Some observations on the prediction of retention in reversed-phase liquid chromatography using the pH as main factor

Abstract

When solutes show acid–base behaviour inside the working pH of a reversed-phase liquid chromatographic (RPLC) column, besides the type and percentage of organic modifier, pH can be included to improve the possibilities of separation. Optimisation based on a retention model is an economical and reliable way to achieve the best resolution. However, the difficulties found in knowing the exact pH values and the dependence of acid−base constants of solutes with mobile phase composition makes the accurate modelling of the retention behaviour as a function of these variables rather difficult. In this work, it is shown that in fact, there is no need to have this information. Based on the elution of several diuretics with acetonitrile–water mobile phases from a C18 column, two simple approaches are proposed to model the RPLC retention, where the pH in aqueous solution (before the addition of organic solvent), or the concentration of strong base added to the acidic species of an acid–base system used to buffer the pH of the mobile phase, are used. Both approaches yield satisfactory predictions of retention factors as a function of pH and concentration of organic modifier.