Benzene in ambient air can be measured by adsorption onto a polymeric sorbents then GC-MS by using thermal desorption. Several other methods can also be used.Take a look at chapter 3 of the link below.

https://www.google.es/url?sa=t&source=web&rct=j&url=http://ec.europa.eu/environment/archives/air/pdf/ppbenzene.pdf&ved=0ahUKEwj0q6KJ-tHKAhWDtBQKHZYSDtoQFggaMAA&usg=AFQjCNEnC3vXNAw91I6YTei-hFW8caBLzQ

For methane you can look at the attached documents

http://opticalengineering.spiedigitallibrary.org/article.aspx?articleid=1100738

http://www3.epa.gov/gasstar/documents/workshops/2010-annual-conf/03neretin.pdf

http://www.readcube.com/articles/10.3390/ani2020160

you could use a flame-ionisation detector (FID) or a photo-ionisation detector (PID) for most hydrocarbons. it is important if you have to distinguish benzene and methane, i.e. if you need to measure both at the same time. in this case, with FID and PID you won't be able to separate the contribution of either of them. a separation method is needed, for example using a GC. 

as said earlier, adsorbent tubes are useful for benzene and other longer hydrocarbons. for methane, that won't work so well. a great advantage is that the sampling volume cane varied, so that also small mixing ratios can be detected. 

You can do benzene and CH4 and many other species by GC-FID if you use whole air sampling in canisters along with pre-concentration as described here:

Colman, J. J., A. L. Swanson, S. Meinardi, B. C. Sive, D. R. Blake, and F. S. Rowland, Description of the analysis of a wide range of volatile organic compounds in whole air samples collected during PEM-Tropics A and B, Anal. Chem., 73, 3723– 3731, 2001.

We manufacture continuous GCs to monitor both in ambient air without pre-concentration. 920-225 for CH4 and 910-131 for benzene.

www.peaklaboratories.com

Thanks all of you for your valuable suggestions. 

Regards

Anand Dev Gupta