High-performance Liquid Chromatography of Some Alkaloids on n-Octadecylphosphonic Acid-modified Magnesia-Zirconia Stationary Phase

The high-performance liquid chromatographic behavior of some alkaloids was studied on an n-octadecylphosphonic acid-modified magnesia-zirconia (C18PZM) stationary phase. The effects of the mobile phase variables such as the methanol content, the ionic strength, and the pH on their chromatographic behavior were investigated. The retention mechanism of the alkaloids in the stationary phase was elucidated. The results indicate that both hydrophobic and cation-exchange interactions contribute to the retention of the alkaloids under most chromatographic conditions. The inherent Brönsted-acid sites and the adsorbed Lewis-base anionic buffer constituents on the accessible ZM-surface Lewis-acid sites play a role in the retention of ionized solutes by cation-exchange interaction. However, at high pH, the retention of the alkaloids depends mainly on the hydrophobic interactions between the solutes and the support. A satisfactory separation of the alkaloids was achieved with the mobile phase of pH 10.1. The C18PZM seems to be an excellent alternative to the silica-based reversed-phase stationary phase for the separation of strongly basic solutes.