Capillary electrochromatography with monolithic-silica columns. II. Preparation of amphiphilic silica monoliths having surface-bound cationic octadecyl moieties and their chromatographic characterization and application to the separation of proteins and other neutral and charged species

Abstract

Three different synthetic routes have been introduced and evaluated for the preparation of amphiphilic silica-based monoliths possessing surface-bound octadecyl ligands and positively charged groups. The amphiphilic silica monoliths (designated as cationic C₁₈-monoliths) have been designed for use in reversed-phase capillary electrochromatography (RP-CEC) with hydro-organic mobile phases. These amphiphilic stationary phases yielded anodic electroosmotic flow (EOF) over a wide range of mobile phase pH. The magnitude of EOF remained constant up to pH 4.0 and then decreased at pH > 4.0 due to the ionization of silanol groups and the subsequent decrease in the net positive surface charge density of the amphiphilic monoliths. The cationic C₁₈-monoliths exhibited reversed-phase chromatography (RPC) behavior toward non-polar solutes (e.g., alkyl benzenes), which parallels that observed with octadecyl-silica (ODS) monoliths. On the other hand, the amphiphilic stationary phases exhibited both non-polar and polar interactions toward slightly polar solutes such as anilines and PTH-amino acids. CEC retention factor k* and velocity factor k*_e, which reflects the contribution of the electrophoretic mobility, were evaluated for charged solutes such as anilines and proteins.