pH Gradient as a Tool for the Separation of Ionizable Analytes in Reversed-Phase High-Performance Chromatography

The aim of this work was to propose a general scheme of optimizing separation of ionizable analytes and to determine conditions of maximal peak compression in pH-gradient reversed-phase high-performance liquid chromatography (RP HPLC). The approximated explicit equation of the linear pH gradient has been developed. It allows predicting retention times for a given organic modifier content, initial value of pH, and the start and steepness of the pH gradient. Also the formula for calculating maximal peak compression is provided. The developed theory was compared with experimental data on the example of a weak acid (ketoprofen) and a weak base (papaverine). Five parameters characterizing analyte retention (log k(w) and S of the ionized and nonionized forms along with pK(a,chrom)) were determined in a series of isocratic experiments carried out at different pH values and with different methanol contents in the eluent. Next, a series of pH gradients of different pH-gradient steepness and of different pH-gradient starting time has been obtained and used to test the validity of our theoretical approach. The conditions of maximal peak width compression have been found. The derived theory was proved to be in a good agreement with the experimental data. The pH-gradient method led to peak compression of up to 0.2, and minimized peak tailing was obtained for the tested analytes. Since the majority of analytical separations are done in an isocratic mode we proposed a means to transfer an isocratic method to a pH-gradient method.