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 specifications are summarized in table 2.

The Detector Output

Most practical detectors must have a linear output, e.g.,

where (y)	is the output of the detector in appropriate units,
(c)	is the concentration of solute in the mobile phase,
and (A)	is a constant.

All are designed to provide a response that is as close as possible to linear for accurate quantitative analysis. However, the output from some detector sensors may not be linearly related to the solute concentration and appropriate signal modifying circuits must be introduced into the detector electronics to provide a linear output (e.g., the output from a light adsorption sensor will be exponential and, consequently, it must be used with a logarithmic amplifier to produce an output that is linearly related to solute concentration).

Table 2 Detector Specifications

Specifications	Units
Dynamic Range	(D R ) g/ml (e.g. 3 x 10 -9 to 6 x 10 -5 )
Response Index	(r) dimensionless
Linear Dynamic Range	(D LR ) g/ml (e.g. 1 x 10 -8 to 2 x 10 -5 )
Detector Response	(R C ) Volts/g or (specific units of measurement/g)
Detector Noise Level	(N D ) usually in millivolts but may be in specific units (e.g. Refractive Index Units)
Sensitivity or minimum detectable concentration	(X D ) g/ml (e.g. 3 x 10 -8 ) but may be in specific units (e.g. Absorption Units)
Total Detection System Dispersion	( ) (ml 2 often ml 2 )
Cell Dimensions	(length (l), and radius (r)), (cm)
Cell Volume	(V D ), ml.
Overall Time Constant (sensor and electronics)	(T D ), seconds (sometimes milliseconds)
Pressure Sensitivity	(D P ) usually in the USA p.s.i, in Europe MPa
Flow Rate Sensitivity	(D Q ) usually in ml/min
Temperature Range	o C