A flame ionization detector (FID) is a scientific instrument that measures the concentration of organic species in a gas stream. It is frequently used as a detector in gas chromatography. Standalone FIDs can also be used in applications such as landfill gas monitoring, fugitive emissions monitoring and internal combustion engine emissions measurement in stationary or portable instruments.
The operation of the FID is based on the detection of ions formed during combustion of organic compounds in a hydrogen flame. The generation of these ions is proportional to the concentration of organic species in the sample gas stream.
FID measurements are usually reported as “as methane”, meaning as the quantity of methane which would produce the same response. Hydrocarbons generally have molar response factors that are equal to the number of carbon atoms in their molecule, while oxygenates and other species that contain heteroatoms tend to have a lower response factor. Carbon monoxide and carbon dioxide are not detectable by FID.
FID measurements are often labelled “total hydrocarbons” or “total hydrocarbon content” (THC), although a more accurate name would be “total volatile hydrocarbon content” (TVHC), as hydrocarbons which have condensed out are not detected, even though they are important for e.g. safety when handling compressed oxygen.