Hi Marco, You can check ...

Hi Marco,

You can check this paper out:

http://www.sciencedirect.com/science/article/pii/S0165993613002574

The detectors I have used are pretty expensive though but you maybe able to borrow one from a university that researches atmospheric gas fluxes. There also might be some cheaper alternatives. The chamber method may be good for you ..I have used this for CO2 measurements.

Because these machines will be automated you can download the data with a time stamp and simply plot the   methane ​and nitrous ​oxide through regression analysis in Excel.

Its not my area of expertise but I do deal with atmospheric gases. I f I knew what you need this for or if you are doing constant measurements, the scale of the project or just a one off I could give you more advice.

Hope this helps a bit

Paul