High-performance liquid chromatography on dynamically modified silica : IV. Effects of variations of pH, ionic strength and buffer composition in eluents containing cetyltrimethylammonium bromide

Abstract

The influence of pH changes as well as variation of ionic strength and buffer composition on the retention of selected test solutes in high-performance liquid chromatography on dynamically modified silica were investigated. Chromatography was performed on columns of bare silica (LiChrosorb Si 60) using eluents consisting of mixtures of methanol and aqueous phosphate buffers and containing cetyltrimethyl-ammonium (CTMA) bromide. With increasing pH the amount of CTMA ions adsorbed on to the silica surface rose, thus increasing the retention due to reversed-phase partition. It was shown that apart from the adsorption of CTMA on to silica, ion exchange plays only a minor role in the retention of cations, except for hydrophobic quaternary ammonium compounds. An increase in the ionic strength of the phosphate buffer caused a decrease in the retention of most ionic test solutes. This effect was shown to be due to interaction with the phosphate ions in the eluent and not to a decrease in the amount of CTMA adsorbed. The addition of inorganic salts, with anions different from the buffer anion, had no effect on retention apart from ionic strength effects. The addition of alkane-sulphonates was shown to lead to a selective decrease in the retention of anionic solutes. The nature of the buffer cation was shown greatly to influence retention and selectivity.