Separation of anthraquinone sulphonic acids by ion-pair liquid chromatography

Abstract

The separation of sulphonic acid derivatives of anthraquinone using reversed-phase high-performance liquid chromatography in the presence of quaternary ammonium salts as ion-pairing agents has been investigated. It is shown that there is a general increase in retention time with increasing size of the quaternary ammonium ion, R 4N +, but that the separation depends critically upon the length of the longest carbon chain. Retention of substrates containing ionisable groups such as −OH and −NH 2 is strongly affected by the pH of the eluent. Severe tailing and double peaks were observed at pH values close to the p K a of the solute. At low concentrations of pairing agent the retention time varies directly with R 4N + concentration, but saturation effects are observed at higher concentration ( ca. 0.06 M for tetrabutylammonium bromide). The practical utility of the separation procedure is demonstrated for the separation of (a) a series of anthraquinone mono- and disulphonic acids using cetyltrimethylammonium bromide as pairing agent in acetonitrile—water (50:50, v/v) as eluent and Spherisorb S5 ODS as the stationary phase, and (b) sulphonated and non-sulphonated derivatives of 1-aminoanthraquinone using an isocratic eluent of methanol—water—tetrabutylammonium hydroxide and Spherisorb S5 ODS as the stationary phase.