The principle of flame ionization detector (FID) is the ionization of molecules of the effluent gas in a hydrogen flame which results in a current increase.
The detector has inlets for hydrogen which is the carrier gas, air or oxygen to burn the hydrogen and the effluent gas from the chromatograph. Hydrogen emerges through a hollow needle and is burnt as it emerges giving a colorless flame. The effluent gas is mixed with hydrogen and the mixture burnt in an excess of air.
The thermal energy of the flame is sufficient to cause ionization of the molecules in the effluent gas. A potential is applied between two electrodes. The increase in current between the two electrodes when ions are formed in the flame is amplified and recorded. The ion current is approximately proportional to the number of carbon atoms which enter the flame.
The FID is relatively simple and highly sensitive but destroys the sample. It is also insensitive to most inorganic compounds, water vapor and air.