The only difference between GC/MS and GC/X is the detector employed; i.e. the separation is the same on the front end.  I'm not sure if that is what you mean, but if you inject it on the GC you'll be able to get a response by GC/MS as well as spectra (if you acquire in scan mode).  Let me know if you have any more questions or if I misunderstood.

You will have problems doing trace analysis for methane with GC-MS because the ubiquitous oxygen has a m/z 16 mass similar to the methane. If you are interested in higher levels then this should not a problem, but to my opinion GC-FID with a polar column is the method of choice.

Actually i have facility of both GC-MS and GC but GC is not in working condition so that why i thought if i characterize biogas sample in GC-MS to check methane composition.

Saadia,

for "general composition" you should use GC-TCD even with a packed column or PLOT carbon molecular sieve of porapak Q will be the best stationary phase. For hydrocarbons you can try to use GC-MS with a PDMS column, but if your purpose is C1-C6 range (the number of isomers is not that big), FID detector is enought (but you need to have the standards for identyfication). For sulfur compounds on ppm level you must have a selectiv detector like FPD.

As above, a PLOT column is normal for this analysis, but I have done this with a split injection on a normal DB5 column with an MS. It is not pretty but depending on what else is in your biogas it should be OK - As Ludwig noted methane has the same parent 16 peak as monatomic oxygen, so you looking at masses of 15 and 13 (about 80% and 8% of parent). It is better with a thick film column, say 0.25mm ID with a film of 0.5µ-5µ, and run the oven as near room temperature as practical. Unfortunately ethane probably won't separate well from methane and will also give peaks at 15 and 13 (roughly 5% and 1% of parent) so look for  26, 27 and 30 (all 25-30% of parent ethane).

Yes of course, GC or GCMS is the same method, the difference is the detector