FID responds to compounds with Carbon and minimum or no Oxygen, thus there is a response for CH4, but not for CO2 and probably not for N20. Electron ionization should ionize N2O, thus an appropriate GC/MS method and a standard mixture of the different gases that you are investigating should provide a GC/MS based quantitative normalization method. N2O and CO2 have same integral mass (44) as previously said by JF Gal, and also very close monoisotopic masses (44.001 Da vs 43.989 Da), thus they must be fully resolved by GC, as no deconvolution of partially or unresolved peaks can be performed by standard GC/MS instruments.

Because GCMS is for both qualitative and quantitative analysis I think it will work .

yes

Yes you can. Follow the same procedure you followed to quantify CO2 and CH4. 

Check this Paper. Determination of CH4, CO2 and N2O in air samples and soil atmosphere by gas chromatography mass spectrometry, GC-MS. http://www.ncbi.nlm.nih.gov/pubmed/15237293

The paper previously suggested is a good option for the determination you need to perform. The link is http://www.ncbi.nlm.nih.gov/pubmed/15237293

Yes, you can use either external or internal standard calibration methods.  

Of course you can

Thank you very much all of you, the answers have been really very helpful

Our lab would try to employ the EPA TO15 method that uses a "summa" canister (6L) that is evacuated to -15 psi vacuum.  These are deployed to the field.  The valve is opened, a gases sample is sorbed onto the wall of the coated canister, brought back to the lab and desorbed and concentrated in the inlet of the GC/MS.  Low mwt samples are challenging, as is getting an appropriate internal standard.  Here are a couple links.

http://www.ialonline.com/Files/Guide_to_Air_Sampling_and_Analysis_070909.pdf

http://www3.epa.gov/ttnamti1/files/ambient/airtox/to-15r.pdf

yes, of course u can.

Dear Jayashree,

yes you can. If you want some sensitivity do not do full scan meassurments, bit selected ion measurments. Typically 20-100 ms dwelltime.

However I would be surprised if you would not get any signal of N2O on the FID as well.

Yes, you can. And the external standard will be needed. In term of sensitivity, the difference of FID and MS detector has to be concerned.

Be careful for the possible interference with CO2.

Most of the intensity is concentrated on the molecular peak,  m/z 44 for both molecules, and fragments are somewhat similar, except for m/z = 30. 

Have you considered GC with a thermal conductivity detector?

N2O at ambient air concentrations are best resolved using GC-ECD and possibly GC-PDHID. TCD and FID do not have the sensitivity at these levels (

FID responds to compounds with Carbon and minimum or no Oxygen, thus there is a response for CH4, but not for CO2 and probably not for N20. Electron ionization should ionize N2O, thus an appropriate GC/MS method and a standard mixture of the different gases that you are investigating should provide a GC/MS based quantitative normalization method. N2O and CO2 have same integral mass (44) as previously said by JF Gal, and also very close monoisotopic masses (44.001 Da vs 43.989 Da), thus they must be fully resolved by GC, as no deconvolution of partially or unresolved peaks can be performed by standard GC/MS instruments.

Patrick is right for the molecular peak. If the two gases are not completely resolved by GC, may be a specific fragment ion (m/z = 30 ?) may be monitored instead of m/z 44. May be MS/MS also?.