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

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

. 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

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

Dispersion Due to Detector Electronics Response Time The response of the detector sensor and the detector electronics is not usually critical unless high speed separations are required. However, fast chromatographic analyses are often necessary when monitoring reaction kinetics and for repetitive analyses involving clinical or forensic tests. Modern solid state electronics using semi conductor devices can be designed to have an extremely fast response and the speed of the electronic signal processing is not usually the limiting factor in fast analyses. However, the sensor, can have a relatively slow response (depending on the property it is measuring) and can certainly effect the chromatographic resolution obtained with high speed separations. In the early LC UV detectors, the cadmium sulfide photo resistor was frequently employed as the UV sensor. This device was particularly slow and its response was measured by Scott et al. (12).

labile materials of biological origin) and the connection to the sample valve and detector should be as short as possible and have a very small diameter to reduce extra column dispersion. Detector and Detector Electronics There is a wide range of detectors available for both GC and LC each having their own particular areas of application. In general the more catholic the response, the less sensitive the detector and the most sensitive detectors are those that have a specific response. The performance of all detectors should be properly specified so that the user can determine which is most suitable for a specific application. Such specifications are also essential to compare the performance of different detectors supplied by alternative instrument manufactures. Detector specifications should be presented in a standard form and in standard units, so that detectors can be compared that function on widely different principles. The more important detector