Factors Controlling Selectivity

The chemical structure and the stereoisomerism of a given stationary phase will not be the only factors that effect chiral selectivity. Selectivity is controlled by two other major factors and they are the stationary phase loading on the column (as already discussed) and the operating temperature. The effect of these two variables on the retention ratio of a pair of enantiomers has been reported by Supelco and their data is shown in figure 47.

Figure 47. Curves Showing the Relationship between Retention Ratio and Stationary Phase Loading and Temperature

The retention ratio of a given pair of enantiomers increases regularly (but not linearly) with the amount of cyclodextrin in the stationary phase mixture. This increase appears to flatten above 15% and the retention ratio then increases only slightly with increase in cyclodextrin content. Temperature has a major effect on selectivity. For example, by reducing the operating temperature from 100 C to 70 C increases the separation ratio of the isomers of a-Pinene from 1.015 to 1.035. However, reducing the temperature will also cause an increase in retention volume and, thus, a longer analysis time. The net result on column optimization becomes even more confused when it is realized that as the retention ratio has been increased, an excess of theoretical plates is now available and the column can be shortened. Reducing the column length, in turn, will reduce the retention volume and analysis times. The net advantage or disadvantage of raising or lowering the column temperature on analysis time must be assessed quantitatively and the optimization procedure will be considered when open tubular columns are discussed.