FID and TCD

[Ruhtra]

Hi there,

I have a question about FID and TCD.

I have two GC machine, one has TCD, another one has FID. Both of them are calibrated with the same calibration gas (only single point calibration). If I run both machines with a sample (The same sample for both machine). Would I get the same result from both machine ? I guess it is not. But how much would the difference be in your opinion ?


Thank you very much for your answer.


Ruhtra


PS. I have no background in GC at all. ( I am Petroleum Engineer).

[chromatographer1]

It depends upon the components of the sample.

TCD will detect gases that a FID won't.

FID is generally more sensitive for gases that can be ionized in a hydrogen flame.

But for any gas in a sample, calibrated with the same calibration gas for both detectors, given that this gas is separated from the other matrix components, should give you the same answer.

best wishes,

Rod

[Ruhtra]

Thank you, Rod.

Apologize. I should mention in previous post. Usually, I analyze natural gas compositions (C1- C8+, N2, CO2).

I have Varian micro GC 4900 and Agilent GC (Cant remember the model).

This was what I did:
1 .I calibrated both machines with the same calibration gas.
2. I reran the calibration gas again (this time I called "calibration test"). Both machines matched the gas composition certification of calibration gas.
3. I ran a sample. The results from both machines were different. For example, C1 from micro GC was about 33% and C1 from Agilent GC was about 48%)

Both machines provided the reproducible results.

What are the parameters that I should look into it to be able to solve this issue ?


Thank you for your help.

Ruhtra

[chromatographer1]

You may be calibrating well outside the linear range of the detector. If you are calibrating with a 1% methane mix and then measuring an approximately 50% methane sample your answers will not be accurate. So tell me what is the calibration mix level of methane.

OR

Are you calibrating on the one peak, or are you calculating your percentage on a analysis of the complete sample?

It sounds like you are measuring based on a percent of the total response of the sample. If your FID doesn't see CO2 then what effect do you think this will affect the methane measurement?

best wishes,

Rod

[Ruhtra]

Thanks, Rod.

-The calibration gas has about 80% C1, 12% CO2, 2.67% N2, the rest are C2-C8.

-Yes, I am calculating the percentage on a analysis of the complete sample.

-I dont think my FID see CO2. I might be wrong as the previous GC operator told me that the FID detector that we have only detect hydrocarbons.

PS.

-My company is in the process of hiring new GC operator right now (It takes 2 months already but I dont have one yet).

-I am in Australia. Today is Saturday. If you need any information that could help me to solve this issue, please let me know. I would provide you on Monday.


Thank you

[chromatographer1]

You cannot perform the analysis using the calculation as you are now doing it.

An FID does not detect CO2, nor does it see N2.

You can not do a complete analysis using a detector that doesn't see all your components.

In addition you should be doing an external calibration on a peak by peak basis, not as a complete area percent basis.

best wishes,

Rod