This is a non-selective technique for measuring total hydrocarbons.  It can be used for methane where no other hydrocarbon species are present or in conjunction with a specially designed catalyst used to oxidise non-methane species contained in the sample.

The flame ionisation detector is a highly sensitive but non-selective sensor for measuring hydrocarbons.  It uses a simple principle where a flame strips hydrogen atoms from the hydrocarbon molecule, forming carbon radicals and electrons.  This is usually a short-lived condition with the charged components quickly recombining, however in an electrostatic field oppositely charged components can be driven towards electrodes.  This creates an effective current within the flame, which is proportional to the number of molecules of hydrocarbon present. The molecular arrangement has an effect on the ionisation mechanism within the flame and therefore the response of the analyser.

A response factor is used to correct the measurement response, or the variation can also be accounted for by calibration using the target gas.