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Unfortunately, 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.

Detectors A large number of LC detectors have been developed over the past thirty years based on a variety of different sensing principles. However, only about twelve of them can be used effectively for LC analyses and, of those twelve, only four are in common use. The four dominant detectors used in LC analysis are the UV detector (fixed and variable wavelength) the electrical conductivity detector, the fluorescence detector and the refractive index detector. These detectors are employed in over 95% of all LC analytical applications. These four detectors will be described and for those readers requiring more information on detectors are referred to Liquid Chromatography Detectors. The subject of detector specifications will not be discussed here but will also be dealt with in detail there. Detector sensitivities and detector linearity will, however, be given for each of the four detectors. The UV Detector The UV detector is by far the most

the lack of an inline sensitive detector. The first, effective inline liquid chromatography (LC) detectors were the refractive index detector reported by Tiselius and Claesson (1) in 1942 and the conductivity detector described by Martin and Randall (2) in 1951. These two devices should have evoked a growth in LC development, but, in the early fifties, gas chromatography (GC) was invented which completely eclipsed the development of LC. It was not until the early 1960s that the renaissance of LC took place, initially based on the use 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

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 katharometer detector,

LC Detectors Based on Refractive Index Measurement   LC detectors range from those that are exclusively non specific (i.e., bulk property detectors, e.g., the refractive index detector) through those that are partially specific (i.e. partial solute property detectors, e.g., the UV detectors) to the totally specific detectors (i.e., solute property detectors, e.g., the fluorescence detector). In general, the sensitivity increases progressively as the detector becomes more specific, the highest sensitivities being obtained from the specific detectors.    Refractive index is a bulk property of the column eluent and so detection depends on the solute modifying the overall refractive index of the mobile phase sufficiently to provide a signal twice that of the noise. Bulk property detectors have an

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 theoretically examined (see Extra Column Dispersion ) and calculated and the results from such calculations are shown in figure 6. The undistorted peak, that