There are many ways of detecting different gases, IR (infra Red), Catalytic, Pellistor, semi conductor, electro chemical sensors.

IR sensors is very useful where there are chemicals, silicon or other elements that will poison sensor elements.
IR sensors will not be effected like the other sensor elements.

You can use IR sensors in petro chemical and Aerosol factories, where other sensor elements will not work in the long term, so you chemicals.

Alex Clay