Effect of stationary phase solvation on shape selectivity in reversed-phase high-performance liquid chromatography

The effect of stationary phase solvation on reversed-phase chromatographic shape selectivity has been investigated using n-hexanol as an additive to methanol-water mobile phases. A wide range of mobile phase compositions was evaluated to normalize for solvent strength selectivity differences. Monomeric C18 stationary phases of both high and low bonding density were synthesized and used to correlate selectivity changes caused by stationary phase ordering with those seen by the addition of n-hexanol. The temperature dependence of retention and selectivity was also investigated using Van 't Hoff plots, which provided insight into the nature of selectivity behavior for estrogens and polyaromatic hydrocarbons. The results showed that using n-hexanol as a mobile phase additive did not provide higher shape selectivity, suggesting that changes in the solvation of the stationary phase did not impart a significant change in the level of surface ordering or morphology. However, n-hexanol did impart solvent selectivity changes in the separation of estrogen diastereomers that could prove useful in future methods development schemes.