An Interphase Model for Retention in Liquid Chromatography

An interphase model for retention is appropriate for biphasic systems in which one phase (the mobile phase) modifies the properties of the other phase through absorption of mobile phase components. This is typical of liquid chromatography, where separation occurs by the distribution of sample components between a bulk mobile phase and an interphase region in intimate contact with the mobile phase. This has profound implications for the interpretation of retention mechanisms since the properties of the stationary phase are those of the interphase region, which can be quite different to those known or perceived for the nonsolvated stationary phase. For reversed-phase chromatography, it is shown that retention properties can be adequately described by the solvation parameter model and visualized as a function of the bulk mobile phase composition by system maps. For normal-phase chromatography, a modified approach is required for inorganic oxide adsorbents to accommodate site-specific interactions (localization of sample and/or mobile phase components) on high energy adsorption sites within the interphase region. This is achieved using a competition model approach to separate out the contributions of solvent and solute interactions with the adsorbent surface and the solvation parameter model to provide insight into the relative importance of various intermolecular interactions on retention and selectivity. The above discussion is set within the framework of thin-layer chromatography although the conclusions are general and equally applicable to column liquid chromatography.