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A Sensitive Determination of Iodine Species, Including Organoiodine, for Freshwater and Seawater Samples Using High Performance Liquid Chromatography and Spectrophotometric Detection

Kathleen A. Schwehr, Peter H. Santschi

In order to more effectively use iodine isotope ratios, 129I/127I, as hydrological and geochemical tracers in aquatic systems, a new HPLC method was developed for the determination of iodine speciation. The dissolved iodine species that dominate natural water systems are iodide, iodate, and organic iodine. Using this new method, iodide was determined directly by combining anion exchange chromatography and spectrophotometry. Iodate and the total of organic iodine species are determined as iodide, with minimal sample preparation, compared to existing methods. The method has been successfully applied to determine iodide, iodate as the difference of total inorganic iodide and iodide after reduction of the sample, and organic iodine as the difference of total iodide (after organic decomposition and reduction) and total inorganic iodide.

Analytical accuracy was tested (1) against certified reference material SRM 1549, powdered milk (NIST), (2) through the method of standard additions, and (3) by comparison to values of environmental waters measured independently by ICP-MS. The method has been successfully applied to measure the concentrations of iodide species in rain, surface and groundwater, estuarine and seawater samples. The detection limit was ~1 nM (0.2 ppb), with less than 3% relative standard deviation for samples determined by standard additions to an iodide solution of 20 nM in 0.1 M NaCl.

This technique is one of the few methods sensitive enough to accurately quantify stable iodine species at nanomolar concentrations in aquatic systems across a range of matrices, and to quantitatively measure organic iodine.

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