Methane as gas can be determined by a TCD detector. Qualitative analysis can be performed by the retention time or peak position in x axis of the chrmatogram. It can be identified without calibration if you have enough experience or are applying a well known technique with standard column for permanent gas such as molecular sieves.

Nevertheless, the only way in this case for an accurate identification of methane and its concentration is by a qualitative and quantitative analysis. For this, you will need a standard, external or internal.

TCD detects the presence of methane by the difference between it and the reference flow in the wheatstone bridge, normally stabilized by Hydrogen flow due to its high thermal conductivity. Peak position identifies the compound and peak area its concentration. To associate an area with a concentration, a calibration curve must be previously obtained.

Best Regards

I would recommend that you read books and articles on determining the concentration of samples using GC analyses.

Please look at the following below links which may help you in your analysis:

https://iopscience.iop.org/article/10.1088/1755-1315/113/1/012023/pdf

https://gcms.cz/labrulez-bucket-strapi-h3hsga3/114db259839c4e9d9d67599c4550283f/5988-9260EN.pdf

https://commons.emich.edu/cgi/viewcontent.cgi?referer=https://www.google.com/&httpsredir=1&article=1064&context=mcnair

Thanks