Retention behaviour of quinolone derivatives in high-performance liquid chromatography. Effect of pH and evaluation of ionization constants

This paper examines the effect of solute ionisation on the retention behaviour of a series of quinolones and evaluates their pKa values using chromatographic data in acetonitrile-water mixtures with acetonitrile percentages of 30, 35, 40 and 50% (v/v). We also compare these pKa values with those previously obtained in acetonitrile-water mixtures from potentiometric measurements. In doing so, the pH values were measured in the hydroorganic mixture, which was used as the mobile phase, instead of in water, taking into account the effect of activity coefficients. The resulting equations permit the chromatographic determination of the pKa values of the quinolones in acetonitrile-water mixtures and also permit the prediction of the effect of pH on their chromatographic behaviour. These equations can be combined with those previously derived, which relate retention to the solvent composition of the mobile phase, to establish a general model that relates the elution behaviour of the solute to significant mobile phase properties: composition, pH and ionic strength.