| A flame ionization detector (FID) works by using a hydrogen/air flame to ionize any carbon present in the mobile phase (made up of a carrier gas and the analyte). This carbon, now in its ionic form, will carry a charge. The carrier gas sweeps these gaseous ions across the space in between two metal plates. One of these plates has a voltage applied to it, and the other plate is connected to a voltmeter. When ionized carbon passes through the inter-plate space, the circuit is complete and a voltage is registered, thus detecting the presence of an analyte.
FIDs are especially useful due to their rugged nature, their high sensitivity (10-13 g/s), their wide dynamic range (107), the fact that they are mass sensitive and not concentration senstive (signal depends on the number of carbon atoms in the analyte). Drawbacks to this type of detector include the fact that it is only weakly sensitive to carbonyl, amine, and alcohol groups. It is also not sensitive to molecules which can not be combusted such as water, carbon dioxide, silicon dioxide, and nitrous oxides. The detector also is destructive to the initial analyte and so a sample can not be reused once run through this detector. | 
