A flame ionization detector (FID) is an analytical instrument used to measure the concentration of organic compounds in gases. It operates by burning a sample in a hydrogen flame, which ionizes the molecules present. The resulting ions are collected and measured, producing a current that correlates with the amount of the substance being analyzed. FID is commonly used in gas chromatography to detect hydrocarbons and other volatile organic compounds. It is highly sensitive and can detect concentrations as low as parts per billion. However, it is not effective for detecting inorganic gases or compounds that do not ionize in the flame.