Chromatographic Data

A successful quantitative analysis requires the solutes to be adequately resolved (i.e. peaks must be separated sufficiently to allow peak areas or heights to be accurately measured). This is necessary whether the measurements are to be made manually or calculated by a computer. Despite chromatographic optimization, some components may still be only poorly resolved and, thus, special techniques must be used.

Resolution

Resolution is defined as the distance between the peaks in units of the average standard deviation of the two peaks. In practice, the standard deviation (s) of two peaks, eluted close together, are almost identical. However, the degree of separation that constitutes adequate resolution must be defined. In figure 26, five pairs of peaks are shown, separated by 2s, 3s, 4s, 5s and 6s, the area of the smaller peak being half that of the larger peak. A separation of 6s would be ideal for accurate quantitative results. However a resolution (6s) would require such high efficiencies and entail such long analysis times as to make many analyses impractical.

Figure 26. Peak Pairs Showing Different Degrees of Resolution

However, accurate quantitative results can be obtained with a separation of 4s and sometimes less. In the following discussion a resolution of 4s will be assumed. It is now possible to calculate the number of theoretical plates necessary to separate a specific pair of solutes.

The Efficiency Required to Achieve a Specific Resolution

Consider the two peaks shown in figure 27. The various parameters of the chromatogram are labeled according to the expressions derived from the Plate Theory. (Introduction to Analytical Gas Chromatography [19])

The difference between the peaks is given by (DV) where,