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Gas Chromatography Detectors - The Response Mechanism of the FID > Page 40
The Response Mechanism of the FID
The ionizing mechanism of the FID was carefully examined by Ongkiehong (15) and Desty (16) in 1960 and it would appear no such detailed evaluation of the detector has been carried out since. The ionization mechanism in the FID flame was originally thought to be similar to the ionization process in a hydrocarbon flame which was studied intensively by Calcote and King  and Schuler and Weber  in the midľ1950s. The present generally accepted explanation is that ions are not formed by thermal ionization but by thermal emission from microscopic carbon particles that are formed during the combustion process. Consequently the dominating factor in the ionization of organic material is not their ionization potential but the work function of the carbon that is transiently formed during their combustion.
The flame plasma contains both positive ions and electrons which are collected on either the jet or the plate depending on the polarity of the applied voltage. Initially, the current increases with applied voltage, the magnitude of which depend on the electrode spacing. The current continues to increase with the applied voltage and eventually reaches a plateau at which the current remains sensibly constant. The voltage at which this plateau is reached also depends on the electrode distances. Curves relating ionization current to applied voltage obtained by Ongkiehong are shown in figure 18.
Distance between the electrodes I, 10 mm and II, 20 mm.
Figure 18. Current Voltage Curves for an FID with Different Electrode Geometry's and Operating with 5 ppm v/v of Butane in Pure Hydrogen