Gas Chromatography - GC Detectors > The Flame Ionization Detector > Page 37
A large number of GC detectors have been developed and made commercially available. In general, GC detectors are 4 to 5 orders of magnitude more sensitive than LC detectors and, thus, are ideal for trace analysis and environmental monitoring. The detectors with the highest sensitivity tend to be specific and sense specific types of sample (e.g., halogenated substances by the electron capture detector). Conversely, those detectors with a catholic response, although highly sensitive compared to LC detectors (e.g. the flame ionization detector) are significantly less sensitive than the specific detectors. The detectors with a catholic response are the most popular and the majority of GC separations are monitored by the flame ionization detector (FID). The most commonly used specific detectors are the nitrogen phosphorus detector (NPD) and the electron capture detector (ECD) The katharometer detector, although having relatively poor sensitivity is widely used in gas analysis.
The Flame Ionization Detector
The FID, invented by Harley and Pretorious (7), and separately by McWilliams and Dewer (8), evolved from the Heat of Combustion Detector developed by Scott (9). The FID detector employs hydrogen as the combustion gas which is mixed with the column eluent (helium, nitrogen or other appropriate gas) and burnt at a small jet situated inside a cylindrical electrode. A potential of a few hundred volts is applied between the jet and the electrode and when a carbon containing solute is burnt in the jet, the electron/ion pairs that are formed are collected at the jet and cylindrical electrode. The current is amplified and fed to a recorder or to the A/D converter of a computer data acquisition system. A diagram of the basic FID is shown in figure 22. During the process of oxidation, oxidized or partially oxidized fragments of the solute are formed in the flame which are thought to generate electrons by thermionic emission. The background current (ions and electrons from the hydrogen flame alone) is very small (1-2 x 10-12 amperes) and consequently, the noise level is also commensurably small (about 10-14 amperes).