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.