| In this thesis, we have developed new theories which for the first time deal in a comprehensive fashion with both the competitive effect of added counter cations and the retention enhancing effect of "ion pairing" anions for the mixed-mode basic pharmaceutical separations in reversed-phase liquid chromatography (RPLC).; In our cationic counterion studies, we disproved the one site model and showed that at least two types of sites must exist one of which is what we call a "hydrophobically assisted" ion-exchange site. Our theory very nicely rationalizes why primary amines are stronger silanophiles than secondary or tertiary amines and why a good silanol blocking agent must have a high hydrophobicity. Unambiguous column classification has been achieved based on a quantitative measurement of the relative contributions of ion-exchange and reversed-phase interactions. As applications of our theory, we compared the separation of basic drugs on several octadecyl silane bonded silica (ODS) phases and a polybutadiene-coated zirconia (PBD-ZrO2) phase. Quite different selectivities were found on these two types of phases due to the very different relative contributions from ion-exchange and reversed-phase interactions. The use of amine counterions with different charges and hydrophobicities enables retention adjustment of bases on the PBD-ZrO2 phase, but has much smaller effect on type B ODS phases.; The addition of relatively hydrophilic anionic additives (e.g. perchlorate and trifluoroacetate) profoundly influences the retention of basic drugs. To understand the mechanisms by which anionic additives perturb retention in chromatography, we studied the formation of ion pairs between a number of prototypical basic drugs and anionic additives using capillary electrophoresis (CE). For the first time, ion pair formation between basic drugs and anionic additives under conditions commonly used in RPLC has been confirmed independently. The ion pair formation constants from CE were used to interpret the chromatographic data. Agreement between the predicted and experimental chromatographic data under various conditions was evaluated. Under certain circumstances (e.g., pH, stationary phase, and nature of anions), we conclude that the ion pair formation in the mobile phase mechanism is dominant and at other conditions it remains a significant contribution. |
