Retention of Ionizable Compounds on HPLC. 2. Effect of pH, Ionic Strength, and Mobile Phase Composition on the Retention of Weak Acids

A new model that relates the retention of a weak acid in HPLC columns with the pH and ionic strength of the mobile phase is derived and tested for different benzoic acids in methanol−water mobile phases. The proposed model uses the pH value in the mobile phase instead of the pH value in water, takes into account the effect of the activity coefficients, and considers different holdup times for the neutral and ionic species. The dependence of the holdup time of the ionic species on the mobile phase properties (pH, solvent composition, and ionic strength) is evaluated. It is demonstrated that the holdup time of the neutral species does not depend on the mobile phase properties, but the holdup time of the ionic species depends on the particular buffer used. The proposed equations can be combined with previously derived equations that relate the retention with the solvent composition of the mobile phase to establish a general model that relates the retention of the solute with the significant mobile phase properties:  composition, pH, and ionic strength.