Principles and Practice of Chromatography - The Basic Column Chromatograph > The Detector Output > Page 60

That is,

where (A) is a constant,

(Cm) 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 which is particularly crucial 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.