The real value of measuring and knowing (a), is that it can also be used to correct for any non-linearity that might exist thus, improve the accuracy of the analysis. Equation (1) can be put in the form,

Log(V) = Log(A) + aLog(c) (31)

A numerical value for (a) can be obtained from the slope of a curve relating the log (V) to the log(c) . An example is given in figure 59.

Figure 59. Graph of Log (V) against Log (c)

Appropriate experimental data for the determination of (a) can be obtained by injecting samples of differing concentrations on to the column and measuring the respective areas or height of the peaks obtained. Reasonable linearity can be assumed if the value of (a) lies between 0.97 and 1.03. If the response index is outside this range then the numerical value of (a) can be used to correct for the non-linearity.

Linear Dynamic Range

The linear dynamic range (D_L) of a detector is that range of solute concentration over which the numerical value of the response index falls within defined limits. For example, the linear dynamic range of a detector such as the FID might be specified as

D_L = 1 x 10^-10 to 1 x 10^-5 g/ml (0.98< a < 1.02)

The dynamic range of a detector (D_R), is that range over which the detector continues to respond to changes in solute concentration. Clearly, it is not the same as its linear dynamic range The dynamic range may extend from 1 x 10^-10 to 1 x 10^-3 g/ml. Detector use, outside its linear dynamic range, is normally restricted to preparative chromatography.