Elucidation of retention behaviors in reversed-phase liquid chromatography as a function of mobile phase composition

Various retention models have been widely used for understanding the retention mechanisms of solutes in reversed-phase chromatography systems. The models have been used to interpret the often-observed linear plots of the logarithms of retention factor k versus the solvent modifier concentration C_M and ln k versus ln⁡C_M. In this study, the retention behaviors of nine solutes as a function of acetonitrile (ACN) concentration were systematically investigated using a commercially available C₁₈ column. The thermodynamic properties of solute adsorptions in neat water, determined using van' t Hoff plots, were investigated. Slightly concave upward and downward retention curves were identified for the plots of ln k versus C_ACN and ln k versus ln C_ACN, respectively. A three-equilibrium-constant stoichiometric displacement retention model was used to interpret the retention behaviors of the solutes. The model was demonstrated to account for the nonlinearity of the retention curves. The linear fits of the ln k versus ln⁡C_ACN and ln k versus C_ACN plots were implied to be more suitably used for high and low ACN concentration ranges, respectively. The model fitted the experimental data satisfactorily over a full range of ACN concentrations; thus, the nonlinearity of ln k versus ln⁡C_ACN plots was implied to mainly be attributed to the weak ACN-sorbent interactions. U-shaped retention curves were observed for acetone, tetrahydrofuran, tert-butanol, and benzyl alcohol at high ACN concentrations, indicating that the retention behaviors of these solutes may involve two types of interactions, with complementary effects on solute retention factor with increasing ACN concentration.