Influence of pH on retention in linear organic modifier gradient RP HPLC

The purpose of this work was to examine the influence of pH on retention of analytes during the linear organic modifier gradient in RP HPLC. We started from the general theory of combined pH/organic modifier gradient and provided an explicit however approximate solution relating gradient retention time and the pH of the eluent. The accuracy of the proposed model was tested by its ability to describe an experimental data set that comprised retention times for a series of monoprotic acids and bases obtained at different pH and for different gradient durations. The basic analytical properties of the equation relating retention time and pH were determined, such as the inflection point and range of retention times. The use of the pH at inflection point as a measure of p K a was discussed and compared to the chromatographic p K a obtained by a fitting to the proposed model. In conclusion, the work provides theoretical results that augment the knowledge on the impact of pH on the gradient retention.