Gas Chromatography Detectors - The Form of Detector Response > Page 3

It will be seen that in specifying the above properties of the detector it is important to employ the correct units which will depend on the mechanism of detection. For example, the katharometer detector responds to the concentration of solute in the gas flowing through it so its sensitivity (minimum detectable concentration) would be defined in terms of g/ml. The FID, on the other hand, responds to the mass of solute flowing though it per unit time and thus, for this 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