Comparison of nonaqueous hydrophilic interaction chromatography with aqueous normal-phase chromatography on hydrosilated silica-based stationary phases.

We investigated the retention behavior of phenolic acids in nonaqueous normal-phase (NP) LC with buffered methanol/acetonitrile mobile phases on hydrosilated silica-based stationary phases. The silica hydride, Diamond hydride, Bidentate C18, and Cholesterol columns showed a higher retention of phenolic acids in the nonaqueous mobile phases than in aqueous NP mobile phases. There are some selectivity differences between the aqueous and nonaqueous mobile phases, but generally the resolution and selectivity are better in the aqueous systems. The retention of the phenolic acids tested decreased with increasing concentration of methanol in the mobile phase, up to 20% v/v methanol. At increased temperatures, the retention factors and peak widths decrease in both NP modes, showing linear ln k versus 1/T plots, due to a single retention mechanism over the temperature range from 25°C up to the column stability limit, however, the best separations are achieved at low temperatures. The enthalpic and entropic contributions to the retention were determined, and the differences between the aqueous and nonaqueous modes are possibly due to the adsorbed water layer.