Sensitive iodine speciation in seawater by multi-mode size-exclusion chromatography with sector-field ICP-MS

Abstract

A simple, fast and sensitive analytical method for iodine speciation in seawater by multi-mode size exclusion chromatography coupled with sector-field inductively coupled mass spectrometry (HPLC-SF-ICP-MS) is described in this paper. Total iodine concentration was directly measured using SF-ICP-MS after 100-fold dilution with 1% TMAH. Separation of iodate and iodide was achieved using an AsahiPak GS-220 multi-mode size exclusion column with a mobile phase of 50 mM malonic acid–37.5 mM TMAH (pH 6.9) with isocratic elution at a flow rate of 1.0 ml min⁻¹. The detection limit was 0.00016 μM (20 ng l⁻¹) for iodate and 0.00024 μM (30 ng l⁻¹) for iodide, and they were ca. 70 times lower than those of the HPLC-ICP-QMS technique. The proposed method was successfully applied to the determination of iodate and iodide in seawaters from the surface down to a depth of 700 m collected from the Pacific Ocean off Aomori Prefecture, Japan. Vertical distributions of total iodine, iodate, iodide, total inorganic iodine and dissolved organic iodine were obtained using the developed method. Iodate was found to be the predominant species. This is the first report on the study of vertical distributions of iodine in seawater using the HPLC-ICP-MS technique.