1. No category

Making Sample Preparation
Easy with Automated
SPME and Static Headspace
Outline of Talk
!
!
!
!
Introduction—sample types for GC
Description of static headspace and solid
phase microextraction (SPME)
! SPME Optimization
! Comparison of two techniques
Automation with the Combi PAL
Summary
GC Sample Types
!
Trace levels of analytes in a “dirty” aqueous
matrix
!
Solids containing volatiles
!
Low levels of analytes in water
!
Low levels of analytes in a clean organic
solvent
Only the last sample type should be injected directly
into the GC without sample preparation
Conventional GC and GC/MS
Sample Preparation
Analytes are usually in an
aqueous matrix which
contains non-volatile material
"
Several Steps
!
!
!
!
Liquid-liquid extraction
Solid phase extraction
Evaporation under nitrogen
Addition of a solvent
Final sample ready
for injection
Static headspace or Solid
Phase Microextraction (SPME)
can be used for most samples
in water or solids
What are they?
Static headspace and solid phase
microextraction (SPME) are both solvent-free
sample introduction methods for GC.
Both techniques are suitable for trace organic
compounds in solids and aqueous solutions.
Static Headspace
Sample containing volatiles is placed in a
closed container.
After equilibrium is reached, an aliquot of the
gas phase is removed and injected into the
GC.
Solid Phase Microextraction
!
A fused silica fiber is coated with an
absorbent such as methyl silicone.
!
The fiber is inserted into the sample or in the
headspace over the sample.
!
Analytes are absorbed into the fiber coating
and then desorbed in a GC injector.
!
Fibers and holders (both manual and
automated) are made by Supelco.
Varian has automated the technique.
SPME
Fiber
Holder
Plunger
Barrel
Color-coded
Screw Hub
Sealing Septum
Retaining Nut
Fiber Sheath
Fiber
Attachment Rod
(pierces septum
of sample vial
and GC injector)
SPME Fiber
SPME Sampling Process
absorb
desorb
Several Fibers are Available for
Different Applications
! Polydimethylsiloxane
" A non-polar phase available
in three phase
thicknesses for a wide variety of slightly polar
compounds.
!
85-micron Polyacrylate (phenols)
!
65-micron Carbowax/divinylbenzene (alcohols)
!
75-micron PDMS/divinylbenzene (amines)
65-micron Carboxen/PDMS (volatiles)
80-micron Divinylbenzene/carboxen/PDMS for
samples with volatiles and semivolatiles
!
!
SPME Applications
!
!
!
!
!
Forensic applications—blood alcohol, arson,
drug analysis
Flavors and contaminants in food
Volatile organics, pesticides, phenols, PNA’s,
explosives in environmental samples
Organic volatiles in pharmaceuticals
Polymers, industrial formulations
Schematic of a Single Extraction of
a Substance From a Solution
CCLL
Distribution
Coefficient
=
Distribution Coefficient =CCG
G
Substances in Water at 25°C
Benzene
4.0
Acetone
551
Ethanol
5260
VG; CG
VL; C0
VL; CL
SPME is a Three- Phase System
K1 = CL/CG
K2 = CF/CL
K3 =CF/CG
C0 VL= CGVG+ CLVL+ CFVF
Conventional Static Headspace Sampling
Response depends on volatility and partition coefficient
After equilibrium
Original sample
SPME Response
Depends on fiber selectivity in addition to partition coefficients
Techniques for Optimization
!
Injector conditions
Selection of the appropriate fiber phase
Achieving clean blanks
!
Saturation with salt
!
Size of sampling vial
!
Mixing or agitation
!
Sampling time
!
Heating
!
!
SPME Chromatograms of BTEX’s
With Splitless Injection
insert: 4 mm id
insert: 0.8 mm id
The inner diameter of the injector insert should be slightly larger
than the outer diameter of the fiber sheath to assure sharp peaks.
Merlin MicrosealTM
Replaces
septum
Nut comes with start switch
!
Designed for 1078/79
injectors
!
Easy installation-replace
septum and nut
!
Eliminates all septum
coring and bleed problems
!
Reguires 23-gauge SPME
fiber or syringe
Absolute Response to Gaseous
VOC’s With SPME and SHS
80000
70000
60000
50000
SHS
Carboxen
PDMS
Tri-phase
40000
30000
20000
10000
0
Chloromethane
200 ppb
Chloroethene
CCl3F
Absolute Response to Later
Eluting VOC’s With SPME and
SHS
3500000
3000000
SHS
Carboxen
PDMS
Tri-phase
2500000
2000000
1500000
1000000
500000
0
1,2,3-TrichloroPropane
200 ppb
1,2-dibromo-3-chloropropane
1,3-butadiene,1,1,2,3,4,4 hexachlorine
Initial Chromatogram of a
New Fiber
Detector: FID
Fiber: 100µ PDMS
Blue: First chromatogram, Black: After conditioning
FID Chromatograms of Empty Vials
Unbaked septum
Baked septum
Vial with no septum
Injected clean needle
No injection
Effect of Addition of Various
Salts on Sensitivity
10
enhancement was 23-fold
No salt
FID Response
8
Sodium chloride
Sodium sulfate
6
Potassium carbonate
4
2
0
MeCl
Benzene
TCE
Chloroform
Toluene
Dioxane
Effect of Agitation on Response of Halogenated Pesticides
(2ppb in water)
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
α-HCH
γ-HCH
β -HCH
Heptachlor
δ-HCH
Aldrin
Heptachlor epoxide
Endosulfan I
4,4’-DDE
Dieldrin
Endrin
4,4’-DDD
Endosulfan II
4,4’-DDT
Endrin aldehyde
Endosulfan sulfate
Methoxychlor
Endrin ketone
4
agitated
10
7
11
6
8
12
9
13
14
1
2
3
5
15 16
17 18
not agitated
SPME Headspace Response of 2 ppb
DDT in Water versus Temperature
30
35
40
45
50
55
60
Temperature °C
65
70
75
80
Static Headspace and SPME
Advantages:
Solvent-free
Can be done manually
Sensitivity to ppb levels or lower
Disadvantages:
Equilibrium techniques (full recovery not
possible)
Matrix effects must be considered in
quantitative work
SPME vs Static Headspace
FEATURES
SHS
SPME
Compounds
volatiles
volatiles, semivolatiles
Selectivity
depends on volatility
and solubility in solvent
can be adjusted by
varying fiber
Sensitivity
ppm to ppb depending
on detector
usually greater than
headspace
Advantages
More rugged technique See above—also easier
than SPME
to focus analytes
(avoids large volume
injections)
Recommendations
!
!
Use static headspace:
! analytes have a narrow boiling point range
and a low boiling point
! SHS meets sensitivity requirements for the
analysis
Use SPME
! analytes have a wide boiling point range
! semivolatiles
! SHS does not meet sensitivity requirements
Features
!
!
Three modes
! Liquid injection
! Static headspace
! SPME
Samples
! Choice of vial sizes (2-mL, 10-mL, 20-mL)
! Can accept up to four sample trays
– up to 392 2-mL vials
– up to 128 10-mL vials
Features
!
Liquid injection
!
!
!
!
!
Headspace
!
!
!
!
Uses standard 10-µL syringe
Can inject large volumes (up to 500-µL)
Can inject into two injectors
Two-solvent syringe wash
Heated gas-tight syringes to 5 mL
Samples heated to 200°C with variable mixing
speeds
Constant heating time for all samples
SPME
!
!
Samples heated to 200°C with variable mixing
speeds
Constant heating time for all samples
Options
!
!
!
!
Cycle Composer software
! Macros can be written to add steps to a cycle
! Examples: derivatization with SPME, add
internal standards with liquid injection
Bar code reader
Cooling of 2-mL samples (4-60°C)
Bakeout station for SPME fibers
Headspace Chromatogram of High
Ethanol Standard (0.50%) followed
by a Blank
ethanol
Blank run
n-propanol
Linearity of Ethanol from 0.00-0.50%
6.00
5.00
corr. 0.9999
4.00
RRF
3.00
2.00
1.00
0.00
0.00
0.10
0.20
0.30
% ethanol
0.40
3 data points at each level (not averaged)
Response factor RSD=0.88%
0.50
0.60
Area Count Data for Highly
Volatile Componds
Compound (BP, °C)
Precision
(% RSD)
Dichlorodifluoromethane (-29.8)
1.18
Chloromethane (-23.7)
0.56
Vinyl chloride (-13.4)
0.41
Bromomethane (3.6)
0.79
Chloroethane (12.3)
0.70
Trichlorofluoromethane (23.7)
0.37
(2 ppm, n=4)
Headspace Chromatogram of
Diesel followed by a Blank Run
Headspace SPME of Pesticides
in Milk
(100 ppb, 80°C)
Blank milk, ions 235+246
DDE
DDD
DDT
Spiked milk, ions 235+246
Spiked milk, ECD
Total Ion Chromatogram of
Volatiles in Water (17 ppb)
Carboxen fiber—60 compounds in water from dichlorodifluoromethane
(B.P. -29.8°C) to 1,2,3-trichlorobenzene (B.P. 221°C)
Conclusion
" With
static headspace, SPME and liquid
injection capability, the Combi PAL allows the
user to handle a wide variety of GC samples
with a minimum of sample preparation.