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, 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

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

of the refractive index of Tiselius and Claesson. Although a significant number of GC detectors were developed over two or three years, the development of LC detectors was much slower, largely due to the fact that low concentrations of solute in a liquid do not change the properties of a liquid nearly as much as they do a gas. In fact, the development of LC detectors was gradual and arduous. In a similar way to the development of GC there has been a continuous interaction between improved detector performance and improved column performance. Initially, separations monitored by detectors with improved sensitivity permitted a precise column theory to be developed and experimentally substantiated. This allowed new columns to be designed with reduced dispersion and higher efficiencies. The improved efficiencies, however, produced small volume peaks, small, that is, compared with the volume of the detector sensor and the dispersion that took place in the conduits of the detector

more sensitive detectors has encouraged the improvement of column performance. In fact, the rapid development of GC in the 1950s was possible because or the swift introduction of high sensitivity linear detectors. Classification of Detectors Detectors can be classified into two types, bulk property detectors and solute property detectors. The bulk property detector measures some bulk physical property of the eluent (such as dielectric constant or refractive index) and the solvent property detector, measures some physical or chemical property that is unique to the solute (such as heat of combustion or fluorescence). Detectors can also be classified as concentration sensitive devices such as the katharometer or mass sensitive devices such as the flame ionization detector (FID). Another method of classification is to define detectors as specific or non-specific. An example of a specific detector would be the nitrogen phosphorous detector (NPD), which as its name implies detects only

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

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