Detector sensitivity or minimum detectable concentration (MDC) is defined as the minimum concentration of solute passing through the detector that can be unambiguously discriminated from noise, conventionally taken when the signal to noise ratio is two and this criteria has been adopted for defining detector sensitivity.

Thus for a concentration sensitive detector, the detector sensitivity ( X D ) is given by

The two important ranges that are specified for a detector are the dynamic range and the linear dynamic range. The dynamic range (D R ) extends from the minimum detectable concentration (i.e. the sensitivity) to that concentration at which the detector no longer responds to any increase. The dynamic range is not usually pertinent to general analytical work but can be important in preparative chromatography. The linear dynamic range or detector linearity is as important as sensitivity for any detector that is to be used for quantitative analysis. It is defined as the concentration range over which the detector response is linearly related to the concentration of solute passing through it.

That is,

where (A) is a constant,

(C m ) is the concentration of solute,

and (V) is the output of the detector.

Because of the imperfections in mechanical and electrical devices practical detectors can only approach this ideal response. A measure of linearity that is specified in numerical terms so that comparisons can be made between detectors can be obtained as follows. It is assumed that for a closely linear detector the response could be described by the following power function

(1)

where (r) is defined as the Response Index

It follows that for a truly linear detector, r = 1, and the proximity of (r) to unity will indicate the extent to which the response of the detector deviates from true linearity. In addition if (r) is not unity but is known then appropriate corrections can be made to the response and improved accuracy can be achieved.

The three remaining important specifications are pressure sensitivity that is particularly critical if multidimensional chromatography is envisaged , flow rate sensitivity which is important if flow programming is contemplated and, of course, the maximum operating temperature to ensure the detector is not damaged by overheating. A GC detector must be situated in a separate oven and maintained at a temperature at least 15°C above that of the column oven to ensure that no solute condenses in the detector and causes detector fowling. In GC any connecting tube between the column and the detector must also be heated to a temperature above that of the oven to prevent condensation.