Innovations in Column Oven Design Enhance GC Performance
Novel oven designs for gas chromatographs enable more efficient chromatography.
A number of essential functional features combine to provide a truly high performance column oven for a gas chromatograph (GC). These features include fast heating, fast cooling, the ability operate over a wide temperature range from near-ambient temperatures, flexible temperature programming, temperature stability with low temperature gradients, and low noise. These key requirements are enabled in a novel column oven design that permits easy access to interface a wide choice of conventional columns to standard injectors and detectors.
This is an example of SOFTcooling to
a temperature limit. Software limits the maximum cooldown rate by throttling
the ambient air intake during cooling, which only affects the initial cooling
rate and will have a low effect on the total time required to completely
cool the oven for the next run. Source for all graphics: PerkinElmer. Click
to enlarge.
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 allow full compatibility
with established methods with support for standard pneumatics so that laboratory
standard operating protocols (SOPs) could be followed without modification. The
new GC oven also supports a migration path towards high speed GC for those GC
users wanting a low-risk opportunity to explore such capabilities.
Novel design for fast cooling
The new GC uses an air-bath oven where a large fan is used to mix the air inside the oven during the heating and to assist with the exchange of the hot air with ambient air during cooling. In this design, 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 (gradients).
A key factor in accomplishing faster sample injection-to-injection time is fast cooling. To achieve fast cooling rates, the new oven was designed with a large circular door mounted concentrically to the large oven fan. During cooling, ambient air enters the inner chamber via this door, which is mounted behind and concentric to the fan, and the hot air exits through the outer wall and out of the oven through a door that opens at the base.
This oven is able to cool from 450 °C to 50 °C in about 1.6 min, which is at least twice as fast as any other conventional GC oven. In addition, the time required to cool down to even lower temperatures has been significantly improved—about 2 min to 40 °C and just 4 min to 30 °C with an ambient temperature of 23 °C. This cooldown 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 allows imimization of the stabilization time to get a steady temperature before the GC coming ready.
SOFTcooling the oven
Fast cooling is a significant and advantageous development, but it should be managed carefully on implementation. One concern is that, in some instances, the carrier gas inside the column contracts during rapid cooling at a rate faster than the carrier gas is 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 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’.
Gas chromatograph of very light crude
oil with high power heater reveals fast response times. Click
to enlarge.
To investigate the fast cooldown rates, the behavior of the carrier gas at the
column-detector interface during column cooling was studied. Essentially, the
carrier gas (helium) was doped with a fixed concentration of methane. The flame
ionization detector (FID) gave a response proportional to the mass flow rate of
methane and hence the mass flow rate of the carrier gas, helium.
The FID signal was monitored during cooling of the fast cooling oven. 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 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.
This SOFTcooling approach 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 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 to the oven. For example, once the cooling rate is reduced to 25 °C/min on a 60 m x 0.25 mm x 1.0 µm 5% Phenyl/Methyl Silicone column, the ghost peaks are eliminated.
In addition to SOFTcooling, the GC oven can be supplied with a higher-power (2000 W) heater to increase the potential programming rates if a higher supply voltage is available. The high power heater rates serve to extend the scope of the GC to a broader range of applications.
High-speed chromatography
The novel GC oven design flows ambient
air through the inner chamber via the circular door, and the hot air exits
through the outer wall and out of the oven through the door that opens to
the base. Click
to enlarge.
The new GC oven enables faster chromatography immediately on existing methods
delivering increased throughput and productivity. For example, for diesel oil,
the separation is complete in 3.8 min; for very light crude the separation is
complete in just under 4 min; for C6–C44 the separation is complete in under 6.5
min and for gas oil, the separation is complete in just over 4 min.
The oven also delivers high-speed
temperature programming, enabling even more time savings with faster chromatography and delivers the capability of chromatography at near-ambient temperatures with practical and acceptable cycle times for highly volatile compounds.
Fast oven cooling requires 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. Fortunately, techniques like SOFTcooling, as implemented on the new GC oven, may be utilized to minimize or even eliminate any impact from these effects produced by fast cooling rates.
Slight SOFTcooling totally eliminates the detector gas ingress effect 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.
—Andrew Tipler,
Senior Staff Scientist,
and Mark Collins,
GC Product Manager,
PerkinElmer Inc.
