Chromatography Forum

Can anybody give me some ideas on where to look to find reasons for discrepancies in results for Hydrocarbon Analysis in Condensates between two different set-up 7890A GC's? Sample injected is 100% sample - no ISTD or sample dilution/prep conducted.

1 GC has a TCD & Packed Column, Auto-Injector, Backflush set-up

The other GC has an FID & Capillary Column, Manual Injector, full run-time.

End results are in volume%

Could it be a volume injection issue, maybe not accounted for in the method?

Are there components detected by one detector that are not by the other? And could be skewing the volume% calculations?

The FID GC results are about half of those obtained from the TCD GC. Could this result if the FID were calibrated based on mol%?

It is known that the TCD GC is calibrated using vol%....not yet known how the FID is calibrated.

Any ideas where else to look? Help Please!

Since you don't know how the FID GC is calibrated why do you have questions about the measurement?

If your injection is split on the FID GC then split discrimination can be a significant factor in a different answer.

Get your FID GC method properly calibrated and then determine if you have a problem or not.

RIght now you don't have any information with it that should concern you. Your TCD GC hopefully is calibrated correctly and should be considered the true analysis until you can prove otherwise.

best wishes,

Rodney George

Because we are conducting side by side project analysis with another lab group. The Other Lab group is the FID GC. Just wanted to get some brainstormed thoughts before meeting with them, so we had some ideas other than a calibration error that we could possibly help them out with. The information that CONCERNS me is that the results don't match for the same sample.
Was simply looking for help in troubleshooting any possibilities to contributing factors. I don't expect the results to ever be identical....however was reading up many differences alone between FID & TCD detection (concentration vs. mass sensitivity)...thought there might be a little more theory out there to help us out....still researching.

Thanks Anyways Rodney

TCD will see gases that an FID will never see.

FID for hydrocarbons have a weight (not mole) factor that is fairly consistent from hexane to higher analogs.

TCD has great differences with response depending upon the conductivity of the gases involved. On a mole basis hexane may have a response several times that of methane.

If you want almost uniform response on a mole basis, try using an Argon or Helium ionization detector.

Is that theoretical enough for you?

Good luck in your research. Review natural gas analysis literature and you will learn a great deal.

best wishes,

Rodney George
consultant