The flame ionisation detector is the almost universal detector for the gas chromatograph, often used in the laboratory.
Its sensitivity makes it suitable for the low ppb to ppm range. Needing a hydrogen flame and pure air as a source of oxygen, the running cost of this detector is higher than for a PID. Hydrogen is shut off when the flame does not burn, the flame temperature is monitored for this. Nitrogen or helium can be used as a carrier gas.
The combination of a PID and an FID gives the opportunity to monitor complex mixtures with satisfaction.
Principle of flame ionisation : in a flame of oxygen and a burning compound ions are formed. When other molecules than hydrogen pass through a hydrogen flame, these will also burn and increase the amount of ions formed compared to pure hydrogen.
The ions and electrons generated by the ionisation are attracted to the positive and negative side of an electrical field in the detector cell. The electrical current in the detector cell changes, and this change is measured.