Chromatography Forum

Hi,

I am running a Fischer-Tropsch synthesis and something is wrong with the methane peak from the TCD. The light gases are analyzed using a HP 6890 gas chromatograph. H2, N2 (internal standard), CO, CH4 and CO2 are analyzed on TCD using a Carbosieve column. Hydrocarbons products were separated with a GS-Alumina PLOT column and detected with a flame ionization detector (FID). CH4 was used to combine TCD and FID analysis. The synthesis gas contained 3% N2 which served as an internal standard for quantification of the products to close the mass balance.
So the methane peak is very low and unstable. All the other gases H2, N2, CO, CO2 seems to be as they should be. The methane peak in FID is fine, but since I need the area from TCD to calculate the methane selectivity and C5+ I need to solve this problem, any ideas?

Best,
ljubezr

What are you using for carrier? This sounds an awful lot like the classic, I want to measure fixed gasses with TCD problem. Hydrogen with helium carrier wont work well, hydrogen with argon carrier will work but the sensitivity to everything else is awful, etc.....

Possible to post a chromatogram?

There is a reason that most systems use a methanizer for CO, CO2 and CH4. Your FID response is good for methane; your TCD response is awful and unlikely to get any better.