Well, my project didn't get into first fifty -
Seems like I didn't emphasise words Arduino and RaspberryPI enough nor list all open-source components of the project. Nevermind, the show must go on.
I asked my friend in local university laboratory to measure the spectral image of my assumed LPG. It doesn't look very much convincing, but the notch around 1555nm looks like I'm on the right track.