, even today, there is no LC detector that has an equivalent performance to the flame ionization detector (FID) used in GC. In general, LC detectors have sensitivities of two to three orders of magnitude less than their GC counterparts and linear dynamic ranges one to two orders of magnitude lower. Only highly specific LC detectors have sensitivities that can approach those of GC detectors. See also the section on detectors in the HPLC supplement. Detector Specifications Detector specifications are like those for GC detectors and are listed as follows, 1. Dynamic Range 2. Response Index or Linearity 3. Linear Dynamic range 4. Detector Response 5. Detector Noise Level 6. Detector Sensitivity or Minimum Detectable Concentration 7. Total System Dispersion 8. Sensor Dimensions 9. Detector Time Constant

GC Detectors 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

of GC Detectors The GC detector is designed to respond to very small quantities of vapor contained in a permanent gas. Because the physical and chemical properties of permanent gases differ widely from those of a vapor, a very wide range of detection methods can be employed including the measurement of standard physical properties such as thermal conductivity and light adsorption to more specific properties such as ionization potentials and heats of combustion. The response of a GC detector can be general or specific but a detector with a catholic response is generally more useful in routine analyses. Aspecificdetector(e.g.,the nitrogen-phosphorus detector (NPD)) can be extremely useful for selectively monitoring compounds such as herbicides and pesticides, when the compounds are not eluted discretely but mixed with a number of other contaminating compounds. GC detectors should be insensitive to changes in flow rate but, unfortunately, few detectors have this

the enthusiastic inventor for some specific application. Very few of these detectors have survived and even fewer are still being manufactured and are commercially available. However, one or two have recently been rediscovered and found suitable for new areas of application. A selected number of this fairly large group of "lost detectors" will be briefly described to illustrate the large variety of sensing techniques that have been applied to GC detection. The Thermionic Ionization Detector Electrons produced by a heated filament can be accelerated by an appropriate potential so that they attain sufficient energy to ionize any gas or vapor molecules in their path. In 1957, the early days of gas chromatography, Ryce and Bryce [29,30] modified a standard vacuum ionization gauge to examine its possibilities as a GC detector. A diagram of the device is shown in figure 47.   Figure 47.  The Ionization Gauge Detector

System Dispersion and Sensor Dispersion One problem common to all detectors is the peak dispersion that takes place in the mobile phase conduits and sensor volumes of the detector. Dispersion of this type is particularly serious in LC where solute diffusivities are 4 to 5 orders of magnitude smaller than those in gasses. In GC however, due to the much higher diffusion rates detector dispersion is minimal and does not significantly effect chromatographic performance. Consequently detector dispersion in GC detectors will not be discussed in this book, but dispersion in LC detectors will be considered in detail in Liquid Chromatography Detectors . Peak Dispersion from the Overall Detector Time Constant Peak dispersion resulting from the time constant of the sensor and its associated electronics can be significant in both GC and LC, particularly when filter circuits are introduced to remove inherent detector noise. The effect of the detector time constant can be

in temperature and pressure. No existing detector fulfills all these specifications but the FID come close to this ideal performance.   Detector Specifications The subject of detector specifications has been touched upon in Principles and Practice of Chromatography and Gas Chromatography but will be treated here in detail. In order to evaluate a detector for use in GC, accurate performance criteria or specifications must be available to assess the pertinence of a particular detector for a given application. Such information is also necessary to permit a rational comparison with other detectors or detectors supplied by competitive manufacturers. The principle characteristics of a GC detector that will satisfy these requirements are as follows.                         1. Dynamic Range                         2. Response Index or Linearity                         3. Linear Dynamic range                         4. Detector Response                         5.