where WB is the baseline peak width and Wh/2 is the peak width measured at half of the peak height.
Valid comparisons of efficiencies can be made only if the same formula is used throughout, as the
computed values of N using each of the above formulae may differ considerably.
The parameter N is universally referred to as the plate number, but an alternative means of quoting
efficiency is in terms of a plate height, H or HETP*. Plate number and plate height are inversely related
by the equation
where L is the length of the column.
Values of N may be many thousands for columns having high efficiencies, the corresponding values of
H being less than 1 mm.
Resolution, Rs, is measured from a chromatogram by relating the peak-to-peak separation to the average
peak width. This is expressed by the equation
where ∆tR is the separation of the peak maxima and W 1 and W 2 are the respective peak widths (Figure
4.12). Because of the Gaussian profile of the peaks, a 100% separation is never attainable (see counter-
current distribution, section 4.1) but an Rs value of 1.5 or more indicates cross-contamination of 0.1% or
less and is known as baseline resolution.
Figure 4.12
Resolution of adjacent peaks* HETP = height equivalent to a theoretical plate. It is derived from the plate theory of distillation which is a
confusing concept having no basis in fact in the context of modern chromatographic separations. Nevertheless the
terms plate number and plate height are still very widely used.