Separation of benzodiazepines using cholesterol-modified fused-silica capillaries in capillary electrochromatography

This paper describes the use of cholesteryl-10-undecenoate as a capillary column modifier for capillary electrochromatography. This bonded phase has shown an entirely different selectivity as compared to octadecyl silica (ODS) phases in reversed-phase liquid chromatography, specifically in the analysis of the benzodiazepines. This difference serves as the basis for the continuing study as well as the present focus on electrochromatography as a relatively new separation technique. To significantly increase the surface area, etching of the inner wall of a 75-micron capillary is performed using ammonium hydrogen difluoride and is then subsequently modified via a silation reaction with triethoxysilane and subjected to hydrosilation with cholesteryl-10-undecenoate to form the bonded phase. The performance of this capillary is compared with unetched cholesterol-modified and bare, fused-silica capillaries. Efficient resolution was noted for the etched capillary showing the effectiveness of the etching process as well as the selective property of the bonded phase.