| Octadecyl bonded silica-based (ODS) columns are by far the most popular stationary phases used in reversed phase liquid chromatography (RPLC) for the separations of basic compounds. The two major modes of interactions of such analytes on silica-based columns are hydrophobic and ion-exchange (Coulombic) interactions. The hydrocarbon of the bonded chains and the ionized residual silanol groups account for the hydrophobic and ion-exchange interactions, respectively.; Polybutadiene coated zirconia (PBD-ZrO2) is a new type of stationary phase characterized by its excellent chemical and thermal stability, as well as high efficiency. When hard Lewis bases, such as phosphate, are added to the eluent, they are adsorbed on the Lewis acid sites on zirconia's surface and cause the surface to become highly negatively charged. Under these conditions, the stationary phase acquires both reversed-phase and ion-exchange characteristics, qualitatively similar to silica-based phases, but quantitatively quite different.; Although the influence of hydrophobic and Coulombic interactions on the chromatography of cationic species is qualitatively well recognized, the quantitative relationship between hydrophobic and ion-exchange interactions remains unclear. The retention mechanisms on ODS and PBD-ZrO 2 phases were studied with a specific set of judiciously selected basic compounds, namely a homologous series of p-alkylbenzylamines (R-C6H4-CH2-NH2). Various quantitative models of mixed-mode retention mechanism on the two types of phases were compared and evaluated. The one-site model in which hydrophobic and Coulombic interactions combine has been disproved. Furthermore, the model equations developed allow us to, for the first time, quantify the relative amount of hydrophobic and Coulombic contributions to retention of any solute on any stationary phase. The data on both ODS and PBD-ZrO2 point to a multi-site model. The results clearly show that while ion-exchange plays an important but minor role (1--10%) on retention of bases on ODS phases, it is by far the dominant factor (>90% in some cases) in retention on PBD-ZrO2. The results also explain the very high sensitivity of PBD-ZrO2 phases compared to silica-based phases to the structure and type (primary, etc.) of the amine solute. |
