Effect of acetonitrile/water mobile-phase composition on adsorption characteristics of reversed-phase liquid chromatography.

The influence of volumetric composition of acetonitrile in a mobile phase on adsorption characteristics of reversed-phase liquid chromatography using octadecylsilyl (ODS)-modified silica gel was studied by the pulse response method and the moment analysis. The results were compared with those obtained for the reversed-phase system consisting of ODS-silica gel and methanol/water mixtures. In both systems, surface diffusion was dominant for intraparticle diffusion in ODS-silica gel particles. The contributions of three mass transfer steps in a column to peak broadening were of about the same order of magnitude. The activation energy of surface diffusion, E(s), was found to be larger than the isosteric heat of adsorption, Q(st). Because similar tendencies were observed for these adsorption characteristics, adsorption mechanisms may be analogous in both chromatographic systems. However, absolute values of the adsorption equilibrium constant, K, the decreasing ratio of hydrophobic surface area, ΔA/A, Q(st), and E(s) for acetonitrile/water systems were smaller than the corresponding values for methanol/water systems. Oppositely, greater values of D(s) were obtained for acetonitrile/water systems. It was concluded that the interaction between ODS ligands and adsorbate molecules was weaker compared with that in methanol/water mobile-phase systems when acetonitrile was used as an organic modifier in a mobile phase of reversed-phase liquid chromatography.