Chrial Gas Chromatography.
by
RPW Scott
part of the
Chrom-Ed Series
Detector Response
Detector response can be defined as the voltage output for unit change in solute concentration. The detector response (Rc) can be determined by injecting a known mass of the chosen solute (m) onto the column and measuring the response from the dimensions of the peak. Assuming the concentration of the solute at the peak maximum is twice the average peak concentration, then the detector response is given by ,
The response of a detector will almost certainly differ between different solutes and between different detectors. Consequently, the response of two detectors of the same type and geometry can only be compared under similar operating conditions.
Detector Sensitivity or the Minimum Detectable Concentration
Detector sensitivity or the minimum detectable concentration (the two methods used for defining the detector sensitivity) is the minimum concentration of an eluted solute that can be discerned unambiguously from the noise. If a peak is considered decisively identifiable then the ratio of the signal to the noise has been customarily taken as two. Although, this ratio originated from electronic theory, it is realistic, and any peak having a signal-to-noise ratio of less than two is seriously obscured by the noise and almost impossible to be sure of its existence let alone its magnitude.
Thus, the minimum detectable concentration is that concentration of solute in the mobile phase that provides a signal equivalent to twice the noise level.
The concentration that will provide a signal equivalent to twice the noise level will also depend on the physical properties of the solute as well as those of the detector used for measurement. Consequently, the detector sensitivity, or minimum detectable concentration must be reported in conjunction with the solute that is used for measurement. Thus for a concentration sensitive detector, the detector sensitivity (XD) is given by
Where (ND) is the noise level of the detector
GC detectors exhibit a wide range of sensitivities. At one extreme the katharometer has a sensitivity of about 1 x 10-6 g/ml and at the other, the electron capture detector can detect certain solutes at concentrations of 2 x 10-13 g/ml.
