GC-FID problem/question (barometric pressure)

Discussions about GC and other "gas phase" separation techniques.

5 posts Page 1 of 1
i'm writing to you because i have a problem with a GC-FID equipped with HS (clarus 580+turbomatrix).
We are copying/building a method for the detection of a lot of solvents (methanol, acetone, benzene, ethanol, acetate, etc...).
We had already built this method onto another GC. The method is perfect and allow us to resolve all peaks.

We are copying that method onto the other GC.
The GCs are the same: same model, same method (HS-autosampler, injection, oven, flow, etc...) and equipped with the same column, but one GC is located at 200 m (altitude) and the other is at 2000 m.
With the 200m-GC we can resolve all peaks, but with the other we can't.
My question (probably stupid question, but i'm thinking about everythinks): the atmospheric/barometric pressure could affect time retention in gaschromatography?
because we have very big difference in time retention, especially for solvent with low boiling point.
I have no other idea for that shifting...

Thank you for your time!

Does Clarus reference atmospheric? I know Agilent does but don't know PE as well.

Best regards,

AICMM wrote:

Does Clarus reference atmospheric? I know Agilent does but don't know PE as well.

Best regards,


I don't konow that...how can i check it?
Yes, things change appreciably with that much of an altitude difference.

Setting aside the headspace for a moment, and just focusing on the GC, The ambient pressure at 200m elevation is roughly 14.4 PSI. At 2000m, it is roughly 11.4 PSI. Since an FID is an ambient pressure detector, the pressure reduction affects your flow, specifically, the linear velocity associated with a particular volumetric flow.

Putting the numbers into a GC method translator such as: https://www.restek.com/ezgc-mtfc , if He is the carrier gas, linear velocity at the same volumetric rate increases from 32.35 cm/sec to 35.16 cm/sec. Certainly enough difference to see in your chromatogram. If you are using hydrogen as the carrier gas, the change is even more pronounced, increasing from 39.5 cm/sec to 44.8 cm/sec.

Retention is probably slightly affected, too, because the vapor pressures will be different, but I'm guessing the change in linear flow can account for many of your issues. You might need to tweak the temperature ramp slightly, but I would first try to attain the same linear flow rate on both instruments and proceed from there.

Good Luck
Inject something that will not be retained -methane or butane from a cigarette lighter will do - on both instruments. What are the retention times ?.

What are the conditions that are set on both instruments, and what are the dimensions of the column ?

Peter Apps
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