Capillary electrochromatography of basic compounds using octadecyl-silica stationary phases with an amine-containing mobile phase

The capillary electrochromatographic (CEC) analysis of basic compounds on o ctadecyl-silica stationary phases (Hypersil ODS and Spherisorb ODS I) was s tudied. A basic drug (fluvoxamine) and one of its possible impurities were used as test compounds. With an eluent of acetonitrile-phosphate buffer (pH 7.0), the compounds could be baseline-separated; however, broad and tailin g peaks were obtained. To minimise detrimental interactions with residual s ilanol groups, the pH of the mobile phase was lowered to 2.5, but the plate numbers were still quite low (<2.6x10(4) plates/m). Addition of a masking agent (hexylamine or triethylamine) to the mobile phase resulted in much be tter peak efficiencies (ca. 1x10(5) plates/m). Therefore, the influence of the amine concentration and pH of the mobile phase on the CEC performance ( peak width, peak tailing, electroosmotic flow, selectivity) was investigate d in detail. Highest efficiencies (2.8x10(5) plates/m) could be obtained wi th the Spherisorb column, while the Hypersil column offered a better select ivity. Furthermore, the results show that the residual silanol groups are ( at least partly) responsible for the separation of the basic compounds and that the amount of injected sample has an unusually large effect on the pea k efficiency. The usefulness of the system for impurity profiling was demon strated with a mixture containing fluvoxamine and its stereoisomer (a possi ble impurity) at the 0.1% level. The general effectiveness of amine additiv es in CEC was illustrated by the separation of a mixture of five structural ly different basic drugs yielding plate numbers in the 1x10(5)-3x10(5) plat es/m range. Comparison with capillary electrophoretic analysis revealed a u nique selectivity of the CEC system which is based on both electrophoretic mobility and chromatographic partitioning. (C) 2000 Elsevier Science B.V. A ll rights reserved.