The flame ionization detector (FID) is an almost universal, highly sensitive detector for all organic compounds with a C-H bond.

The FID measures the conductivity of a flame. With only the usual carrier gases and the detector gases hydrogen (as fuel) and air (as oxygen supplier) present, the conductivity is almost zero. When an organic component enters the flame plasma, the flame breaks these compounds into ions and electrons and the conductivity of the flame increases.

This change in conductivity is measured using two electrodes: the flame tip is the cathode and a cylindrical collector surrounding the flame is used as the anode. After amplification, the resulting current is fed to a recorder or a data system.

The flame tip is positioned in the detector block. This block should be heated to avoid any condensation of sample components. In practice it is advised to use a temperature some 20^oC above the maximum temperature of the temperature program. Temperatures below 150^oC should not be used to avoid condensation of the water vapour formed in the flame.

The response of the FID depends on the number of ionizable carbon atoms in the sample components: carbon atoms bonded to hydrogen.

The FID responds to almost all  organic compounds with a C-H bond, and is therefore called a universal detector.

The FID is not sensitive towards compounds that do not contain C-H bonds. The response is more or less directly related to the number of carbon atoms. The presence of the so called hetero atoms (O, S, N, P) in an organic compound reduces the sensitivity of the FID considerably. On the other hand, if a compound contains a large number of C-atoms, the influence of a few hetero-atoms will only have a marginal effect.