Comparison of octadecyl-bonded alumina and silica for reversed-phase high-performance liquid chromatography

Abstract

Chromatographic properties of an octadecyl-bonded alumina (ODA) high-performance liquid chromatographic stationary phase are described and compared with those of commonly used octadecylsilica (ODS) stationary phases. Under reversed-phase conditions, selectivity of the ODA phase is shown to be similar to that of ODS, resulting in similar elution orders for most compound mixtures. The higher stability of the ODA material to alkaline solvents and the absence of acidic silanol sites on its surface allow a more efficient separation of organic bases with simpler mobile phases than is generally possible with ODS. Comparison of the correlation of the capacity factors of 31 organic compounds on the ODA and ODS stationary phases indicates a higher degree of hydrogen-bonding solute-stationary phase interactions on ODA than on ODS. Multiple-ring aromatic compounds are more strongly adsorbed on the ODA stationary phase than on ODS. Differences in the retention behavior of benzylamine on the ODA and ODS stationary phases with increasing percentages of methanol in the mobile phase are reported. These are interpreted in terms of the differences in solute-stationary phase interactions on the two phases resulting from the chemical properties of their silica and alumina backbones.