I can only provide a general answer to you as there are a lot of missing information (GCMS manufacturer and etc.). I assume that your GC is set up for gas analysis.

Based on my experience, this is how I would go about it. May not be the best way to do it though.

You will need the following supplies/items for calibration:

1. Obtain a Standard gas from a supplier (ie: Linde or Air Products) with known concentration.

2. GCMS Syringe - Leur Lock Gas-Tight syringes (comes with push button valve to lock, prevent gas from leaking after sampling). Size wise, depends on your target Limit of detection and GCMS system. I suggest 1000uL and 100uL but please adjust accordingly based on your calibration curve. The air tight syringe is to ensure noting is leak to the atmosphere after sample is drawn and prior to injection.

3. Get sealed (crimped) headspace vials (comes in 10 ml or 20ml volume) filled with Nitrogen gas or other inert gases that does not interfere with the target gases. Inject a certain concentration of the standard gas bought from the supplier into the vials for dilution. You will need and do serial dilution from there. So your volume is always 10x or 20x dilution when you inject 1 ml of gas into the vial. These will be your gas with standard concentration/calibration points (with known concentration - usually in ppm).

4. Inject all the gas standard with known concentration into GCMS and obtain the chromatogram. In the software, key in the concentration value for peak (it is calculated based on area under the peak). Sorry not able to provide a more specific answer here. GCMS software wise, it defers based on the manufacturer of the GCMS you are using. Please find out from your supplier/manufacturer or the software manual. To setup the calibration curve, usually 3 points calibration is used (3 concentrations). You probably need to use SIM mode as this is a Quantitative analysis.

5. After calibration, run your unknown sample. You may need to dilute your sample depending on the concentration of your calibration curve.

Hope this will help you in design the analysis or as a starting point.

Thank you Kuok Sum Cheong sir for clearing the doubt.

One more query, you are saying we have to dilute our standard? Say we have methane or carbondioxide standards of 100ppm 1000 ppm and 2%. Why we need to dilute the standards? can't we just use them as it is.

And how can we came to know what standard concentration we have to use for unknown gas samples from waterbodies

If you already have the fixed concentrations and it is within the range you need, no dilution is needed.

Thank you Kuok Sum Cheong.

As for how do you know what standard concentration you need, try making one run of your sample from the water body. You should get some idea of the concentration by looking at the peak height by comparing it with the results from your standards. assuming that it is linear.