E.S.I. uses analyzers that operate on various principles.
Non-Dispersive infrared for Co & co2
paramagnetic for o2
chemiluminescence for nox
heated flame ionization detector for voc's
f.i.d. with a gas chromatography column for the seperation of methane & ethane
fouier transform infrared for nearly everything!
electro-chemical for periodic nox, co and zzzz testing.