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Gases&Technology
FEATURE
Maximizing Sample Throughput in Gas
Chromatography
A N D R E W
T I P L E R
A N D
M A R K
C O L L I N S
By focusing on critical design elements, a significant time savings can be realized.
Introduction
In today’s economy, improving efficiency and maximizing productivity
are the keys to success. As with many
analytical techniques, most gas chromatography (GC) analyses utilize
established methods. The goal in
designing a new high performance GC
oven was to provide significant benefits in terms of sample throughput
while at the same time allowing full
compatibility with established methods. Further, the design should support standard injectors, detectors,
pneumatics and columns, so that
established methods and laboratory
standard operating protocols (SOPs)
could be followed without modification. In addition, a new GC oven
design should support a migration
path towards faster chromatography
for those GC users wanting a lowrisk opportunity to explore such
capabilities.
Figure 1: GC oven design showing the air flow paths
GC Oven Design
The new GC uses an air-bath oven where a large fan is
used to mix the air inside the oven during heating in
order to assist with the exchange of the hot air with ambient during cooling. In this design (patent pending) a novel
dual-walled oven approach has been adopted. The inner
chamber of the oven holds the GC column, and injector
and detector ports. This inner chamber is surrounded by
a second wall that serves to reduce the heat loss from the
inner chamber during heating and so improves the heating rate and minimizes temperature non-uniformity (gra-
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dients). A key factor in accomplishing faster sample injection-to-injection time is fast cooling. To achieve fast cooling rates, the oven was designed with a circular door
mounted concentrically to the large oven fan as shown
in Figure 1.
During cooling, ambient air enters the inner chamber
via the door, which is mounted behind and concentric to
the fan. The hot air exits through the outer wall and out
of the oven through a door that opens at the base and to
a vent. During analysis, the dual-walls of the oven further
insulate the inner oven from the outer oven walls and
therefore helps minimize heat losses during heating, and
also heat coming into the oven from the walls during
operation at a low starting temperature.
January/February 2007
Gases&TechnologyFEATURE
Fast Cooling
This oven design allows cooling
from 450°C to 50°C in about 1.6
minutes. To perform this analysis, a
thermocouple was placed inside the
oven to record the data during cooling. Figure 2 demonstrates the cooling performance of the oven. In addition, the time required to cool down
to even lower temperatures has been
significantly improved—about
2 minutes to 40°C and just 4 min-
A key factor in
accomplishing
faster sample
injection-toinjection time is
fast cooling.
utes to 30°C with an ambient temperature of 23°C. This cool-down
performance allows chromatography
at these near ambient temperatures
to become practical. Most ovens will
cool to 30°C but this may take many
minutes to achieve.
The cooling algorithm includes a
1-minute stabilization time to
achieve a steady temperature. The
need for additional equilibration
time is thus eliminated. While such
cooling performance is a very welcome development, it does introduce
some undesirable effects.
The first concern is that, in some
instances, the carrier gas inside the
column contracts during rapid cooling at a rate faster than the carrier gas
entering into the column inlet from
the injector. This has the resultant
effect of producing a partial vacuum
at the column outlet. As the column
outlet normally resides inside a detector, vapors inside that detector will be
drawn back into the column during
Figure 2: GC oven-cooling performance curves
Figure 3: System for doping helium carrier gas with methane
Figure 4: Flow rate versus response calibration
January/February 2007
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Gases&TechnologyFEATURE
Figure 5: Flow rate into detector with the fast cooling GC oven
Column: 60 m x 0.25 mm x 1.0 µm 5% Ph/Me Silicone
Oven: Clarus 600 GC, cooling from 280°C to 50°C
Carrier Gas: Helium at 25 psig
Injector: 100 mL/min split at 375 °C
Detector: Flame ionization at 400 °C
Figure 7: Example of the use of SOFTcooling to limit the oven cooling
rate to 300 °C/min using conditions given for Figure 5
rapid cooling. Such vapors may be
hostile to the still hot column.
Secondly, some columns generate
significant stationary phase bleed
when operated at temperatures close
to their specified limit. A fast cooling
oven may “chill” this bleed so that it
collects in pockets along the column.
When the column is next-temperature
programmed, these focused areas of
bleed will manifest themselves as
“ghost peaks”.
The system shown in Figure 3 was
used to study the behavior of the carrier gas at the column-detector interface during column cooling. Essentially, this system dopes the carrier
gas (helium) with a fixed concentra-
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Figure 6: SOFTcooling to a temperature limit
Figure 8: Example of the use of SOFTcooling to limit the oven cooling rate to a given temperature threshold
tion of methane. The flame ionization
detector will give a response proportional to the mass flow rate of
methane and hence the mass flow
rate of the carrier gas, helium, as
shown in Figure 4.
Using the test rig shown in Figure
3, the FID signal was monitored during cooling of the experimental fast
cooling oven with the result shown in
Figure 5. The signal disappears completely soon after the onset of cooling,
indicating that the flow of carrier gas
into the detector has actually
stopped. At this point, the temperature of the column is still very high.
Oven-control software called
SOFTcooling™ has been developed to
limit the maximum cool-down rate by
throttling the ambient air intake during cooling. Such an algorithm will
only affect the initial cooling rate and
will have a low effect on the total time
required to completely cool the oven
for the next run as shown in Figures
6 and 7.
SOFTcooling
The SOFTcooling approach
described above may also be used to
mitigate the ghost peak effects. In
this instance, the oven must be
cooled at a much slower rate to allow
dissipation of the column bleed so
that “focused” condensation within
January/February 2007
Gases&TechnologyFEATURE
Figure 9: Examples of different SOFTcooling rates from 350°C to
250°C to assess their effect on the creation of ghost peaks; GC conditions are provided below
Column: 60 m x 0.25 mm x 1.0 µm 5% Ph/Me Silicone
Oven: Clarus 600, 50 °C(1min) – 20°C/min – 350°C(15min)
Carrier gas: Helium at 25 psig
Injector: 100 mL/min split at 375 °C
Detector: Flame ionization at 400 °C
Figure 10: Heating rate at 230 V with fast 2000-watt heater
Figure 12: Very light crude oil with high-power heater
Figure 11: Diesel oil with high-power heater
the column does not occur. Once a
temperature has been reached at
which column bleed has effectively
disappeared from the carrier gas,
ballistic cooling of the GC oven may
be resumed. The SOFTcooling algorithm is essentially the same as
before except that now a temperature threshold is applied as shown
in Figures 8.
Figure 9 shows “chromatograms”
of ghost peaks at different cooling
rates. Once the rate is reduced to
25°C/min on this 60 m x 0.25 mm x
1.0 µm 5% Phenyl/Methyl Silicone
column, the ghost peaks are eliminated.
January/February 2007
GC oven heating rates
If a higher supply voltage is available, the GC oven can be supplied with
a higher-power (2000-watt) heater to
increase the potential programming
rates as shown in Figure 10.
High-speed chromatography
Figures 11–14 show examples of
fast chromatography showing excellent peak shape. For diesel oil, the
separation is complete in 3.8 minutes; for very light crude the separation is complete in just under 4
minutes; and for C6–C44 the sepa-
ration is complete in under 6.5
minutes.
Figure 14 provides an example of
fast chromatography showing excellent peak shape for a gas oil cut.
The separation is complete in just
over 4 minutes.
Autosampler pre-rinse to
speed up the analytical
cycle time
Having significantly improved the
oven cool-down time, the next step
was to address the autosampler
rinsing and priming of the syringe.
Figure 15 shows the timing of a
typical analysis. The chromatogra-
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Gases&TechnologyFEATURE
Figure 13: C6–C44 ASTM D2887 calibration standard with highpower heater
phy normally occupies the bulk of
the time and we can reduce this
because of the increased oven heating rates. In terms of analytical
throughput this is the only productive time. For temperature programming, the oven cooling time may be
significant; we have reduced this
dramatically on the Clarus 600 GC.
The rest of the time is spent inbetween runs performing diagnostic
checks, equilibrating the system
and, most significantly, preparing
the autosampler for the next injection. The autosampler will start to
operate once the GC becomes ready.
As a result, the GC sits idle and
ready for injection but still has to
wait for the autosampler to go
through its various steps in rinsing
the syringe and priming it with
sample.
By changing the system timing to
initiate autosampler operation in
advance of the GC becoming ready,
reduction in analytical cycle time can
be achieved by several minutes. The
system now calculates the length of
time to process all the steps in the
GC necessary to become ready and
together with the time needed to prepare the syringe with sample, and is
so able to start the autosampler at
the optimum moment: the autosampler injects at the same time the GC
becomes ready.
In practice though, we start the
last step (Sample Pump) when the
GC is ready; this is to prevent the
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Figure 14: Gas oil cut with high-power heater
syringe from sitting with sample for
extended periods if there is an interruption to the normal operation.
The timing diagram now looks
like the chart shown in Figure 16.
Note how the ready time is much
By changing the
system timing to
initiate autosampler
operation in
advance of the GC
becoming ready,
reduction in
analytical cycle time
can be achieved.
reduced from the last chart. This
feature can save several minutes
under some conditions.
Conclusions
In improving analysis, all aspects
of high speed GC were taken into
consideration including the use of
short
columns,
narrow-bore
columns, thin film columns, fast
temperature programs, and high
carrier gas flow rates.
The GC oven enables faster chromatography immediately using existing methods and delivering
increased throughput and productivity: typically 3.5 minutes for each
temperature-programmed cooling,
and typically 1.5 minutes for each
autosampler injection.
The GC oven also delivers highspeed temperature programming,
enabling even more time savings
with faster chromatography and the
capability of chromatography at
near-ambient temperatures with
practical and acceptable cycle times
for highly volatile compounds.
The analytical sample time can be
reduced by several minutes if the
system timing is changed to initiate
autosampling while the GC is getting
ready, a period of time that is nonproductive. The system calculates
that period of time and compares it
with the period needed to prepare
the syringe with sample, then initiates the autosampler so it is ready to
inject at that moment when the GC is
ready for its new sample.
Fast oven cooling, although simple in concept, does require some
special care in its implementation.
As illustrated in this paper, fast cooling can introduce two significant
problems that can affect the integrity
of the column and the analytical
data. Techniques like SOFTcooling,
as implemented on the GC oven,
may be utilized to minimize or even
eliminate any impact from the effects
produced by fast cooling rates.
Slight SOFTcooling totally eliminates the detector gas ingress effect
January/February 2007
Gases&TechnologyFEATURE
Figure 15: GC run timing
and stronger SOFTcooling is
required to eliminate the ghost peak
effect. This is only necessary to
reduce the column temperature to a
point where bleed levels are very
low. Ballistic cooling may then
resume.
Acknowledgements
The authors wish to acknowledge
and thank the entire development,
engineering and manufacturing team
at PerkinElmer for the innovative
design, development and production
of this oven that facilitates high
speed GC.
Andrew Tipler is Senior Staff
Scientist,
Chromatography
Business Unit,
PerkinElmer Inc.,
710 Bridgeport
Ave., Shelton,
CT 06484. He
has been with
January/February 2007
Figure 16: Final GC run timing
PerkinElmer for 24 years.
Throughout that time, he has been
responsible for developing new
technologies and applications for
the company’s GC product line—
these include gas chromatographs,
headspace samplers and thermal
desorption systems. He has been
awarded 9 patents for innovations
in GC and has presented papers at
many international symposia. He
can be reached at 203-925-4600
or [email protected]
Mark Collins, Ph.D., Gas
Chromatography Product
Manager, Chromatography
Business Unit, PerkinElmer LAS.
He has been with PerkinElmer for
11 years. Throughout that time,
he has been responsible for product and marketing management
for the company’s GC product
line. These include portable analytical instruments, gas chromatographs, GC/MS systems,
headspace samplers and thermal
desorption systems. He has
presented
papers at many
international
symposia. He
can be reached
at 203-9254600 or
[email protected]
Note: Gases and Technology
periodically publishes articles about
the technology of new products or
innovative technologies introduced
into existing products. This is to
explain the technology in a non-commercial way to inform possible endusers of the technology that may suit
their application. Gases and
Technology does not verify the test
results noted, nor does Gases and
Technology endorse these products.
The technology is presented for
information purposes only.
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