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", although a more accurate name would be "total volatile hydrocarbon content", as hydrocarbons which have condensed out are not detected, even though they are important for e.g. safety when handling compressed oxygen.
Advantages and disadvantages
Advantages
Flame ionization detectors are used very widely in gas chromatography because of a number of advantages.
Cost: Flame ionization detectors are relatively inexpensive to acquire and operate.
Low maintenance requirements: Apart from cleaning or replacing the FID jet, these detectors require little maintenance.
Rugged construction: FIDs are relatively resistant to misuse.
Linearity and detection ranges: FIDs can measure organic substance concentration at very low and very high levels, having a linear response range of 107 g/s.
Disadvantages
Flame ionization detectors cannot detect inorganic substances and some highly oxygenated or functionalized species like infrared and laser technology can. In some systems, CO and CO2 can be detected in the FID using a methanizer, which is a bed of Ni catalyst that reduces CO and CO2 to methane, which can be in turn detected by the FID. The methanizer is limited by its inability to reduce compounds other than CO and CO2 and its tendency to be poisoned by a number of chemicals commonly found in gas chromatography effluents. Another important disadvantage is that the FID flame oxidizes all oxidizable compounds that pass through it; all hydrocarbons and oxygenates are oxidized to carbon dioxide and water and other heteroatoms are oxidized according to thermodynamics. For this reason, FIDs tend to be the last in a detector train and also cannot be used for preparatory work.
Alternative solution
An improvement to the methanizer is the Polyarc reactor, which is a sequential reactor that oxidizes compounds before reducing them to methane. This method can be used to improve the response of the FID and allow for the detection of many more carbon-containing compounds. The complete conversion of compounds to methane and the now equivalent response in the detector also eliminates the need for calibrations and standards because response factors are all equivalent to those of methane. This allows for the rapid analysis of complex mixtures that contain molecules where standards are not available.
Operation
To detect these ions, two electrodes are used to provide a potential difference. The positive electrode doubles as the nozzle head where the flame is produced. The other, negative electrode is positioned above the flame. When first designed, the negative electrode was either tear-drop shaped or angular piece of platinum. Today, the design has been modified into a tubular electrode, commonly referred to as a collector plate. The ions thus are attracted to the collector plate and upon hitting the plate, induce a current. This current is measured with a high-impedance picoammeter and fed into an integrator. The manner in which the final data is displayed is based on the computer and software. In general, a graph is displayed that has time on the x-axis and total ion on the y-axis. The current measured corresponds roughly to the proportion of reduced carbon atoms in the flame. Specifically how the ions are produced is not necessarily understood, but the response of the detector is determined by the number of carbon atoms hitting the detector per unit time. This makes the detector sensitive to the mass rather than the concentration, which is useful because the response of the detector is not greatly affected by changes in the carrier gas flow rate.
Description of a generic detector
The design of the flame ionization detector varies from manufacturer to manufacturer, but the principles are the same. Most commonly, the FID is attached to a gas chromatography system. The eluent exits the gas chromatography column and enters the FID detector’s oven. The oven is needed to make sure that as soon as the eluent exits the column, it does not come out of the gaseous phase and deposit on the interface between the column and FID. This deposition would result in loss of eluent and errors in detection. As the eluent travels up the FID, it is first mixed with the hydrogen fuel and then with the oxidant. The eluent/fuel/oxidant mixture continues to travel up to the nozzle head where a positive bias voltage exists. This positive bias helps to repel the reduced carbon ions created by the flame pyrolyzing the eluent. The ions are repelled up toward the collector plates which are connected to a very sensitive ammeter, which detects the ions hitting the plates, then feeds that signal to an amplifier, integrator, and display system. The products of the flame are finally vented out of the detector through the exhaust port.
