Comparison of the performance of conventional C18 phases with others of alternative functionality for the analysis of basic compounds by reversed-phase high-performance liquid chromatography

The performance of ten reversed-phase columns which included conventional C-18 phases, phases with embedded polar groups, short chain and cyano phases, and a high-pH stable phase, was evaluated with a variety of basic compounds of low and high pK(a). The aim of the work was to determine if these alternative phases offered any advantages over conventional C-18 phases for the analysis of basic compounds. Mobile phases which were unbuffered, buffered with phosphate at a pH of 7.0 and 3.0, and modified with either methanol or acetonitrile, were investigated. Phases with embedded polar groups exhibited reduced hydrophobicity, somewhat different selectivity, and greater inertness towards basic compounds compared with C-18 phases prepared on the same silica. Phases with shorter alkyl chains also produced improved peak shape for basic compounds; selectivities were similar for alkyl bonded phases but completely different for a cyanopropyl phase at pH 7.0. At high pH (pH 11.0) a novel bidendate phase gave improved peak shapes for some bases together with different selectivity compared with operation at pH 7.0. However, a contributing factor to this improvement may be the silanol masking ability of buffer components utilised in addition to reduced ion-exchange interactions with ionised silanols. (C) 1999 Elsevier Science B.V. All rights reserved.