An amino-endcapped octadecylsilane silica-based mixed-mode stationary phase for the simultaneous separation of neutral and ionizable components in fixed-dose combinations
An amino-endcapped octadecylsilane silica-based stationary phase (ODS–APS) was prepared for simultaneous separation of neutral and ionizable compounds in fixed-dose combinations. The packing was prepared by a three-step vapor deposition method which involves the reaction of silica particles with (3-aminopropyl)trimethoxysilane and octadecyldimethylchlorosilane in an environmentally friendly process. Physicochemical characterization including Fourier transform infrared spectroscopy, solid-state 13C CP/MAS NMR spectroscopy and elemental analysis confirmed the successful bonding of both octadecyl and amino-endcapped moieties on the ODS–APS phase. The chromatographic performance of the packing was thoroughly evaluated. In addition, the retention mechanism of acidic compounds on the synthesized phase was systematically evaluated by studying the effects of pH and ionic strength of the mobile phase. Finally, two fixed-dose combinations, losartan potassium/hydrochlorothiazide and B-complex vitamin tablets, were analyzed on the ODS–APS phase. The results demonstrated that the ODS–APS packing provides better peak shape and higher selectivity for separating basic and acidic analytes under neutral conditions compared with the traditional ODS phase. The ODS–APS phase also exhibits superior performance for the simultaneous separation of neutral and ionizable drugs in fixed-dose combinations.