Preparation and retention mechanism of a mixed-mode reversed-phase/weak-cationic-exchange chromatographic packing

Abstract

A "mixed ligand" octyl-carboxylic co-bonded silica (OCS) packing was prepared by the method of "click chemistry". The resulting OCS packing was characterized by elemental analysis (EI) and Fourier transform infrared spectroscopy (FT-IR) to prove the successful immobilization of octyl and carboxylic groups on the surface of silica gel. Then the mixed-mode reversed-phase/weak-cationic-exchange (RP/WCX) retention mechanism of the OCS packing was quantitatively probed by studying the retention factors of a homologous series of three cationic surfactants on the mixed-mode stationary phase column as a function of the ammonium concentration in the eluent or the number of methylene groups in the solute. The one-site and two-site mixed-mode retention models of the three cationic surfactants on the OCS phases were studied by investigating the logarithm and reciprocal relationships of retention factors and salt concentrations, demonstrating that the two-site model was more suitable for the description of individual RP or WCX contribution to total retention was obtained according to the mathematical equations of two-site retention mechanism, which can provide some valuable guidance for the separation of real samples. This study developed the qualitative model of retention mechanism of the mixed-mode OCS packing, and a series of standard basic mixtures were well separated on the OCS packing, demonstrating the great application potential of OCS packing for the separation of various basic compounds.