A flame ionization detector (FID) is an instrument used for detecting the presence of hydrocarbons, specifically butane, hexane, and other carbon-containing compounds that might be present in the measured sample. The instrument is connected to a gas chromatograph by a tube called a capillary, and features a chamber with a flame. Gases are injected into this chamber from one source, while hydrogen and oxygen are added from another. An electric ignition component is used to ignite the flame inside the flame ionization detector; the subsequent combustion of hydrogen and oxygen creates a charged current between the flame jet, which acts as an electrode, and another electrode in the chamber.

The capillary column inserted into the instrument is connected to the gas chromatograph, a device used to analyze the chemical composition of a sample of gas. This is the instrument with which the flame ionization detector response is measured. The cylindrical electrode that surrounds the flame collects ions formed from the combustion process when a voltage is applied between the two. A current is generated, and then amplified while the output is collected by electronic data collectors. Each gas type has a specific base current and flow rate, and when this is charted, operators can determine the gas present by consulting a guide that shows the flow rates that different gases typically have.

When the fuel is added to a flame ionization detector, it is activated at a predetermined flow rate. After the air is pumped in, the flame is ignited and left for an hour to stabilize and burn continuously, for the most accurate results. Shutting off the flow for the fuel turns off the flame, and then the inflow for other gases is turned off. A portable flame ionization detector works in a similar way and is commonly used to monitor volatile organic compounds (VOCs). With these, flame ionization detector sensitivity can be adversely affected if the temperature changes quickly or there are intense electrical fields present in the environment.

A flame ionization detector is only able to detect compounds that are organic. The instrument is used commonly for pharmaceutical production and analyzing pesticides. It is possible to measure methane or even toxic compounds such as hydrogen cyanide because they contain carbon molecules. Inorganic gases are difficult to detect with an FID. Ammonia, for example, does not have carbon in its molecular structure, so it may go unnoticed by the flame ionization detector.