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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

range of a detector is that concentration range over which it will give a concentration dependent output. The units are dimensionless.   2. The Response Index – (r) – The response index of detector is a measure of detector linearity and would be unity for a truly linear detector. In practice the value of (r) should lie between 0.98 and 1.02. If (r) is known, quantitative results can be corrected for any non linearity. 3. Linear Dynamic Range – (DL) – The linear dynamic range of a detector is that concentration range over which the detector response is linear within defined response index limits. It is also dimensionless and is important when the components of a mixture cover a wide concentration range. 4. Detector Response – (Rc) – The detector response can be defined as the detector output per unit change in concentration (e.g.  volts/g/ml) or, as the detector output per unit change of physicalpropertybeingmeasured (e.g.fortheFID, volts/gram of carbon/sec). In

  efficiencies has demanded higher detector sensitivities which has provoked the development of more sensitive detectors. In turn, the 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

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 popular and useful LC detector that is available to the analyst at this time. This is particularly true if multi-wavelength technology is included in this class of detectors. Although the UV detector has some definite limitations (particularly

) using the numerical value of the response index. Thus. in effect, the useful linear dynamic range of a detector for quantitative purposes can be significantly extended by employing correction procedures when using the response index. It should be pointed out that the logarithmic dilution method should not be used if the linearity is to be measured by the method recommended by the E19 committee of the ASTM. Detector Response There are two ways of defining detector response, either as detector output (usually in mv) per unit change in solute concentration or as the detector output per unit change in the units of detector measurement (e.g. the sensitivity of a conductivity detector would be defined in terms of detector output per unit change in electrical conductivity). The detector response (RD) is determined by injecting a known mass (m ) onto the column and measuring the peak height (h)  in (mv), then

and, as the detector is required only to monitor the separation, they need not have a linear response. They do need to tolerate high flow rates and thus, must have low flow impedance. Analytical detectors can be used for preparative purposes but a portion is usually split from the column eluent, diluted with more mobile phase and then passed through the detector. In practice this becomes a rather clumsy procedure. The most commonly used detector in preparative GC is the thermal conductivity detector (hot wire detector). Even this detector, however, is often too sensitive and has too high a flow impedance. Under such circumstances, the procedure mentioned above must be employed. The eluent from the preparative column is split and a small portion diverted through the detector (sometimes with further dilution with carrier gas to reduce sensitivity). In LC, the refractive index detector is probably the most useful of the analytical detectors for preparative work, but even at its