, 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

) 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

from those of the carrier gas. This detector was used extensively in the early days of GC for the analysis of hydrocarbon gases. There was much discourse and dissent with regards to the exact mechanism of detection involved in the katharometer and even today it is considered to respond to a number of different physical properties of the eluent gas with no one property playing a major role.   The Flame Thermocouple Detector The "flame thermocouple detector" was the next detector to be reported which was developed by Scott [8] and was, in fact, the forerunner to the flame ionization detector FID. A diagram of the flame thermocouple is shown in figure 8.   Figure 8  The Flame Thermocouple Detector

of stainless steel or titanium (reputed to provide greater stability for 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

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

where the signal crosses from positive through zero to negative. The Dynamic Range of the Detector A detector has two response ranges, the dynamic range and the linear dynamic range and the two range are not synonymous. The dynamic range of a detector is that concentration range over which a concentration dependent output is produced. The minimum of the range will be the concentration at which the output is equivalent to twice the noise level and the maximum that concentration where the detector no longer responds to a concentration increase. The dynamic range is usually given as a concentration ratio and is thus, dimensionless. Detector Linearity The linear dynamic range of a detector is that concentration range over which the detector output is linearly related to solute concentration.         Thus,                           y= Ac           (c) is the concentration of solute in the mobile phase passing    and (A) is a constant