REVERSED-PHASE HPLC SELECTIVITY AND RETENTION CHARACTERISTICS OF CONFORMATIONALLY DIFFERENT BONDED ALKYL STATIONARY PHASES

Monomeric C8 and C18 stationary phases with bulky side ('sterically protecting') groups have a lower concentration (micromoles/meter) and carbon content than corresponding dimethyl-substituted phases. Compared with dimethyl-substituted packings, sterically protected diisobutyl- and diisopropyl-C18 stationary phases show higher column efficiency and better peak symmetry for basic drugs at pH 7. The bulky sterically protecting groups may differently shield unreacted silanol groups on the silica support surface, which causes a stronger retention of basic drugs. Alternatively, the bulky side groups may provide a ligand conformation that produces different column selectivities and higher retention than dimethyl-substituted phases. Monomeric C18 packings with bulky, sterically protecting side groups are highly stable in aggressive low pH and high-temperature environments. A densely coated dimethyl-C18 silica-based packing showed unexpected stability in a pH 9 mobile phase environment.