Controlling the retention of clopenthixol and other basic drug substances by reversed-phase ion-pair chromatography on bonded-phase materials using two counter-ions of opposite charge.
In the reversed-phase chromatography of nitrogen-containing bases on chemically bonded ODS-silica, peak tailing and prolonged retention are often considerable problems. These effects are due to residual silanols on the surface of the column material and may be remedied by adding suitable amines or quaternary ammonium ions to the eluent as anti-tailing agents. However, further addition of an anionic compound is often needed to achieve a suitable retention. The retention mechanism in such systems is complex as interaction takes place between the anionic compound and the solute molecules, anti-tailing agent and column packing material. The influence of the nature of the anti-tailing agent and anionic counter-ion on the retention of cis- and trans-clopenthixol and of other basic drug substances was investigated and it was found that both the retention and the selectivity were greatly affected.