Flame ionization detector

Did You Know?

• The first flame ionization detectors were developed simultaneously and independently in 1957 by McWilliam and Dewar at ICIANZ in Australia and by Harley and Pretorius at the University of Pretoria in South Africa.
• In 1959, Perkin Elmer Corp. included a flame ionization detector in its Vapor Fractometer.

Operating principle

The FID detects ions formed during combustion of organic compounds in a hydrogen flame, with ion generation proportional to the concentration of organic species in the sample gas stream. Two electrodes provide a potential difference: the positive electrode is the nozzle head where the flame is produced, and the negative electrode is positioned above the flame. Originally a tear-drop shaped or angular piece of platinum, the negative electrode is now a tubular collector plate. Ions are attracted to the collector plate, inducing a current measured by a high-impedance picoammeter and fed into an integrator. The current corresponds roughly to the proportion of reduced carbon atoms in the flame, making the detector sensitive to mass rather than concentration, so changes in carrier gas flow rate have little effect.

Response factor

FID results are typically given as the equivalent amount of methane that would generate the same signal. The current produced varies by chemical, based on its elemental makeup. For hydrocarbons, the response factor generally matches the number of carbon atoms (more carbon means a stronger signal), while oxygen-containing compounds and other molecules with heteroatoms usually yield a lower response. Carbon monoxide and carbon dioxide are invisible to an FID. These measurements are frequently called "total hydrocarbons" or "total hydrocarbon content," but a more precise term is "total volatile hydrocarbon content," since condensed hydrocarbons are not detected.

Description

The FID design varies by manufacturer but follows the same principles. The eluent exits the gas chromatography column and enters the detector's oven, which prevents the eluent from depositing on the interface. The eluent is mixed with hydrogen fuel and then oxidant, traveling to the nozzle head where a positive bias voltage exists. This bias repels oxidized carbon ions created by the flame pyrolyzing the eluent. The ions are repelled toward collector plates connected to a sensitive ammeter, which detects the ions and feeds the signal to an amplifier, integrator, and display system. Flame products are vented through the exhaust port.

Advantages and disadvantages

Flame ionization detectors are widely used in gas chromatography due to several advantages: they are relatively inexpensive to acquire and operate, require little maintenance beyond cleaning or replacing the FID jet, have rugged construction, and offer a linear response range of 10^7 g/s, measuring organic substance concentrations from very low to very high levels. However, they cannot detect inorganic substances and some highly oxygenated or functionalized species. CO and CO2 can be detected using a methanizer, a bed of Ni catalyst that reduces them to methane, but the methanizer is limited by its inability to reduce other compounds and its tendency to be poisoned by common gas chromatography effluents. The FID flame oxidizes all oxidizable compounds, so FIDs are typically last in a detector train and cannot be used for preparatory work. The Polyarc reactor, a sequential reactor that oxidizes compounds before reducing them to methane, improves response and allows detection of many more carbon-containing compounds, eliminating the need for calibrations and standards.