The role of modifiers in high selectivity chiral separations on Pirkle-type columns

Atypically high selectivity (alpha > 2) chiral separations have been observed on Pirkle type supports. These separations were examined in terms of their response to various modifiers, thermodynamic behavior and LSER modeling. Atypical thermodynamic and LSER results were observed along with large changes in selectivity between modifiers. The use of bulky modifiers gave increased selectivity resulting from dramatic increases in the retention of the second eluting enantiomer. These findings lead to the conclusion that increased retention is due to an inability of bulky modifers to penetrate a tight complex between selector and this enantiomer.