Analysis of non-ultraviolet-absorbing dialkyl-type cationic surfactants by liquid chromatography with an on-line ion-pair extraction detector

Abstract

Dialkyl-type cationic surfactants, i.e., dialkyldimethylammonium, dialkyl(2-hydroxyethyl)methylammonium and dialkylmethylpoly(oxyethylene)ammonium salts, were analysed by liquid chromatography with an on-line ion-pair extraction detector. The separation was achieved by reversed-phase chromatography with a eluent of 0.1 M ammonium acetate in methanol—acetonitrile—deionized water (60:60:5). The post-column detection system was based on the on-line ion-pair extraction of the cationic surfactants with a counter ion of Bromophenol blue into an immiscible organic phase of hexane which was monitored by a double-wavelength spectrophotometer (605 nm and 670 nm). By the proposed method, the type and alkyl chain distributions of the cationic surfactants in commercial products were distinguished within 15 min without any pre-treatment of samples and with a detection limit each of approximately 15 nmol.