Effect of trimethylsilane pre-capping on monomeric C18 stationary phases made from high-purity type-B silica substrates: Efficiency, retention, and stability

Silica-based monomeric C18 stationary phases are the most widely used in reversed-phase liquid chromatography (RPLC), and methods to improve the efficiency and chemical stability of such phases are always being investigated. Previous work showed that stationary phases made by pre-treating the silica surface with small amounts of trimethylsilane (TMS) reagents prior to C18 silanization showed vast improvements in the chromatographic efficiencies of both polar and non-polar analytes. It was concluded that this “pre-capping” step improved efficiency by selectively neutralizing the most reactive highly acidic silanol sites, producing a more energetically homogenous surface prior to exhaustive derivatization that subsequently yielded a more evenly distributed alkyl bonding arrangement. These previous studies were performed on Type-A silica, an older variety of silica gel material that contains higher levels of metal impurities than the Type-B silica used today. The purpose of the work presented here is to investigate the effects of trimethylsilane pre-capping on monomeric C18 stationary phases made from high-purity Type-B silica substrates. The results show little or no efficiency improvements for non-polar compounds, but a more noticeable improvement was observed for some drug compounds and bases under buffered conditions, with the magnitude of the improved efficiencies correlating with metal impurity content and physical parameters of the silica substrate. The results lend supporting evidence that metal impurities are the primary source of highly acidic silanols, but they also suggest a means to improve efficiencies for compounds of interest. Pre-capping also resulted in a decreased retention due to a decrease in carbon loading. In addition, hydrolytic stability of the stationary phases was investigated at high pH and elevated temperature, with results indicating a slight decrease in hydrolytic stability with increased pre-capping.