BACKGROUND As the main source of iodine intake, water is of practical significance to detect the content of iodide accurately. At present, the common methods for the analysis of iodide in water samples include ion chromatography, gas chromatography, colorimetry, spectrophotometry. The analysis results of different methods can be affected by the actual sample matrix and experimental conditions. Our project team organized 38 laboratories to determine the content of iodide in natural water samples by ion chromatography, starch spectrophotometry, catalytic reduction spectrophotometry, and inductively coupled plasma-mass spectrometry (ICP-MS), the results showed that there were significant differences among the measured values by different methods, and the data were obviously dispersed.

OBJECTIVES To discuss the reasons for the differences between the results of different methods and give suggestions on the selection of iodide analysis methods under different conditions, based on the principles and conditions of each method.

METHODS Four analysis methods, including ion chromatography, starch spectrophotometry, catalytic reduction spectrophotometry, and ICP-MS, were used to determine the content of iodide of the groundwater sample by 38 laboratories. High performance liquid chromatography-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS) was used to determine the content of iodide by our own laboratory.

RESULTS (1) HPLC-ICP-MS can be used to effectively separate iodide and iodate ions in water samples. Through the quality control of the experimental process by blank samples, standard reference materials and replicate samples, the analytical results of this method were accurate and reliable. The determination results of iodide in JSH-2 samples were 78.32μg/L.　　(2) The determination values of iodide by ion chromatography (83.38μg/L) were consistent with those by HPLC-ICP-MS (78.32μg/L). However, the data of ion chromatography between different laboratories were obviously dispersed. The determination values of iodide by the starch spectrophotometry (92.95μg/L), catalytic reduction spectrophotometry (101.84μg/L) and ICP-MS (103.13μg/L) were higher than those by HPLC-ICP-MS.　　(3) The reasons for the differences between the results of different methods were discussed, based on the principles and conditions of each method: 　　① The determination results of ion chromatography may be affected by the selection of experimental conditions, such as sample pretreatment, chromatographic column and detector, and other interfering components.　　② It is not considered whether there is iodate ion in the sample itself in the starch spectrophotometry. Therefore, when there are iodate ions in the water sample, the determination result is actually the total content of inorganic iodine.　　③ The standard working curve of the catalytic reduction spectrophotometry is bent downward as a whole and does not show a good linear relationship. Therefore, when the response value of iodide is high, the concentration value will be high.　　④ ICP-MS is only used to determine the total content of elements, and cannot be used for elemental speciation analysis. There are iodate ions in the samples of this experiment, so the determination results of ICP-MS should be the total content of iodine, rather than the content of iodide.

CONCLUSIONS When iodate ions are present in water samples, starch spectrophotometry and ICP-MS may lead to a high result of iodide determination. Among them, starch spectrophotometry is usually suitable for samples with an iodide concentration of 25-500μg/L, and ICP-MS can be combined with other separation techniques to improve method selectivity. When iodate ions are present in water samples, catalytic reduction spectrophotometry and ion chromatography can be used to analyze the concentration of iodide, but the influence of sample concentration level, matrix interference components, experimental conditions and other factors should be considered. HPLC-ICP-MS can be applied to the quantitative analysis of iodine species in water samples, which can avoid the influence of iodate ions on the accuracy of iodide determination results.