Rationalization of the selection of the type of the organic modifier(s) for selectivity optimization in reversed-phase ion-pair chromatography

Abstract

The selection of the type and/or concentration of the organic modifier(s) in reversed-phase ion-pair liquid chromatography (RP-IPC) can be rationalized by considering the nature of the solutes in the sample mixture. The separation of a few typical sample mixtures containing ionic and/or uncharged solutes and requiring the variation or optimization of the type and/or the concentration of the organic modifier(s) is discussed. The examples demonstrate the utility of the sample composition-based parameter selection method and highlight the importance of organic modifier optimization in RP-IPC. The problems associated with the use of the solvent strength parameters in the determination of the initial eluent compositions prior to mobile phase optimization in RP-IPC are discussed. Eluent composition which were predicted to lead to identical pairing ion surface concentrations at constant ionic strength ( i.e., to create isopotential conditions) agree reasonably with the eluent composition data found experimentally to be isoeluotropic in different RP-IPC systems. Comparison of the different transfer rules reveals that those defined for reversed-phase systems overestimate the isoeluotropic eluent compositions in RP-IPC. When ionized solutes and oppositely charged pairing ions are present, one may need to decrease the predicted organic modifier concentrations by as much as 0.15-02. volume fraction units in oder to obtain identical retention conditions. The main reasons why transfer rule equations could not be obtained for RP-IPC systems include the different in k' vs. organic solvent concentration behaviors of ionic and uncharged solutes in the regular RP mode and the different adsorption behavior of the pairing ion(s) as the organic modifier is changed in the eluent systems.