Determination of Iodide and Iodate in Seawater by HPLC with UV Detection

Iodine is an essential nutrient in seawater, seafood, and iodine-enriched foods, such as iodized table salt. The most common forms of iodine in the diet are iodide and iodate, with additional iodo-organic compounds providing a small fraction of the bio-available iodine. Iodine deficiency affects thyroid hormone production and leads to developmental diseases, goiter, and paralysis (1).

Because iodide and iodate are essential sources of iodine, these anions must be determined in a variety of matrices. In seawater, for example, iodide amounts can range from less than 1 μg/L to greater than 60 μg/L with the measured concentrations dependent on water depth, oxygen concentration, and the biological mediation of the iodide/iodate equilibrium (2). Determination of iodide and iodate in saline matrixes is challenging due to the high chloride concentrations in samples. In seawater, the matrix is further complicated by high concentrations of carbonate and sulfate. This high ionic strength matrix makes analysis of samples containing iodide and iodate difficult.

The analysis method described here is specific, sensitive, and rapid. Iodide and iodate are separated on the Acclaim® Mixed-Mode WAX-1, a silica-based column that incorporates hydrophobic, weak anion-exchange, and ion exclusion properties. Consequently, retention of basic, neutral, and acidic molecules can be independently or concurrently adjusted by changing mobile phase ionic strength, pH, or organic solvent content. The ability to control these properties leads to unique and adjustable selectivity. This column effectively separates anions with the high capacity necessary for the analysis of iodide and iodate in saline matrices, minimizing the need for sample pretreatment.

Experimental

A Dionex UltiMate® 3000 system with a HPG-3200M pump, a TCC-3200 column compartment, a WPS-3000TSL Micro autosampler, and a PDA-3000 detector were used. An Acclaim Mixed-Mode WAX-1, 5 μm 2.1 × 150 mm column was used for all separations with the conditions in Figure 1. Chromeleon® Chromatography Management Software was used for system control and data processing.

Figure 1

Results and Discussion

Figure 1 shows the separation of iodate and iodide in a 1:10 dilution of synthetic seawater on the Acclaim Mixed-Mode WAX-1 column. A wavelength of 223 nm was determined by collecting data between 210–500 nm and choosing the wavelength with the best signal to noise (S/N) ratio for iodide. Iodide and iodate can be determined simultaneously in less than 15 min. Iodide is well separated from all other peaks and iodate is easily identified. Although this sample was diluted to determine iodate, no sample dilution is necessary for iodide determination.

Linear response to iodide and iodate was confirmed between 10–250 and 500–7600 μg/mL (ppb), respectively. The standards were prepared in deionized (DI) water without the added complexity of matching the sample matrix. Determinations of iodide and iodate in saline matrixes were accurate and precise with recoveries ranging between 93–113% and peak area RSDs +/- 1.2%. The data shows the advantage of the high capacity and adjustable specificity of the Acclaim Mixed-Mode WAX-1 column to determine analytes in high ionic strength samples.

References

(1) Iodine Status Worldwide: WHO Global Database on Iodine Deficiency, de Benoist, B.; Anderson, M.; Egli, I.; Takkouche, B.; Allen, H.; eds. Department of Nutrition for Health and Development, World Health Organization, Geneva, 2004.

(2) Tian, R.C.; Nicolas, E.; Iodide Speciation in the Northwestern Mediterranean Sea: Method and Vertical Profile, Marine Chemistry, 1995, 48, 151.

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