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. 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 10. Pressure Sensitivity 11. Flow Sensitivity 12. Operating Temperature Range In general the specifications are the same for both GC and LC detectors with the exception of detector dispersion. Although, detector dispersion has a minimal

peak height (or the peak area) 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

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

detector, the sensitivity would be defined in units of g/sec. The Form of Detector Response There are three different forms of detector response, namely, proportional, differential and integral. A proportional response is one that is directly related to the concentration of solute in the mobile phase passing through it. All detectors with a proportional response are designed to give as near a linear response as possible. In many detectors, the actual sensor does not give a proportional response. Thus suitable electronic circuitry must be employed to modify the signal from the sensor so that the actual detector output is proportional to the solute concentration in the mobile phase passing through it. For example a sensor with a logarithmic response would be modified by an exponential amplifier to give an output linearly related to the solute concentration. The different types of detector response are shown in figure 1. Figure 1. Different Types of Detector Response

has a truly linear response (despite manufacturers claims) but most detectors will have a response approaching that of linear. It is difficult to apply a standard to detector linearity, but the Response Index (1) does help comparisons to be made between one detector with that of another. Providing the response of the detector approaches linearity then its response can be described by the following simple equation,                                        y = Acr     where (r) is the response index and the other symbols have the For a truly linear detector, r=1, and the extent to which (r) deviates from unity would be a measure of its non linearity. Curves relating the detector output to different solute concentrations passing though it for different response values are show in figure 2.   Figure 2 Curves Relating Detector Output to Solute Concentration for Different Response Indices

where (A) is a constant, (Cm) is the concentration of solute, and (V) is the output of the detector. Because of the imperfections in mechanical and electrical devices practical detectors can only approach this ideal response. A measure of linearity that is specified in numerical terms so that comparisons can be made between detectors can be obtained as follows. It is assumed that for a closely linear detector the response could be described by the following power function (1) where (r) is defined as the Response Index It follows that for a truly linear detector, r = 1, and the proximity of (r) to unity will indicate the extent to which the response of the detector deviates from true linearity. In addition if (r) is not unity but is known then appropriate corrections can be made to the