Determination of thiosulfate, thiocyanate and polythionates in a mixture by ion-pair chromatography with ultraviolet absorbance detection

A sensitive ion chromatographic method has been developed for the determination of mixtures of thiosulfate, thiocyanate and polythionates (tri-, tetra-, penta- and hexathionate). The proposed method is based on the separation of the sulfur anions on an octadecylsilica (ODS) column with an acetonitrile-water mobile phase containing tetrapropylammonium salt (TPA) as an ion-pairing reagent and the ultraviolet absorption detection of the sulfur anions. When an acetonitrile-water (20:80, v/v) solution (pH 5.0) containing 6 mM TPA was used as a mobile phase at flow-rate of 0.6 ml min(-1), the sulfur anions were resolved within 22 min. The detection limits defined at S/N=3 and 230 nm were very low for all anions, except trithionate: 30 nM for thiosulfate, 60 nM for thiocyanate, 20 nM for tetrathionate, 15 nM for pentathionate and 18 nM for hexathionate. The proposed method gave recoveries ranging from 95.0 to 105.0% when applied to the determination of polythionates added to hot spring waters.