An investigation on the cation-exchange and Lewis-base interactions of a n-octadecylphosphonic acid-modified magnesia-zirconia stationary phase

The cation-exchange and Lewis-base interactions of a n-octadecylphosphonic acid-modified magnesia-zirconia (C18PZM) stationary phase have been studied in greater detail than previous studies, which are characterized by the chromatography of some basic drugs of high pKa and some hard Lewis base analytes, respectively. The effect of mobile phase constituents such as pH, type of buffer and buffer concentration on the surface chemistry properties on C18PZM are elucidated. The results show that at an intermediate pH, the cation-exchange sites are due to both the chemisorbed phosphonate and the adsorbed buffer anion, i.e. acetate, at the hard Lewis acid sites on the C18PZM phase; at high pH, the cation-exchange interactions mainly result from the chemisorbed phosphonate rather than the inherent zirconol groups. The chromatography of the hard Lewis base analytes indicates that a strong Lewis acid-base interaction still exists on C18PZM even in the presence of Lewis base buffer (such as acetate or phosphate) in the mobile phase or a high-pH eluents is used. Moreover, the chemisorbed n-octadecylphosphonic acid (C18P) is found to slowly bleed out of the C18PZM column when exposed to a phosphate buffer. As expected, the resulting mobile-phase phosphate modified C18PZM column (p-C18PZM) exhibits significantly stronger cation-exchange interactions towards the test basic solutes than the “original” C18PZM column. And the p-C18PZM column demonstrates a tremendous improvement on the chromatography of the Lewis base analytes over the “original” C18PZM column. An approach designed to overcome the Lewis acid sites problem by attaching methylphosphonic acid (MPA) onto the C18PZM column, however, is not successful in improving the chromatography of the troublesome Lewis base solutes.