Semi-empirical description of a retention of small organic probes on AtlantisTM Premier BEH C18 AX mixed-mode column under varying pH and organic modifier conditions

Mixed-mode chromatography is a widely used analytical method that utilizes multiple separation mechanisms in one chromatographic column; frequently a reversed-phase and ion-exchange interactions. Adequate understanding of how the individual interactions influence the overall retention of the analyte is important for fast and effective design of mixed-mode-based methods. Previous attempts to derive semi-empirical mathematical models of mixed-mode chromatography were limited to a narrow range of analytes and did not consider the effect of organic modifier in the mobile phase. In this paper, we attempt to improve upon the previous models by extending the range of classes of analytes to weak acids and bases and including the effect of organic modifier. An improved semi-empirical model was derived that captures all important retention trends of the combined effect of reversed-phase and ion-exchange retention mechanisms for all tested classes of analytes and up to 30% (v/v) organic modifier content.