Capillary Column Design and Choice

In an analysis the column is required to separate the solutes of interest and complete the analysis in the minimum time. Stereoisomers are chemically very similar and, thus, have very small retention ratios. As a consequence, very high column efficiencies will be required and the stationary phase must exhibit the maximum selectivity.

Purnell (30), derived an equation that allowed the calculation of the column efficiency (n) required to separate a pair of solutes having a separation ratio (a) where the first of the pair is eluted at a capacity ratio (k').

i.e., ( 22

Using equation (22) curves can be constructed relating efficiency required to achieve a separation to separation ratio for a series of (k') values. The calculated curves are shown in figure 51.

The efficiency required to achieve a separation is seen to increase rapidly as the retention ratio becomes smaller. Equally important, it is also seen that significantly higher efficiencies are necessary if the solutes are eluted at low capacity ratios (k). As a consequence, the optimum operating conditions that are necessary to achieve a particular separation rapidly is quite difficult to identify. To develop the optimization process further it is necessary to refer to the basic capillary column theory as developed by Golay.

Figure 51. Graph of Efficiency Necessary for Resolution against Retention Ratio