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 ph ase on adsorption characteristics of reversed-phase liquid chromatogra phy using octadecylsilyl (ODS)-modified silica gel was studied by the pulse response method and the moment analysis. The results were compar ed with those obtained for the reversed-phase system consisting of ODS -silica gel and methanol/water mixtures, In both systems, surface diff usion was dominant for intraparticle diffusion in ODS-silica gel parti cles, The contributions of three mass transfer steps in a column to pe ak broadening were of about the same order of magnitude. The activatio n energy of surface diffusion, E-s, was found to be larger than the is osteric heat of adsorption, Q(st). Because similar tendencies were obs erved 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 hy drophobic surface area, Delta A/A, Q(st), and E-s for acetonitrile/wat er systems were smaller than the corresponding values for methanol/wat er systems, Oppositely, greater values of D-s were obtained for aceton itrile/water systems, It was concluded that the interaction between OD S ligands and adsorbate molecules was weaker compared with that in met hanol/water mobile-phase systems when acetonitrile was used as an orga nic modifier in a mobile phase of reversed-phase liquid chromatography .