.4
Overview o f t h e analogue s y s t e m
PROBE
TO LOGIC
SET pH
FROM SENSOR
•
FROM SYSTEM
START OR
LOGIC
1
^1
TO L O G I C
R E S E T TIMER
1
1
TO LOGIC
LEVEL
DETECTOR
FROM L O G I C
•
E J E C T TIMER
1
^1
TO L O G I C
FROM L O G I C
•
WASH TIMER
1
^1
TO L O G I C
A/S TIMER
1
1
TO INTERFACE
FROM LOGIC
176
F i g u r e 5.2.5
Analogue,
t i m e r s and l e v e l d e t e c t o r c i r c u i t
diagram
MQ4
A C3
4 TO I C 3 - Q CN
^ CCNTRXLCR
set
in «i
IN LO
,
MAIN ANALOGUE CIRCUIT
4
HH
mi
CA 3 M ( £
oun^JT TO
CCWTROLLER
,
7555
1
r r t vKxa cr PT w« CT
JL
tCLeCICD rCR M M
TIrCR
FRCM
CCWTROLLER
1(X
TIMER CIRCUIT DIAGRAM
LEVEL DETECTOR CIRCUIT
This
section
of the c i r c u i t r y
also
incorporates
a
level
d e t e c t i o n c i r c u i t . Since t h e system i s d e s i g n e d t o n e u t r a l i z e
samples c o n t a i n i n g
trace q u a n t i t i e s of analyte,
t h e use o f
i n v a s i v e probes t o d e t e c t t h e l e v e l i n t h e r e a c t i o n v e s s e l was
thought undesirable.
The use o f an i n f r a r e d d e t e c t i o n system
was developed which t r a n s m i t s a s i g n a l t o t h e l o g i c c i r c u i t r y
when
the desired
described
volume
has • e n t e r e d
the vessel.
The i s
more f u l l y l a t e r . *
Three t i m e r s a r e i n c o r p o r a t e d i n t o t h e system, one p r o v i d i n g
a r e s e t d e l a y , one p r o v i d i n g a wash p e r i o d and a n o t h e r t i m i n g
the
e j e c t i o n o f the analyte.
The i n p u t t o t h e r e s e t
timer
comes f r o m e i t h e r from a system s t a r t up c i r c u i t o r from t h e
l o g i c c o n t r o l . The i n p u t s t o b o t h
t h e wash and e j e c t
timers
b o t h o r i g i n a t e from t h e l o g i c c o n t r o l . A l l t i m e r o u t p u t s a r e
r e t u r n e d t o t h e l o g i c c o n t r o l system. There i s a l s o a t i m e r
to
operate
circuitry,
t h e autosampler
and an o u t p u t
with
going
an i n p u t
the l o g i c
t o the i n t e r f a c e u n i t .
p e r i o d o f t h e r e s e t t i m e r was a r b i t r a r i l y
was
from
seconds.
The wash p e r i o d
seconds,
and t h e e j e c t p e r i o d was a d j u s t e d
The
s e t a t about t e n
set t o approximately
to
twenty
approximately
t h i r t y seconds, a f t e r measuring t h e t i m e t a k e n f o r t h e system
t o empty, and then a l l o w i n g a f u r t h e r 15% f o r e r r o r .
Whilst
the f i n a l instrument
w i l l be under computer c o n t r o l ,
and w i l l t h e r e f o r e be adapted t o r e c e i v e i t ' s t i m i n g from t h e
computer,
system.
timers
were r e q u i r e d
The CMOS v e r s i o n
f o r t h e development
of the ubiquitous
178
of the
555 t i m e r
was
s e l e c t e d f o r use.
The
555
t i m e r s are s i m p l e t o use
and
v e r y i n e x p e n s i v e - t h e CMOS v e r s i o n used here have
are
similar
b e n e f i t s , and a l s o consume l e s s power, and do n o t s u f f e r from
s p i k e problems found i n t h e i r b i p o l a r c o u n t e r p a r t s . Since t h e
timers
are
r e q u i r e d t o be
triggered
by
a
positive
going
t r a n s i t i o n , and t h e IC r e q u i r e s a n e g a t i v e g o i n g s i g n a l ,
the
circuitry
The
around
TRl
was
designed
to
achieve
this.
c i r c u i t r y around TR2 i n v e r t s - t h e o u t p u t s i g n a l from t h e t i m e r ,
making i t c o m p a t i b l e w i t h t h e r e q u i r e m e n t s o f t h e r e s t o f t h e
l o g i c system.
5.2.2.2
S e t t i n g up p r o c e d u r e f o r the analogue system
Once
system
the
had
been
c o n s t r u c t e d as
per
the
circuit
diagram, t h e system r e q u i r e d s e t t i n g up. B e f o r e t h e i n t e g r a t e d
c i r c u i t s were f i t t e d
i n t o t h e i r sockets, i n i t i a l
continuity
checks were c a r r i e d t o check t h a t t h e pcb l a y o u t was
Once these checks had been made, t h e system was
correct.
powered up,
and t h e o p e r a t i o n o f t h e 12 v o l t r e g u l a t o r checked. Then t h e
6.5
v o l t s u p p l y was
r e g u l a t o r R2,
was
e s t a b l i s h e d , by m o n i t o r i n g t h e o u t p u t o f
and a d j u s t i n g p o t e n t i o m e t e r P2 u n t i l 6.5
measured r e l a t i v e t o t h e 0 v o l t s r a i l .
The
volts
mains s u p p l y
was t h e n d i s c o n n e c t e d , and t h e i n t e g r a t e d c i r c u i t s f i t t e d .
One
lead
to
was
disconnected
effectively
remove
s e t t i n g up procedure
from
the
buffer
i t from
the
circuit.
control
The
i s g i v e n i n Table 5.2.2.
179
so
rest
as
of
the
T a b l e 5.2.2
ACTION
S e t t i n g up p r o t o c o l
CONTROL
f o r t h e analogue
system
MONITOR
ADJUST
TO READ
AICl pin 4
P3
0. OmV
P6
O.OmV
PANEL METER
P9
0.00
AICl pin 4
P4
+516 mV
BUFFER
7.00
SETTINGS
SHORT INPUT
TEMP - 100
SLOPE - MAX
w i t h DVM
AIC2 p i n 4
w i t h DVM
INPUT +37 9inV
Refit
to
lead
buffer
TEMP - 0
SLOPE - MAX
w i t h DVM
TEMP - 0
PANEL METER
SLOPE - MAX
CONTROL
control short
input
INPUT
TEMP - 25
-175inV
SLOPE - 50%
INPUT
TEMP - 25
-70mV
SLOPE - MAX
INPUT
TEMP - 25
+ 70mV
SLOPE - MAX
PANEL METER
P8
10.50
PANEL METER
13.99
PANEL METER
0.01
180
Once
the
setting
up
analogue s e c t i o n was
calibrator
procedure
USA).
measured,
and
5657-00,
that
t e m p e r a t u r e . The
the b u f f e r
display.
control
was
The
c a l i b r a t o r was
c a l i b r a t o r was
control
7,
and
p a n e l meter d i s p l a y
both
places
measuring
of
system
control
adjusted to t h i s
on t h e p a n e l meter
The
calibrator
t h e r e a d i n g a g a i n checked.
the corresponding
noted.
was
t h e n s e t t o pH 3, and t h e slope
t h e n set t o a l l v a l u e s
1,3,5,7,9,11,13, and
to
Chicago,
s e t t o pH 7, i . e . 0 o u t p u t ,
set -to g i v e 7.00
c a l i b r a t o r was
was
of
the
meter
the temperature
a d j u s t e d t o g i v e a r e a d i n g o f 3.00.
calibrator
analogue
u s i n g a pH
Cole-Parmer,
r e a d i n g s e t on
t h e n r e s e t t o pH
correct
completed,
For t h i s check, t h e ambient temperature
o f t h e analogue system. The
and
been
checked f o r accuracy
(Model
Illinois,
had
on
The
i t ' s range i e
values
shown on
pH
the
I n a l l cases, t h e r e a d i n g s were
decimals,
was
indicating
functioning
that
the
correctly
and
accurately.
5.2.3
The
c o n t r o l system
The d e s i g n o f t h e l o g i c system was t h e n developed. The
system i s shown i n F i g u r e
5.2.6. I n i t i a l l y ,
initial
t h e use
of
a
m i c r o p r o c e s s o r was c o n s i d e r e d as t h e main c o n t r o l d e v i c e , but
as w i l l
be
observed
from Table
5.2.1, t h e r e are t e n
steps
o v e r a l l i n t h e f u l l c y c l e f o r n e u t r a l i z a t i o n o f a sample. This
suggested
the
novel
use
of
a decade
181
counter
as
the
main
c o n t r o l l i n g device,
are
both
design
very
and t h i s was
inexpensive
p r o b l e m was
such a way
pursued s i n c e these d e v i c e s
(around
50p)
and
therefore to configure
reliable.
The
the c i r c u i t r y
as t o make each stage t r i g g e r t h e decade
in
counter
c l o c k and t h e r e b y advance t h e process t o t h e subsequent stage.
The CD 4017
i s a CMOS decade -counter, which has t e n s p i k e - f r e e
o u t p u t s , a l l bar one
o f which are h e l d low. A p o s i t i v e g o i n g
p u l s e on t h e c l o c k i n p u t causes t h e c o u n t e r t o advance, t a k i n g
the next
output
high,
and
r e t u r n i n g the
s t a t e , p r o v i d i n g the master r e s e t and
held
low.
device,
I t was
therefore
2 - 'set d e s i r e d pH'
the
low
c l o c k enable p i n s
are
as
the
and t h e r e s t o f t h e l o g i c b u i l t around
S e l e c t o r s w i t c h 31 has
of
selected
former t o t h e
pH
probe'
potentiometer,
main c o n t r o l
it.
t h r e e p o s i t i o n s , 1 - 'probe set
and 3 - ' s t a r t ' . D u r i n g
and
s w i t c h Sic
the
'setting
causes p i n 15
182
of
up',
the ' s e t t i n g
the
desired
up
pH'
( t h e master r e s e t )
F i g u r e 5.2.6
L o g i c c o n t r o l c i r c u i t diagram
.•12
3d
II
00 K
•
A/S
MONO
LEVEL
r-/6
^
CO
V2, V3
WASH
V2 .V4
CD 4 0 1 7
iCl
=
IC2
IC6
j CD
!C3
[C8
1 CD 4081
40106
\CA
V4 . V5
VI .V4.V5.V6.P
EJECT
TIMER
ICS
) CD 4071
on t h e decade c o u n t e r ,
t o go h i g h . T h i s causes o u t p u t 0 t o go
high,
outputs
and a l l o t h e r
t o go l o w . I n these
switch
p o s i t i o n s , a c a p a c i t o r i s charged v i a a r e s i s t o r from t h e 12
volt
line.
allows
Moving s e l e c t o r s w i t c h S I t o t h e s t a r t p o s i t i o n
this
timer's
charge t o f l o w t h r o u g h
trigger
circuit,
and
a diode i n t o
thereby
the reset
the reset
period
commences. T h i s p u l s e a l s o d r i v e s p i n 8 on IC5c h i g h , sending
t h i s gate's o u t p u t h i g h , which i n i t i a t e s a monostable t i m e r
to
s i g n a l t h e autosampler
t o advance t o t h e f i r s t
sample
p o s i t i o n . P i n 12 o f IC3d i s h e l d h i g h by t h e h i g h o u t p u t on
I C l p i n 3, and a t t h e end o f t h e r e s e t p e r i o d , IC3d p i n 13
a l s o goes h i g h . T h i s causes a h i g h t o t r a n s f e r t h r o u g h
three
f u r t h e r OR-gates, IC4d, IC4b and IC5b w i t h t h e o u t p u t o f t h e
latter
gate
controller.
being
This
thereby causing
The
connected
t o the clock
p o s i t i v e going
input
of the
s i g n a l advances t h e c l o c k ,
o u t p u t 1 t o go h i g h , and o u t p u t 0 l o w .
h i g h on o u t p u t
1 causes a h i g h on IC4a p i n 3, and t h i s
s i g n a l i s used t o s w i t c h on v a l v e V2 and t h e pump. I t a l s o
a c t s as a g a t i n g s i g n a l on IC3a p i n 2, the o t h e r i n p u t t o t h i s
gate b e i n g t h e l e v e l d e t e c t o r o u t p u t . When t h e d e s i r e d
of s o l u t i o n i n the r e a c t i o n vessel
i s reached, t h i s
goes h i g h , sending a p o s i t i v e g o i n g s i g n a l t h r o u g h
gates
onto
the clock
i n p u t , thereby
advancing
level
output
t h r e e OR-
the counter
a g a i n . Since o u t p u t 1 now goes low, t h e pump and v a l v e V2 a r e
switched o f f .
The h i g h l e v e l on o u t p u t 2 s w i t c h e s on v a l v e s V2 and V3 which
184
a l l o w s t h e ammonia l a d e n argon t o e n t e r t h e sample s o l u t i o n ,
and
enables
AND-gate IC3c. I t a l s o p u t s a h i g h on p i n 9 o f
IC5c, and thence onto t h e i n p u t t o t h e autosampler
so t h a t i t i s advanced t o a wash r e s e r v o i r .
monostable,
The o t h e r i n p u t
t o t h i s gate i s r o u t e d from t h e o u t p u t o f t h e pH comparator,
and when t h e
d e s i r e d sample pH i s achieved
t h i s goes h i g h .
This p r o v i d e s a h i g h l e v e l on t h e o u t p u t o f IC3c, which
then
passes t h r o u g h IC4c, IC4b and IC5b, thus advancing t h e c o u n t e r
once a g a i n .
Since
o u t p u t now goes low, t h e ammonia
supply
ceases.
Output 3 now goes h i g h , which p r o v i d e s s w i t c h i n g f o r v a l v e s
V I , V4, V5, V6 and t h e pump, and a l s o causes t h e o u t p u t o f
gate
IC7b t o go h i g h . This
initiates
the eject
t i m e r , and
passes t h r o u g h a lOOK r e s i s t o r , v i a a lOOnF c a p a c i t o r t o two
i n v e r t e r g a t e s , IC6a and IC6b. The purpose o f t h e r e s i s t o r c a p a c i t o r combination i s t o provide a time delay i n the high
level
signal
r e a c h i n g t h e AND-gate IC8c. T h i s i s necessary
because t h e e j e c t t i m e r t a k e s a f i n i t e time t o s w i t c h t o i t ' s
low
( t i m e r on) c o n d i t i o n . I f t h e d e l a y were n o t p r e s e n t , t h e
gate IC8c would have h i g h l e v e l s on b o t h i n p u t p i n s b e f o r e t h e
t i m e r had s t a r t e d , and as a r e s u l t , t h e o u t p u t from gate IC8c
would go h i g h i m m e d i a t e l y , i n s t e a d o f a t t h e end o f t h e t i m i n g
p e r i o d . ICG and i t s c o u n t e r p a r t IC2 are b o t h CD40106's, and
these a r e S c h m i t t i n p u t i n v e r t e r s , which
the
slowly
rising
capacitor f i l t e r .
IC8c
goes h i g h ,
input signal
produced
a r e able t o accept
by t h e r e s i s t o r -
Once t h e e j e c t p e r i o d i s over, p i n 10 o f
causing
the outputs
185
o f IC7c
and IC5b t o
f o l l o w , r e s u l t i n g i n t h e advance o f t h e c o u n t e r a g a i n .
Output
4 o f I C l now
goes h i g h , and
s i n c e i t i s one
of
two
o u t p u t s which a l l o w t h e v e s s e l t o f i l l w i t h wash s o l u t i o n , t h e
two o u t p u t s are f e d t o t h e OR-gate IC5a. E i t h e r i n p u t t o t h i s
gate causes a h i g h on t h e o u t p u t , which t h e n passes t o IC4a,
a n o t h e r OR-gate. The h i g h o u t p u t from t h i s gate s w i t c h e s
on
v a l v e V2 and t h e pump and gates IC3a. This gate a w a i t s a h i g h
from t h e l e v e l d e t e c t o r and when t h i s
i s r e c e i v e d , sends a
s i g n a l t h r o u g h IC4d, IC4b and IC5b t o advance t h e c o u n t e r .
The
resulting
outputs
which
high level
have
on
output
identical
5 i s a g a i n one
of
two
f u n c t i o n s ( e n a b l i n g t h e wash
f u n c t i o n ) , and so are f e d t o an OR-gate IC5d. The o u t p u t from
t h i s gate t r i g g e r s t h e wash t i m e r and s w i t c h e s on v a l v e s
and V4,
and a l s o passes t h r o u g h a r e s i s t o r - c a p a c i t o r
network
b e f o r e e n t e r i n g IC2e and IC2d,
i n v e r t i n g Schmitt gates.
function
t h e same as
above.
o f these
Once t h e
devices
wash
are
t i m e r p e r i o d i s over,
appears on p i n 5 o f IC3b, which
IC4c,
IC4b, IC5b and
those
V2
The
described
a high
i s t r a n s m i t t e d on
level
through
thus t o t h e c l o c k i n p u t o f I C l , thus
i n i t i a t i n g a f u r t h e r change i n t h e c o u n t e r o u t p u t .
W i t h o u t p u t 6 now
high, the e j e c t cycle i s i n i t i a t e d w i t h a
h i g h on t h e i n p u t t o IC7a p r o g r e s s i n g t h r o u g h t o s w i t c h on
v a l v e s V4 and V5,
and thence t h r o u g h IC7b t o s t a r t t h e e j e c t
t i m e r . Once t h i s t i m i n g p e r i o d i s over, gate IC8c goes h i g h ,
and t h i s s i g n a l progresses
t h r o u g h IC4c and IC5b t o t r i g g e r
186
a n o t h e r change i n s t a t e o f t h e CD 4017*s o u t p u t .
The whole wash c y c l e i s t h e n r e p e a t e d , w i t h o u t p u t s 7, 8 and
9 g o i n g h i g h i n t u r n , these h a v i n g s i m i l a r r e s u l t s t o o u t p u t s
4,5
the
and 6 g o i n g h i g h . The e f f e c t s are as d e s c r i b e d above. At
end
of
initiating
second
wash
cycle,
t h e r e s e t t i m e r , and
Switching
forces
this
selector
output
0
and
0
goes
t h e whole process
s w i t c h SI back
high,
output
from
prevents
the
start
the
high,
repeats.
position
process
from
continuing.
5.2.4
The
When any
interface electronics
e l e c t r o - m e c h a n i c a l d e v i c e s are designed,
the
rated
c u r r e n t o f t h e d e v i c e i s a compromise between t h a t r e q u i r e d
t o cause t h e d e v i c e t o change s t a t e , and t h e c u r r e n t r e q u i r e d
t o h o l d t h a t d e v i c e i n t h e on p o s i t i o n . I n g e n e r a l , t h e h o l d
current
i s much l e s s t h a n t h e s w i t c h i n g c u r r e n t ,
so i f t h e
device i s d r i v e n continuously w i t h the f u l l s w i t c h i n g current,
much energy
i s wasted, and premature
failure
o f the
device
w i l l occur, due t o t h e e f f e c t s o f h e a t i n g on t h e c o i l and t h e
r e s t o f t h e mechanism concerned. W i t h these f a c t s i n mind, t h e
s w i t c h i n g v a l v e s were e v a l u a t e d t o d e t e r m i n e t h e h o l d c u r r e n t .
The
valves
used were
a l l rated
at
v o l t a g e passed a p p r o x i m a t e l y 200 mA
t h e v o l t a g e , i t was
12
volts,
and
at
this
o f c u r r e n t . By l o w e r i n g
found t h a t t h e v a l v e s s t i l l
held
their
'on' p o s i t i o n down t o about 4.5 v o l t s . I t was d e c i d e d t o a l l o w
187
a 50% s a f e t y margin on t h i s f i g u r e , and d e s i g n t h e i n t e r f a c e
in
such a way t h a t t h e v a l v e r e c e i v e d t h e f u l l
a
second
o r two, so
accomplished,
that
efficient
12 v o l t s f o r
switching
could
be
and t h e n t o lower t h e v o l t a g e t o a p p r o x i m a t e l y
6 volts t o provide the hold current.
The d e s i g n used i s shown i n F i g u r e 5.2.7. The i n p u t s i g n a l (a
+12
a
volts)
f r o m t h e l o g i c c o n t r o l u n i t f i r s t passes t h r o u g h
resistor-capacitor
frequency
n o i s e which
network,
which
may be p r e s e n t ,
filters
and t h e r e b y
f a l s e s w i t c h i n g . The s i g n a l i s t h e n processed
inverter
gates,
r i s e and f a l l
which
have t h e e f f e c t
i n o u t p u t , even though
o u t any h i g h
prevents
by two Schmitt
of providing a rapid
they receive a slowly
changing s i g n a l from t h e r e s i s t o r - c a p a c i t o r network R l - C l . The
second i n v e r t e r , G2,
s i m p l y a l t e r s t h e s i g n a l back from low
to h i g h . T h i s +12 v o l t s i g n a l t h e n feeds two r e s i s t o r s , R3 and
R5. R5 i s a c u r r e n t l i m i t e r t o p r o t e c t t h e base o f TRl, and
this
transistor
allowing
which
i s an NPN
c u r r e n t t o pass t h r o u g h
power
type
switches
on,
t h e l o a d v a l v e 1. A t t h e
moment o f ' s w i t c h on', t h e base o f t r a n s i s t o r
TR2 i s a t 0
v o l t s , and s i n c e TR2 i s a PNP t r a n s i s t o r , t h i s means t h a t t h i s
transistor
i s fully
conducting.
Thus
the current
flowing
t h r o u g h t h e l o a d and TRl f l o w s t h r o u g h TR2 t o 0 v o l t s , and
thus t h e l o a d i s s u b j e c t t o t h e f u l l
12 v o l t s
(less
^coisat)
which
i s approximately 1 v o l t )
from
G2 causes c u r r e n t t o f l o w t h r o u g h R3, and c a p a c i t o r C2
begins
t o charge,
. The +12 v o l t s i n p u t s i g n a l
c a u s i n g t h e v o l t a g e on t h e base o f TR2 t o
r i s e , t h e r e b y s w i t c h i n g t h i s t r a n s i s t o r o f f . The c u r r e n t i s
188
F i g u r e 5.2.7
Interface electronics c i r c u i t
diagram
I2v
D
LOAD
Gl
CO
INPUT
K
FROM
l>
CONTROLLER
I
1
R]
G2
R5
L
r
TRl
R3
TR2
CI
ZDl
RA
C2
now
volt
f o r c e d t o f l o w t h r o u g h t h e zener diode ZDl, which i s a 5.1
device.
The
combined v o l t a g e drop
of
ZDl
and
TRl
is
t h e r e f o r e a p p r o x i m a t e l y 6 v o l t s , and t h i s i s v o l t a g e used t o
h o l d t h e v a l v e i n t h e 'on' s t a t e . Diode Dl p r o v i d e s 'back emf
protection'
to
TRl,
this
being
necessary
to
due
reverse
i n d u c e d v o l t a g e s generated by t h e c o l l a p s i n g magnetic
field
i n the c o i l of the valve.
When t h e i n p u t
from t h e c o n t r o l l e r r e t u r n s t o 0 v o l t s ,
l o s e s base d r i v e ,
immediately
and t h e r e f o r e s w i t c h e s o f f , and t h e v a l v e
c l o s e s . C2
discharges
v o l t a g e on t h e base o f TR2
t a k e s about
required
to
TRl
t h r o u g h R4,
and
thus
the
r e t u r n s t o 0 v o l t s . This d i s c h a r g e
t h r e e seconds, b u t s i n c e none o f t h e v a l v e s are
switch
off
and
then
turn
back
on
again
i m m e d i a t e l y , t h i s i s o f no consequence.
Using t h i s i n t e r f a c e , no v a l v e f a i l u r e was encountered d u r i n g
t h e months o f development and e v a l u a t i o n o f t h e u n i t .
Other
u s e r s o f these v a l v e s a t t h i s U n i v e r s i t y r e p o r t t h a t when used
a t t h e nominal r a t i n g , t h e v a l v e s are prone t o a h i g h f a i l u r e
r a t e , o f t e n w i t h i n weeks o f f i r s t
usage.
The i n t e r f a c e u n i t a l s o c o n t a i n s two r e l a y s which are used t o
s u p p l y s w i t c h i n g f o r t h e autosampler
190
and t h e pump.
5.2.5
The
Design of the r e a c t i o n
proposed
reagent
use
dictated
of
gaseous
the design
vessel
ammonia
and
as
the
neutralizing
construction of a
reaction
v e s s e l which would p e r m i t t h e d i s s o l u t i o n o f t h e ammonia i n
an e f f i c i e n t manner, and which would a v o i d t h e gas coming i n t o
d i r e c t c o n t a c t w i t h t h e pH probe. Such c o n t a c t would o b v i o u s l y
have unwanted e f f e c t s upon t h e response o f t h e probe.
I t was
also considered
v i t a l t h a t some k i n d o f a g i t a t i o n or
s t i r r i n g be i n c o r p o r a t e d , so as t o a s s i s t i n t h e d i s s o l u t i o n
o f t h e ammonia, and p r o v i d e a more u n i f o r m s o l u t i o n f o r t h e
probe t o measure. A number o f d i f f e r e n t designs were i n i t i a l l y
considered,
difficult
some o f
to
fabricate,
e f f e c t s ' , or simply
The
which
were
ruled
out
likely
to
suffer
had
either
from
very
'memory
inefficient.
i d e a o f a 'bubble pump' system was
clearly
as
a number o f advantages. An
considered,
and
this
i n i t i a l design
was
f a b r i c a t e d i n house, and e v a l u a t e d . The d e s i g n f o r t h i s u n i t
is
shown i n F i g u r e 5.2.8. I t worked w e l l when t h e volume o f
sample was
l a r g e enough t o g i v e a l e v e l above t h e t o p o f t h e
bubble t u b e , b u t d i d not p e r f o r m w e l l w i t h s m a l l volumes. I n
these cases, t h e gas bubbles went up t h e i n s i d e o f t h e main
v e s s e l , i n s t e a d o f f o r c i n g a s l u g o f t h e l i q u i d up t h e bubble
t u b e . I t was
a l s o r e a l i s e d t h a t t h e d e s i g n d i d s u f f e r from a
s m a l l dead volume i n the o u t l e t tube, and a l t h o u g h t h i s
f e l t t o be n e g l i g i b l e , t h e m o d i f i e d d e s i g n o b v i a t e d
191
these
was
F i g u r e 5.2.8
F i r s t bubble-pun?) r e a c t i o n v e s s e l d e s i g n
pH PROBE
SaUTION
VENT
<
2«m I.O.—>
MAIN
V
13M
I.D.
< — 2 « . I.D.
VESSa
3 2 » I.D.
4oi 1.0.
Ill
ARGOJ 8 AmONIA
2«» I.D.
)
OJTUT
192
problems.
T h i s i s shown i n F i g u r e 5.2.9. T h i s v e s s e l has
no
dead voliome, and was found t o capable o f a pumping a c t i o n w i t h
e x t r e m e l y s m a l l volumes o f sample.
5.3
The
Evaluation
complete
attached,
solution.
system
and
The
the
was
first
reservoir
analogue
system
assembled,
filled
was
S e c t i o n 5.2.2.1. Before t h e probe
slope
and
temperature
manner. The
requirement
processing
controls
d e s i r e d pH was
had
them,
been
a
one
test
with
s e t up
was
were
t h e argon
0.880
as
ammonia
described i n
fitted,
s e t up
the b u f f e r ,
i n t h e noirmal
then s e l e c t e d . Since t h e
of
taking
solution
of
'real'
1%
initial
samples
nitric
employed i n t e s t s t o determine t h e performance
supply
acid
and
was
o f t h e system.
25ml o f t h i s s o l u t i o n were i n t r o d u c e d t o t h e v e s s e l , and t h e
l e v e l d e t e c t o r a d j u s t e d on t h e bubble tube t o t h i s h e i g h t .
s o l u t i o n was
t h e n emptied
The
from t h e v e s s e l , and a number o f
sample i n t r o d u c t i o n runs c a r r i e d o u t , t o e v a l u a t e t h e accuracy
of
t h e l e v e l d e t e c t i o n system. The r e s u l t s are shown i n Table
5.2.3.
193
F i g u r e 5.2.9
F i n a l reaction v e s s e l design
SAMPLE
INLET
VENT
2mm I . • .
MAIN
VESSEL
32mm
I.D.
^
2mm
I .•
>
4mm
I .•
ARGON AND
AMMONIA
INLET
4mm
OUTLET
194
I .•
T a b l e 5.2.3
E v a l u a t i o n o f t h e r e p r o d u c i b i l i t y of the level
d e t e c t i o n system showing mass o f
water a d m i t t e d
SAMPLE
WEIGHT (g)
1
25.01
2
25.14
3
24.96
4
25.31
5
25.17
6
25.22
7
25 .09
8
24 .92
9
25.06
10
24.91
MEAN
25.08
0.1326
195
1
As c a n be s e e n , t h e l e v e l d e t e c t i o n s y s t e m p r o v e d r e p r o d u c i b l e
w i t h good
The
precision.
system
was
neutralizing
then
tested
f o r i t ' s effectiveness i n
25ml s a m p l e s o f 1 % n i t r i c
acid.
The s y s t e m was
s e t i n r u n mode, a n d t h e gas f l o w a d j u s t e d t o p r o v i d e a r a p i d
steady
stream o f bubbles
i n t o t h e bubble
t u b e . I n t h i s mode,
t h e m e t e r m o n i t o r s t h e pH o f - t h e s o l u t i o n , a n d i t was
observed
t h a t f o r some m i n u t e s , no change o c c u r r e d i n t h e r e a d i n g . The
pH r e m a i n e d b e l o w 1 f o r a b o u t
to
climb.
reached
a value
times,
pH
the preset
supplying
vessel,
From
2,
four minutes,
the readings
value
increased
o f 8.5. A t t h i s
t h e ammonia s w i t c h e d
before
and argon
rapidly,
point
alone
process
a n d gave r e a d i n g s
was r e p e a t e d
and
the valve
entered the
a l t h o u g h The pH c o n t i n u e d t o c l i m b , f i n a l l y
o f 10.2. T h i s
starting
reaching
a further
three
i n t h e range 10.1 t o 10.4. C l e a r l y
t h e system worked, b u t r e q u i r e d f i n e t u n i n g t o o b t a i n c l o s e r
c o n t r o l o f t h e pH o f t h e
The
resulting
solution.
n e x t s t e p was t o r e d u c e t h e gas f l o w t h r o u g h t h e s y s t e m ,
i n t h e hope t h a t t h i s w o u l d p r o v i d e a more m a n a g e a b l e change,
whilst
time
still
facilitating
neutralization within a
p e r i o d . The t i m e t a k e n t o a c h i e v e
increased
slightly,
and
a p p r o x i m a t e l y one pH u n i t
the
final
reasonable
t h e p r e s e t pH v a l u e
pH
values
were a l l
above t h e s e t v a l u e .
I t was t h e n f o u n d t h a t b y s e t t i n g t h e d e s i r e d pH t o 7.4, f i v e
test
solutions
a l l ended
with
196
pH
readings
o f 8.5
± 0.16.
Further adjustment of the c a r r i e r f l o w r a t e r e s u l t e d i n c l o s e r
a g r e e m e n t b e t w e e n t h e p r e s e t v a l u e and
the
cost of longer n e u t r a l i z a t i o n times. With a n e u t r a l i z a t i o n
t i m e o f a p p r o x i m a t e l y 25 m i n u t e s ,
was
that obtained, but at
within
0.1
five test
Whilst
though
were -encouraging,
c o n t r o l o f t h e pH
the
t h e a u t o m a t e d s y s t e m r e q u i r e d no
method
reduce
this
approached.
of
reducing
the
time
flow
proportionately
to
implement
further investigate digital
-
i t was
control
was
above
time
confirmed
pH
reached,
even
taken
to
would
be
s t a g e s , and
the
then
d e s i r e d pH
is
control
is difficult
decided
therefore
to
( i . e . valves e i t h e r being
2.
that
I t was
the
rate
therefore
of
i n w h i c h t h e pH
change
decided
to
increases
apply
a
and
time
changed,
rapidly
two
pH
step
2
sufficient
achieve
was
t h e n more s l o w l y i n t h e f o r m o f s h o r t p u l s e s .
interval
between these
pulses
was
a l s o used
a l l o w f u r t h e r a g i t a t i o n o f t h e s o l u t i o n by t h e a r g o n f l o w ,
To
to
One
c o n t r o l s y s t e m , a d m i t t i n g t h e ammonia r a p i d l y u n t i l
longer
taken
off).
A f t e r f u r t h e r o b s e r v a t i o n s o f t h e way
it
the
operator input.
as
Unfortunately proportional
expensive
on o r
each o f
appeared excessive,
i n t r o d u c e t h e ammonia r a p i d l y i n t h e i n i t i a l
and
o f t h e sample
u n i t s o f t h e d e s i r e d v a l u e on
results
accurate
possible
to
pH
solutions.
these
achieve
pH
the f i n a l
time f o r the probe t o
the
above
the
to
and
respond..
analogue
197
A
system
was
modified
slightly,
a n d a s i m p l e s t a g e was added t o t h e c o n t r o l
system.
The m o d i f i c a t i o n s a r e shown i n F i g u r e 5 . 3 . 1 . As may be seen,
the
alteration
additional
with
analogue
i t ' s inverting
divider
This input
inverting
the
operational amplifier
comparator,
potential
to
involves
configured t o perform
input
held
b e t w e e n t h e +6.5 v o l t
effectively
input
circuitry
an
as a
a t 200mV v i a a
and ' i n l o *
rails.
r e p r e s e n t s a pH v a l u e o f 2. The n o n -
i s f e d from t h e output o f t h e probe
signal
a m p l i f y i n g a n d p r o c e s s i n g s y s t e m . The p r o b e o u t p u t s y s t e m i s
t h e r e f o r e m o n i t o r e d by two l e v e l d e t e c t i n g c i r c u i t s ,
to
trip
a t pH 2, a n d t h e o t h e r o p e r a t i n g a t t h e s e t pH.
The c o n t r o l
circuit
timing
system which
bursts
every
first
one s e t
switched
10 s e c o n d s ,
inverter
comparator.
was a l s o m o d i f i e d b y t h e a d d i t i o n
gate
t h e ammonia o n f o r one s e c o n d
and a p a i r
i s f e d from
of inverter
the output
g a t e s . The
o f t h e new
When t h e pH o f t h e sample i s l e s s t h a n
comparator's
output
of a
i s l o w , and t h u s
the output
i n v e r t e r i s h i g h . This i s used t o d r i v e t h e i n t e r f a c e
2,
this
from t h e
circuit,
so t h a t t h e v a l v e r e m a i n s open, c o n t i n u o u s l y a d m i t t i n g ammonia
to
t h e bubble
output
from
t u b e . When t h e pH o f t h e s a m p l e r e a c h e s 2, t h e
t h e comparator
goes
high,
that
of the first
i n v e r t e r g a t e goes l o w , a n d t h e v a l v e c l o s e s . The o u t p u t
the
second gate however i s a l s o i n v e r t e d ,
signal
i s used t o operate
the
and t h i s
timing c i r c u i t .
This
from
positive
circuit
t u r n s t h e v a l v e o n f o r 1 s e c o n d a n d t h e n o f f f o r 10 s e c o n d s .
T h i s t e c h n i q u e p r o v i d e d a much c l o s e r c o n t r o l o f t h e u l t i m a t e
pH o f t h e s o l u t i o n ,
as may be o b s e r v e d
198
i n Table
5 . 3 . 1 . The
F i g u r e 5.3.1
Additional c i r c u i t r y
t o in5>rove c o n t r o l o f pH
and sait?>le throughput
6V5
33a<
AlCS
ANALOGUE
l^*vr
o
J
7u '*0'15,
I
[ XHO^T))
22K
47K
IN-UOO 1
V3 DRIVE O-
OUTPUT TO VALVE V3
Ot4K7
4K7
1
47K
22K
ev
2 PULSE HCOTH
3
5B»<
IBu
IGBn
BV
199
T a b l e 5.3.1
1% n i t r i c a c i d s a n ^ l e s n e u t r a l i z e d to a t a r g e t
s e t t i n g of pH 8
FINAL
SAMPLE
1
06
2
8. 09
3
8, 11
4
8. 10
5
8. 08
MEAN
8. 08
200
pH
modified
the
system
adjustment
adjustment
also
a
1%
nitric
8 minutes,
6 samples p e r hour.
the
a more
acceptable
o f t h e pH. The a v e r a g e
of
approximately
provided
reaction
vessel
acid
giving
time
solution
a throughput
time f o r
taken
for
t o pH
8
o f about
the
was
5 to
Further m o d i f i c a t i o n s t o the design o f
may
be
possible
to
increase
this
figure.
5.4
Discussion
There
a r e v e r y many a p p l i c a t i o n s f o r t h i s
system
since the
m a j o r i t y o f aqueous s a m p l e s a r e a c i d i f i e d u p o n c o l l e c t i o n t o
p r e v e n t " p l a t e o u t " . F o r many a n a l y t i c a l d e t e r m i n a t i o n s , t h e s e
samples r e q u i r e n e u t r a l i z a t i o n b e f o r e a p a r t i c u l a r
technique
may be e m p l o y e d .
In the selenium
HPLC
column
s p e c i a t i o n s y s t e m d e s c r i b e d i n C h a p t e r 3, an
i s employed
present. Since
t o separate
t h e column i s unable
t h e selenium
to tolerate highly
species
acidic
s a m p l e s , t h e pH a d j u s t i n g s y s t e m c o u l d e a s i l y be l i n k e d t o t h e
s p e c i a t i o n system t o a d j u s t t h e samples p r i o r t o t h e a n a l y s i s
b e i n g made.
The
atomic
f l u o r e s c e n c e cadmium a n a l y s i s s y s t e m d e v e l o p e d
t h e U n i v e r s i t y o f Plymouth^^, e m p l o y s s o d i u m t e t r a e t h y l
201
at
borate
to
ethylate
the
cadmium
species
present,
following
the
e q u a t i o n shown below^**:
Cd^*
High
a c i d i t y and
+
2BEt4"
-
CdEtj
the presence o f
decompose t h e e t h y l a t i n g a g e n t ;
the
f l u o r e s c e n c e s i g n a l . The
control
of
s a m p l e pH.
+
2BEt
transition
tend
to
w i t h a r e s u l t a n t decrease i n
system t h e r e f o r e
Such an
metals
requires close
a n a l y t i c a l system i s
another
o b v i o u s a p p l i c a t i o n f o r t h e n e u t r a l i z i n g u n i t d e s c r i b e d above.
5.5
Summary
T h i s s t u d y has r e s u l t e d i n t h e d e v e l o p m e n t o f an a u t o m a t e d
line
system f o r the adjustment
on-
o f a s a m p l e ' s pH.
The
results
i n almost
negligible
c o n t a m i n a t i o n o f t h e sample. C a r e f u l d e s i g n o f t h e
electronics
a gaseous n e u t r a l i z i n g r e a g e n t
use
a s s o c i a t e d w i t h t h e measurement and c o n t r o l o f t h e s y s t e m
r e s u l t e d i n a s i m p l e , r e l i a b l e , a c c u r a t e and i n e x p e n s i v e
of
has
unit,
w h i l s t m a i n t a i n i n g c o m p u t e r c o n t r o l c o m p a t i b i l i t y . The use
an
interface unit to drive
t h e s w i t c h i n g v a l v e s has
in
a considerable extension i n valve l i f e
202
and
of
resulted
reliability.
Chapter 6
Conclusiom and
further work
203
Conclusions
6.1
The
and
f u r t h e r work
Conclusions
need
f o r more a u t o m a t i o n
i n analytical
laboratories
is
s e l f - e v i d e n t when t h e number o f s a m p l e s r e q u i r i n g a n a l y s i s i s
c o n s i d e r e d . The
sources
-
demand
is
demands f o r i n c r e a s e d t e s t i n g come f r o m many
industry,
only
governments,
likely
to
hospitals
continue
etc.,
increasing.
and
this
Ever
more
s e n s i t i v e i n s t r u m e n t a t i o n means t h a t c h e m i c a l p r o c e s s e s i n t h e
environment
detail.
As
and
i n medicine
may
more i s u n d e r s t o o d
p l a y e d elements
be
studied
in
ever
more
o f these processes,
the
roles
and compounds a t t r a c e l e v e l s become a p p a r e n t ,
and t h u s t h e n e e d t o a n a l y s e f o r t h e s e t r a c e s becomes r o u t i n e .
Developments i n t h e r e s e a r c h l a b o r a t o r y pass t o t h e a n a l y t i c a l
l a b o r a t o r y w i t h a s t o n i s h i n g speed.
W i t h i n t h i s t h e s i s , a number o f d i f f e r i n g t e c h n i q u e s h a v e b e e n
s t u d i e d , b u t one u n d e r l y i n g t h r e a d has b e e n common t h r o u g h o u t ,
that
of
selenium,
automation.
and
above. E a r l i e r
this
Much
element
workers
work
has
been
illustrates
were
content
c o n c e n t r a t i o n s o f t h e element
well
carried
out
on
t h e p o i n t s made
t o merely
obtain
i n a s a m p l e , b u t as
total
knowledge
o f s e l e n i u m c h e m i s t r y has c o n t i n u e d t o i n c r e a s e , i t has become
apparent
that
of c r i t i c a l
t h e form o f t h e element
i n t h e sample i s a l s o
i m p o r t a n c e , and h e n c e t h e n e e d f o r s p e c i a t i o n
has
emerged. I n a d d i t i o n t h e l e v e l s o f d e t e c t i o n r e q u i r e d b o t h f o r
t o t a l c o n c e n t r a t i o n and i n d i v i d u a l s p e c i e s , c o n t i n u e s t o p l a c e
204
f u r t h e r demands on t h e a n a l y s t .
Until
very
r e c e n t l y , i t was o n l y p o s s i b l e
i n o r g a n i c species
o f selenium
t o determine
i n a g i v e n s a m p l e by d i f f e r e n c e
when u s i n g h y d r i d e g e n e r a t i o n , s i n c e o n l y s e l e n i u m ( I V )
w i t h t h e reductant t o form t h e hydride. Using t h i s
the
sample
is
determined.
process
repeated
Selenium(VI)
and
selenium(VI)
t o give
i s thus
concentration
many
i n two
from
years
a
determined
the t o t a l .
as t h e most
systems d e s c r i b e d
i n this
T h i s system employed a
selenium (IV)
selenium
with
spectroscopy
by
T h i s m e t h o d has been
convenient
method
The f i r s t
thesis relied
a
a
quartz
detection
system.
i n which
indirectly,
determination
employed
o f t h e automated
upon t h i s
technique.
m i c r o w a v e p r e - r e d u c t i o n s t e p as p a r t
upon
way
former
availablef o r
of
t o a hydride
furnace-atomic
Since
automation
such
absorption
instruments
t h e system
may
be
minimum a d d i t i o n a l c o s t . The d r a w b a c k s o f t h i s
the
first
and t h e
subtracting the
w i d e l y a v a i l a b l e i n most l a b o r a t o r i e s ,
that
is
concentration.
o f a computer c o n t r o l l e d o n - l i n e system, coupled
represents
technique,
t o selenium(IV),
total
t r a c e d e t e r m i n a t i o n s o f selenium.
system
reacts
The o t h e r sample* i s t h e n s u b j e c t e d t o a r e d u c t i o n
t o convert
analysis
for
split
the
selenium(VI)
described
achieved
at
technique are
is
carried
and t h a t d e t e c t i o n l i m i t s a r e l i m i t e d
ng ml"^ r a n g e due t o t h e l a c k o f s e n s i t i v i t y
are
out
t o t h e low
of the detector.
These p r o b l e m s were t h e n a d d r e s s e d b y t h e d e v e l o p m e n t o f t h e
s e c o n d s y s t e m , w h i c h e m p l o y e d HPLC t o a c h i e v e
205
the separation
of
the i n o r g a n i c selenium
the
column
e m p l o y e d f o r t h e s t u d y w o u l d be d e s t r o y e d b y a h i g h l y
acidic
sample,
post
column
by
microwave
followed
unaffected
generation
but
sensitive
present.
acidification
heating
reduced
techniques
hydride being
species
the
then
was
which
Since
employed.
left
the
selenium{VI) .
followed, with
c a r r i e d o u t by
atomic
d e t e c t i o n s y s t e m was
This
was
selenium(IV)
Nomal
hydride
detection of
f l u o r e s c e n c e . T h i s more
necessary
due
t o the
limited
sample v o l u m e ( 1 m l ) i n j e c t e d o n t o t h e HPLC c o l u m n , w h i c h
subsequently
limits
and
diluted
o f 0.2
and
selenium(VI)
by
0.3
the
the a d d i t i o n
of the acid.
was
Detection
ng ml"^ w e r e o b t a i n e d f o r s e l e n i u m { I V )
respectively.
O t h e r w o r k c o n d u c t e d on t h e d e t e r m i n a t i o n o f s e l e n i u m i n c l u d e d
the
of
s t u d y o f s i g n a l enhancement due
nitric
acid
i n the
t o the presence o f t r a c e s
m a t r i x , when h y d r i d e
generation
was
e m p l o y e d . T h i s enhancement was
shown t o be l i n k e d t o t h e t y p e
of
since
pump t u b i n g e m p l o y e d ,
but
the
order
s a m p l e s w e r e p r e p a r e d a l s o h a d an e f f e c t , i t was
i n which
the
f e l t that
the
p o s s i b i l i t y o f t h e f o r m a t i o n o f a n i t r o - s e l e n i u m complex c o u l d
not
be
ruled
mechanism was
out,
although
outside the
further
scope o f
does i l l u s t r a t e h o w e v e r , j u s t how
a l l parameters
work
this
to
elucidate
thesis.
much c a r e has
This
the
study
t o be p a i d t o
i n o n - l i n e systems, even t h e s e l e c t i o n o f such
mundane i t e m s as pump t u b i n g , when c a r r y i n g o u t d e t e r m i n a t i o n s
at
The
t r a c e and u l t r a
trace levels.
i n v e s t i g a t i o n i n t o the conditioning of hydride generation
206
apparatus
also
if
when
determining
d e m o n s t r a t e d an
automated
employed.
data
important
handling
Conditioning
i n j e c t i o n o f two
selenium
1 mg
ultra
trace
aspect of instrument
methods
in
ml'^
at
this
are
case
to
was
be
levels
design,
subsequently
achieved
by
samples o f s e l e n i u m ( I V ) , and
the
then
a l l o w i n g t h e s y s t e m t o run f o r a t l e a s t f i f t e e n m i n u t e s b e f o r e
starting
to a n a l y s e samples.
the a c t i v e
sites
of t h e d r i f t
Clearly,
the
i n some d e t a i l
into
T h i s study
s i x minutes
much l e s s
achieve
resulted
i f optimal
with o n - l i n e systems.
investigation
within
and
w h i c h had p r e v i o u s l y been
selenium(VI).
i.e.
i n t h e system,
the e f f e c t
of
filling
i n the
removal
observed.
f u n d a m e n t a l a s p e c t s of t h e c h e m i s t r y a l s o need t o be
considered
obtained
T h i s had
than
reduction.
the
i s to
be
T h i s i s w e l l demonstrated
by
kinetics
indicated
Thus
of
that
a t 70*" C under our
the
perfoinnance
the
reduction
the process
of
occured
laboratory conditions
t h i r t y m i n u t e s commonly employed t o
i t was
possible
to
decrease
the
r e d u c t i o n p e r i o d , m i n i m i s i n g p o s s i b l e l o s s e s , and r e d u c i n g t h e
chances
of
understand
contamination.
However,
it
is
important
to
t h a t t h e s o l u t i o n has t o be a t 70°C f o r s i x m i n u t e s
- t h e t o t a l h e a t i n g time w i l l be dependant upon the method o f
h e a t i n g employed. The a c t i v a t i o n energy f o r t h e r e a c t i o n under
our c o n d i t i o n s was
found t o be
90.4
k J mol"^.
O r g a n o - s e l e n i u m compounds r e p r e s e n t a c h a l l e n g e t o t h e
spectroscopist,
since i t i s d i f f i c u l t
to break
their
atomic
carbon-
s e l e n i u m bonds. T h i s i s r e q u i r e d i f h y d r i d e g e n e r a t i o n i s t o
207
be e m p l o y e d i n t h e i r s u b s e q u e n t d e t e r m i n a t i o n , and
represents
one
of
the
most
since
s e n s i t i v e techniques
this
currently
a v a i l a b l e , t h e b r e a k i n g o f these bonds i s n e c e s s a r y . Microwave
heating of seleno-methionine
i n h y d r o c h l o r i c a c i d appeared t o
h a v e no e f f e c t . The
a c t i o n o f UV
discharge
lamp was
shown t o b r e a k t h e c a r b o n - s e l e n i u m b o n d s ,
and
enable
thus
the
generation coupled
the
time
was
excessive,
line
of
taken
system,
the
selenium • t o
be
determined
under the
experimental
at s i x t y minutes,
conditions
the
distance
prevailing
distance
from
were r e d u c e d ,
and
was
the
the
tubes
for
the
been a t e c h n i q u e
destruction
of
w h i c h has
organic
on-
diameter
to
a capillary
t o c a r r y t h e s a m p l e , t h e n o n - l i n e o p e r a t i o n w o u l d be
P y r o l y s i s has
hydride
f o r i n c o r p o r a t i o n i n t o an
s a m p l e v o l u m e s were l a r g e , as
I f this
by
mercury
t o atomic fluorescence d e t e c t i o n . Although
s a m p l e t u b e s and
source.
from a h i g h i n t e n s i t y
the
used
feasible.
g e n e r a l l y been used
samples.
In
this
study,
p y r o l y s i s has
been employed i n t h e d e t e r m i n a t i o n o f mercury.
The
was
of
technique
dense
s u c c e s s f u l l y employed f o r t h e
inorganic
successful
samples
however
samples.
This
could
employed
i n the
when
be
oven
accurately weighing
such
applied
as
to
determination
sediments.
low
I t was
density
organic
due
simply
to the
low
sample
used
i n the
study.
The
difficulty
s a m p l e s o f a few
milligrams is
not
volume
in
obvious,
and w h e t h e r s u c h s m a l l s a m p l e s a r e r e p r e s e n t a t i v e o f t h e m a i n
batch
i s also h i g h l y
Finally,
questionable.
t h e d e v e l o p m e n t o f a pH
208
a d j u s t i n g system which c o u l d
be e m p l o y e d on s a m p l e s w h i c h r e q u i r e d
determinations
t o be
subsequently
trace
carried
and u l t r a
o u t on
trace
them
was
a c h i e v e d . T h i s work r e s u l t e d i n a low c o s t , d e d i c a t e d system
which
could
concerning
be
easily
the overall
interfaced
to a
reliability
computer.
Problems
of the switching
valves
were o v e r c o m e b y t h e d e s i g n o f a n o v e l i n t e r f a c e w h i c h r e d u c e d
the
d r i v e v o l t a g e a p p l i e d once t h e v a l v e h a d s w i t c h e d . The use
of
a b u b b l e pump s y s t e m e n a b l e d t h e ammonia t o be added t o t h e
s o l u t i o n away f r o m t h e m a i n b o d y o f t h e l i q u i d ,
thus
c o n t a c t b e t w e e n t h e gas and t h e pH e l e c t r o d e , w h i l s t
caused a g i t a t i o n
o f t h e sample s o l u t i o n .
This
avoiding
i t also
apparatus
was
e m p l o y e d s u c c e s s f u l l y i n c o n j u n c t i o n w i t h t h e HPLC-microwave
s e l e n i u m s p e c i a t i o n s y s t e m d e v e l o p e d as p a r t o f t h i s
This
thesis
therefore
describes
a
range
of
thesis.
techniques
employing h y d r i d e g e n e r a t i o n , p h o t o l y s i s , p y r o l y s i s , microwave
reduction,
of
which
all
have
o f w h i c h have been o p e r a t e d o n - l i n e ,
been,
or
could
easily
be
and a l l
automated.
The
o p e r a t i o n o f t h e s y s t e m s have a l l b e e n c h e c k e d b y t h e u s e o f
r e a l s a m p l e s , as w e l l as p r e p a r e d
7.1
standards.
F u r t h e r work
A l t h o u g h much w o r k has been c a r r i e d o u t on h y d r i d e g e n e r a t i o n ,
there
are s t i l l
many a r e a s i n w h i c h f u r t h e r
work
b e n e f i c i a l . Any s t u d y o f t h e l i t e r a t u r e on t h i s
quickly establish that there i s s t i l l
209
would
subject
be
will
d i s a g r e e m e n t upon many
fundamental
aspects o f t h e t e c h n i q u e . Thus, s i n c e t h e use
h y d r i d e g e n e r a t i o n lends i t s e l f t o o n - l i n e systems,
s t u d y o f t h e fundamentals
of
of
further
would c l e a r l y a i d t h e development
such systems.
F u r t h e r i n v e s t i g a t i o n o f t h e s i g n a l enhancement e f f e c t due t o
the
presence
selenium,
of
traces
of
nitric
acid
when
t o f u l l y e l u c i d a t e * t h e mechanisms i n v o l v e d would be
b e n e f i c i a l . W h i l s t t h e n a t u r e o f t h e t u b i n g was
a
determining
major
cause
variations
of
the
enhancement,
the
found t o be
reasons
for
the
i n t h e enhancements depending upon t h e o r d e r i n
which t h e reagents were added s h o u l d be
determined.
Other work o f i n t e r e s t s t a r t e d i n t h i s s t u d y b u t not r e p o r t e d
i n t h i s t h e s i s includes the generation of hydrides
from
the
technique
sample
would
contamination
which
solution
seen
of the
are c u r r e n t l y
to
by
electrolytic
offer
sample by
added, and
the
methods.
potential
impurities
directly
to
i n the
This
obviate
reagents
c o u l d p r o v i d e a cheaper
and
p r o b a b l y a more c o n s i s t e n t method o f h y d r i d e g e n e r a t i o n .
Further
development
system s h o u l d
result
of the
automated HPLC-microwave-HG-AFS
i n s h o r t e r a n a l y s i s times, along
lower d e t e c t i o n l i m i t s . Other e l u e n t s may
s e p a r a t i o n i n t h e HPLC stage, and
with
provide a quicker
f u r t h e r refinement of the
r e s t o f t h e system s h o u l d be capable o f i m p r o v i n g t h e o v e r a l l
sensitivity.
210
The p h o t o l y t i c b r e a k i n g o f t h e selenium-carbon bond i n selenomethionine
simple
a
was
v e r y encouraging,
and i t would appear t o be a
t a s k t o produce an o n - l i n e system i n c o r p o r a t i n g such
step.
The
production of
determining organic
becomes a r e a l i t y ,
a
selenium,
s p e c i a t i o n system capable
selenium(IV)
and
of
selenium(VI)
p r o v i d i n g the p h o t o l y s i s step i s able t o
d e a l w i t h t h e t r i m e t h y l selonium i o n , which i s more d i f f i c u l t
to d e s t r o y than the seleno-methionine
Further
development
of
the
pH
bonds.
a d j u s t i n g system
c a r r i e d out - no d i f f i c u l t y s h o u l d be encountered
the
e l e c t r o n i c s to deal
with alkaline
could
be
i n modifying
samples as
well
as
a c i d i c ones, and so a comprehensive pH a d j u s t i n g system c o u l d
be
constructed.
I t should
be
the
possible
neutralizing
f o r example
agent
for
to
use
hydrogen
c h l o r i d e as
samples.
I n s p i t e o f t h e d e s i r e t o d e s i g n t h e e x i s t i n g system
u s i n g t h e s i m p l e s t and most i n e x p e n s i v e c i r c u i t r y
a
new
design
increased
count,
flexibility
and
microprocessor
etc.,
within
employing
a
PIC
microprocessor
( i f required), a
programmable
setting
up
alkaline
available,
would
reduced
offer
component
procedures.
The
c o u l d a l s o be used t o p r o v i d e a l o g o f samples
which a l t h o u g h
the t e x t ,
not
r e q u i r e d f o r t h e system
described
c o u l d have p o s s i b l e a p p l i c a t i o n s i n o t h e r
systems.
F i n a l l y , development o f computer s o f t w a r e d e d i c a t e d t o o n - l i n e
systems c o u l d be c a r r i e d o u t . A l t h o u g h t h e s o f t w a r e employed
throughout
t h i s s t u d y has
proved
211
reliable,
i t was
developed
as a g e n e r a l system. The problems observed i n t h e d r i f t o f t h e
standards
when
concentrations
software.
determining
could
I f such
be
selenium
overcome
a programme
by
at
ultra
trace
s u i t a b l e design
allowed
the running
of
of a
s t a n d a r d whenever r e q u i r e d by t h e a n a l y s t , and c a l c u l a t e d t h e
c o n c e n t r a t i o n s o f samples between standards on t h e b a s i s o f
a t i m e r e l a t e d d r i f t , i t c o u l d e f f e c t i v e l y compensate f o r t h e
drift.
In
addition,
the - f a c i l i t y
to
integrate
traces
c o n t a i n i n g two o r more p l a t e a u t y p e peaks i n d i v i d u a l l y w i t h
e i t h e r a computer d e r i v e d b a s e l i n e , o r one s e l e c t e d by t h e
analyst
would be o f g r e a t
value.
Such s o f t w a r e
facilities
e x i s t i n p a r t i n commercially
a v a i l a b l e programmes, b u t do n o t
appear t o have been g a t h e r e d
t o g e t h e r i n t o one programme as
yet.
212
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Appendix 1
Touchstone Instrument Signal Peak Data Acquisition
T h e B o r l a n d P a s c a l c o d e b e l o w is a simplified extract from T o u c h s t o n e ,
s h o w i n g the a p p r o a c h to p e a k r e a d i n g of A A hydride g e n e r a t o r s i g n a l s . T h e
actual program c a t e r s for m a n y other t e c h n i q u e s a n d i n s t r u m e n t s .
C o m m e n t s a r e in italics b e t e e n b r a c e s .
{Comment}
procedure ADPoint;/T/?/s procedure
point}
is called repeatedly
to read and plot each
var ADSum,ReadsDone:integer;
begin
for ReadsDone:=1 to A D R e a d i n g s do {Normally
ADReadings=4}
ADSum:=ADSum;=ADRead;
{ADRead returns a signed integer the mean of 16 12 bit A/D readings}
fit also synchronises to the computer clock at 18,2Hz}
A D S u m : = A D S u m div A D R e a d i n g s ;
ADSum.=fiiter(ADSum);fRtinn/ng Mean Filter, number set in Method-Filter}
A b s R e a d : = A D S u m / A D F a c t o r * A6Mu\i',{Convert to Absorbance for this AA}
'\nc{x)\{next X axis pixel position}
ShowSignal;/P/o/ Point and show actual reading}
er\6;{Procedure
ADPoint}
Procedure P l o t A D ; / R e a d s baseline then peak data using ADPoint to plot on
screen}
begin
XAx\sf\Aax =54S;{Maximum
allowed X axis value in pixels}
T i m e E n t C a l c ; f S e / up ZeroPlot and ADReadings to Vapour Gen timers}
x:=0;pc
Pixel Counter}
PeakArea:=0;
PeakHeight:=-1;
W r l t e B a r G R C R E A D I N G B A S E L I N E S I G N A L _Esc-Abort');
O K : = F A L S E ; f S e / TRUE only if a valid result measured}
BaseLine:=0;
repeat
{Get BaseLine during delay}
ADPoint;
{Read A/D, filter, and plot}
BaseLine:=BaseLine+AbsRead;
{Add result to BaseLine}
until keypressed or (x > ZeroPlot);
{Until Delay Time Done}
B a s e L i n e : = B a s e L i n e / (ZeroPlot+1);
{Calculate
Mean}
drawstring('BaseLine= '+RealToString(BaseL[ne)+' Abs');
223
{Valve Switches Here}
WriteBarGR('Measuring Signal Peak');
PeakArea:=0;
{Set Peak Initial values}
PeakHeight:=-1; {Ensure invalid result unless A/D
PACount:=0;
read}
repeat
ADPoint;
{Read A/D, fiter and plot on screen}
A b s R e a d : = A b s R e a d - B a s e L i n e ; {Subtract
BaseLine}
if A b s R e a d > PeakHeight then PeakHeighl:=AbsRead;{Upcfa^e PkHeight}
P e a k A r e a : = P e a k A r e a + A b s R e a d ; {Add to Peak Area running total}
Inc(PACount); {Increment count of readings summed in PeakArea}
until k e y p r e s s e d or (x > XAxisMax) or (nothydrunning);
{Normally stops when vapour generator cycle complete}
if P e a k A r e a < 0 then P e a k A r e a : =0; {Negative
value
meaningless}
if not keypressed then begin {Scale and display the peak results}
PeakArea:=100 * P e a k A r e a / PACount;f>Ac(/i;s/ to timescale
selected}
G o t o X Y G R (51.3); DrawStringC P e a k Area: '+RealToString(PeakArea));
G o t o X Y G R (51.3); DrawStringC Peak Height: •+RealToString(PeakHeight)+'Abs');
ok:=TRUE;
en6,fif not keypressed}
en6:{Procedure
PlotAD}
Copyright Spinoff 1 9 8 7 - 9 4 .
T o u c h s t o n e is distributed by P S Analytical Ltd.
224
CONFERENCES AND COURSES ATTENDED
Sixth
Biennial
National
Atomic
Spectroscopy
Symposium,
U n i v e r s i t y o f Plymouth, J u l y , 1992.
X X V I I I C o l l o q u i u m Spectroscopicum I n t e r n a t i o n a l e , U n i v e r s i t y
o f York, June, 1993.
Research and Development T o p i c s , U n i v e r s i t y o f B r a d f o r d , J u l y ,
1993.
Seventh
Biennial
National
Atomic
Spectroscopy
Symposiiam,
U n i v e r s i t y o f H u l l , J u l y , 1994.
Research and Development
Topics, U n i v e r s i t y o f H e r t f o r d s h i r e ,
J u l y , 1994.
Atomic
S p e c t r o m e t r y Updates
- Atomic
Spectroscopy
Group,
U n i v e r s i t y o f B r i s t o l , March, 1995.
Erasmus Eurocourse - F r o n t i e r s i n A n a l y t i c a l Chemistry : Trace
Environmental Analysis,
University
225
o f Plymouth,
September,
1995.
Royal
Society o f Chemistry lectures
speakers a t t h e U n i v e r s i t y o f
and
lectures
by
invited
Plymouth
V a r i o u s weekly r e s e a r c h l e c t u r e s a t t h e U n i v e r s i t y o f Plymouth
226
PUBLICATIONS
S e l e n i u m s p e c i a t i o n - a flow i n j e c t i o n approach en?>loying
l i n e microwave r e d u c t i o n f o l l o w e d by h y d r i d e
f u r n a c e atomic a b s o r p t i o n
spectrometry
P i t t s L.J.,
&-Hill
Analyst,
W o r s f o l d P.J.
1994,
Investigation
119,
into
S.J.,
the
S.J.
kinetics
P i t t s L.J.
of
seleni\im{VI)
reduction
atomic f l u o r e s c e n c e d e t e c t i o n
& Worsfold
Journal o f A n a l y t i c a l Atomic
Selenium
generation-quartz
2785.
using hydride generation
Hill
on-
speciation
P.J.
Spectrometry,
using
high
1995,
10,
performance
409.
liquid
chromatography-hydride g e n e r a t i o n atomic f l u o r e s c e n c e w i t h onl i n e microwave
P i t t s L.J.,
reduction
F i s h e r A.,
W o r s f o l d P.J.
J o u r n a l o f A n a l y t i c a l Atomic
& Hill
Spectrometry,
227
S.J.
1995,
10,
519.
PRESENTATIONS
An
investigation
of s i g n a l
enhancement e f f e c t s
due t o t h e
p r e s e n c e o f n i t r i c a c i d on t h e d e t e r m i n a t i o n o f selenixim u s i n g
hydride generation
X X V I I I C o l l o q u i u m S p e c t r o s c o p i c u m I n t e r n a t i o n a l e , Y o r k , June
1993.
The
d e t e r m i n a t i o n o f seleni\im u s i n g h y d r i d e g e n e r a t i o n - an
investigation
of
presence of n i t r i c
Research
signal
enhancement
effects
due
to the
acid
& D e v e l o p m e n t T o p i c s , B r a d f o r d , J u l y , 1993.
Selenium s p e c i a t i o n i n aqueous san5>les - an o n - l i n e t e c h n i q u e
en5>loying
microwave
pre-treatment
followed
g e n e r a t i o n - q u a r t z f u r n a c e atomic a b s o r p t i o n
7th.
Biennial
N a t i o n a l Atomic
by
hydride
spectrometry
S p e c t r o s c o p y Symposium,
Hull,
J u l y , 1994.
The
determination
o f mercury
i n sediments
using
on-line
p y r o l y s i s , p r e - c o n c e n t r a t i o n and atomic f l u o r e s c e n c e d e t e c t i o n
228
7th.
B i e n n i a l N a t i o n a l Atomic S p e c t r o s c o p y
July,
The
Symposium, H u l l ,
1994.
development
of
an
on-line
system
en5)loying
microwave
reduction f o r seleniiun s p e c i a t i o n
Research
& Development T o p i c s , Watford, J u l y ,
1994.
Selenixun s p e c i a t i o n - a flow i n j e c t i o n approach e n ^ l o y i n g
l i n e microwave r e d u c t i o n f o l l o w e d by h y d r i d e
furnace atomic absorption
FACSS XXI,
The
S t . L o u i s , USA,
determination
of
on-
generation-quartz
spectroscopy
October,
mercury
in
1994.
sediments
using
on-line
p y r o l y s i s , p r e - c o n c e n t r a t i o n and atomic f l u o r e s c e n c e d e t e c t i o n
FACSS XXI,
S t . L o u i s , USA,
October,
229
1994.