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. 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

.   The Flame Ionization detector Without doubt, the Flame Ionization Detector (FID) is the most useful GC detector available and by far that most commonly used in GC analyses. The FID has a very wide dynamic range, a high sensitivity and (with the exception of a few low molecular weight compounds) will detect all substances that contain carbon. The first FID was described about the same time by Harley and Pretorious (12), and McWilliams and Dewer (13). The FID is an extension of the flame thermocouple detector and is physically very similar, the fundamentally important difference being that the ions produced in the flame are measured as opposed to the heat generated. Hydrogen is mixed with the column eluent and burned at a small jet. Surrounding the flame is a cylindrical electrode and a relatively high voltage is applied between the jet and the electrode to collect the ions that are formed in the flame. The resulting current is amplified by a high impedance amplifier and

for use with capillary columns. The nitrogen phosphorous detector (NPD) is also appropriate for capillary columns (the function of which is very similar to that of the FID) for the same reason. The micro-argon detector employs a scavenger flow which also, in effect, reduces the sensor volume, so this detector can also be used with capillary columns. The FID and the NPD detector will be described here. For a more detailed treatment of GC detectors see book 4 of this series.   The Flame Ionization Detector   The FID, invented by Harley and Pretorious (6), and separately by McWilliams and Dewer (7), evolved from the Heat of Combustion Detector developed by Scott (8). The FID operates in the following manner. Hydrogen is mixed with the column exit gas (which may be helium, hydrogen or any other appropriate gas) and then burnt at a small jet which is situated inside a cylindrical electrode system.     Figure 13. The Flame Ionization Detector

capacity and thermal conductivity from those of the carrier gas. This detector was used extensively in the early days of GC for the analysis of hydrocarbon gases. There was much discourse and dissent with regards to the exact mechanism of detection involved in the katharometer and even today it is considered to respond to a number of different physical properties of the eluent gas with no one property playing a major role.   The Flame Thermocouple Detector The "flame thermocouple detector" was the next detector to be reported which was developed by Scott [8] and was, in fact, the forerunner to the flame ionization detector FID. A diagram of the flame thermocouple is shown in figure 8.   Figure 8� The Flame Thermocouple Detector

1 x 10-9 to 5 x 10-9 amp, the noise level is about 1.2 x 10-13 amp and the ionization efficiency is about� 0.07%. It is claimed to be about 10 times more sensitive than the flame ionization detector and to have a linear dynamic range of 105. An example of the use of a pulsed helium discharge detector for monitoring the separation of some aromatics on a capillary column is shown in figure 37. The pulsed helium discharge detector appears to be an attractive alternative to the flame ionization detector and would eliminate the need for three different gas supplies. It does, however, require equipment to provide specially purified helium, which diminishes the advantage of using a single gas.   The Electron Capture Detector Lovelock's work on ionization detectors culminated in the invention of the electron capture detector (25).� However, the electron capture detector operates on an entirely different principle from that of the argon detector. A low energy b-ray source is used

90zirconium. The ionization currents formed (collected by appropriate electrode potentials) are arranged to oppose one another and consequently any variation in pressure or temperature of the two cells will be balanced out. The differential signal resulting from the presence of a solute in the column eluent sensor cell is amplified and recorded. The sensitivity of the detector was similar to that of the katharometer i.e. about 1 x 10-6 g/ml. Unfortunately, the practical lifetime of this detector was also relatively short as it was eclipsed by both the FID and the argon ionization family of detectors subsequently introduced by Lovelock. The Emissivity Detector Another interesting early detector was the emissivity detector developed by Grant. an interesting and innovative extension of the flame thermocouple detector.