In a flame ionization detector hydrogen gas is added to the gas coming off the column. This mixture is then passed through a mixed with air and burned.
The electrodes, through which the current is measured, are formed by the grounded jet and the collector. A voltage difference of about 400 V is applied across these electrodes, and the ions formed in the flame provide a means through which a current flows between the two electrodes. This current, i, flows through an external resistor, R, over which a voltage drop (i x R) results that is measured and recorded.
When other materials are present in the effluent gas, the number of ions formed in the oxidation of these materials changes, and as a result of that the current between the collector and jet changed.
An FID responds proportional to the number of CH2 groups introduced to the flame. There is no response for fully oxidized carbons, e.g., carbonyl, carboxyl, or ether groups. The response from carbons attached to hydroxyl and amine groups is lower.
The insensitivity of the FID to moisture (H2O) and permanent gases (CO, CO2, CS2, SO2, NH3, N2O, NO, NO2, SiF4, and SiCl4) can be a big advantage when small traces of organic material present in a background of these compounds need to be measured.