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Comprehensive Handbook of Iodine: Nutritional, Biochemical, Pathological and Therapeutic Aspects
Academic PressCopyright © 2009 Elsevier Inc.
All right reserved.
Chapter OneDetermination of Iodine in Seawater: Methods and Applications
Hua-Bin Li Department of Botany, The University of Hong Kong, Pokfulam Road, Hong Kong, China
Xiang-Rong Xu Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China
Feng Chen Department of Botany, The University of Hong Kong, Pokfulam Road, Hong Kong, China
The ocean is a huge reservoir of iodine. The transfer of iodine from the sea to the atmosphere, and subsequent deposition onto soils and incorporation into plants and animals, is one of the main pathways for iodine to enter into the human food chain. Kelp, being a good source of iodine, is taken as a food and nutritional supplement. Iodine is also extracted from seaweeds or seawater, and added to edible salt to supplement the intake of iodine for people who live in iodine-deficient areas. Iodine exists mainly as iodide and iodate, along with a small fraction of organic iodine compounds in seawater. The distribution of iodide and iodate in seawater varies with depth and geographical location. The determination of iodine in seawater is helpful in understanding the marine environment. This chapter provides an overview of the methods employed for the separation and determination of iodine in seawater, including capillary electrophoresis, ion chromatography, high- performance liquid chromatography, gas chromatography, spectrophotometry, ion-selective electrode, polarography, voltammetry, atomic emission spectrometry, and neutron activation analysis. The advantages and limitations of these methods are assessed and discussed.
AES Atomic emission spectrometry CE Capillary electrophoresis GC Gas chromatography HPLC High-performance liquid chromatography IC Ion chromatography ICP-AES Inductively coupled plasma atomic emission spectrometry NAA Neutron activation analysis UV Ultraviolet
Iodine is an essential component of the thyroid hormones that play an important role in human development, growth, and metabolism, especially of the brain. Iodine deficiency in humans can cause several diseases or problems, which include spontaneous abortion, increased infant mortality, cretinism, goiter, and mental defects (Li et al., 2001; Rong and Takeuchi, 2004). Seawater is a huge reservoir of iodine. One of the major pathways for the entry of iodine into the human food chain involves the transfer of iodine from the sea to the atmosphere, its subsequent deposition onto soil and incorporation into plants and animals (Chance et al., 2007). Kelp, a type of macro algae in the ocean, is a rich source of iodine and is thus used as a food and nutritional supplement. Iodine in seaweeds is mainly iodide, with a very small fraction of iodine present as iodate and organoiodine in the form of monoiodotyrosine and diiodotyrosine (Martinelango et al., 2006). Marine algae are also used as natural sources of iodine in the feeding of freshwater fish, which on consumption would improve the iodine intake of man (Schmid et al., 2003). Generally, iodine is extracted from seaweeds or seawater, and added to edible salt to supplement the intake of iodine for people who live in iodine-deficient areas. The main pathways for the entry of iodine into the human food chain are shown in Figure 1.1. Iodine in seawater can also have an impact on the global biogeochemical cycle of iodine, affecting the supply of iodine to the atmosphere from the oceans. Iodine atoms are involved in atmospheric ozone depletion and aerosol formation reactions in the marine boundary layer, and hence have an influence on the earth's radiative balance and weather, which in turn may affect human health (Chance et al., 2007).
Iodine is ubiquitous in seawater having a total dissolved concentration of about 400–500 nmol · l-1 in most places, where it exists mainly as iodide and iodate along with a small fraction of organic iodine compounds. The distribution of iodide and iodate in seawater varies with depth and geographical location. The thermodynamically stable form of iodine in seawater is iodate, which is the dominant form in most of the oceans. Iodide in the oceans is produced by biologically mediated reduction of iodate, also favorable under reducing conditions. Up to 50% of the dissolved inorganic iodine may be present as iodide in surface seawater. Organic iodine constitutes less than 5% of the dissolved iodine in the open ocean, but a large fraction (40–80%) of the dissolved iodine may be present in an organic form in estuarine and coastal waters (Chance et al., 2007; Ito et al., 2003).
The determination of iodine in seawater helps in understanding the marine environment. A variety of analytical methods have been proposed for the quantitative determination of iodine in seawater. This chapter discusses the methods employed for the separation and determination of iodine in seawater. These methods include capillary electrophoresis (CE), ion chromatography (IC), high-performance liquid chromatography (HPLC), gas chromatography (GC), spectrophotometry, ion-selective electrode, polarography, voltammetry, atomic emission spectrometry (AES), and neutron activation analysis (NAA). The advantages and limitations of these methods are also assessed and discussed. Since iodine is present in the ocean at trace levels and the matrices of seawater are complex, especially in estuarine and coastal waters, the methods developed for the determination of iodine in seawater are usually of high sensitivity and selectivity (Chen et al., 2007; Li et al., 2001). Thus, these methods could also be used for the determination of iodine in other samples, such as food, blood, and urine samples, either directly or with minor modifications.
CE is based on the different mobilities of ions in an electric field, and is used for the analysis of charged species. It is a separation technique of high efficiency with low sample and solvent consumption (Li et al., 2004). CE is a powerful tool for separation and quantification of inorganic ions. However, when the concentrations of target analytes are very low in samples, such as iodide and iodate in seawater, the method's performance turns out to be challenging because of the inherent limitations of concentration sensitivity. Incorporation of an online preconcentration technique helps address this challenge, and transient isotachophoresis appears to be one of the most viable options enabling high-sensitivity detection of trace-level inorganic analytes. The sample components can be enriched from diluted solutions by advanced concentration factors, i.e., up to 100 or more (Ito et al., 2003). Thus, preconcentration techniques using transient isotachophoresis were widely adopted in a number of capillary electrophoretic methods for the determination of iodide and/or iodate in seawater.
Capillary electrophoresis was used for the determination of iodide in seawater, human urine and serum, and cooking salt (Pantuckova and Krivankova, 2004). The best separation results of iodide from other macro- and microcomponents in the tested matrices were obtained when host–guest interaction with alpha-cyclodextrin or ion-pairing with polyethylenimine was employed. Due to the relatively high cost of cyclodextrin, only the method using polyethylenimine was developed. The samples were injected into a fused-silica capillary coated with polyacrylamide and filled with the optimized background electrolyte composed of 20 mmol · l-1 KH2PO4 and 0.7% polyethylenimine. The detection limits at 230 nm were 0.17 µmol · l-1 for seawater, 0.14 µmol · l-1 for human urine, 0.17 µmol · l-1 for human serum, and 89 nmol · l-1 for cooking salt, respectively. The relative standard deviations of the peak area and height in all matrices ranged between 0.93% and 4.19%.
Capillary zone electrophoresis with transient isotachophoresis as the online concentration procedure was developed for the determination of iodide in seawater (Yokota et al., 2003). The effective mobility of iodide was decreased by the addition of 10 mmol · l-1 cetyltrimethylammonium chloride to an artificial seawater background electrolyte so that transient isotachophoresis functioned. The detection limit of iodide was 3.0 µg · l-1. The relative standard deviations of peak area, peak height, and migration time for iodide were 2.9, 2.1, and 0.6%, respectively. The proposed method was applied to the determination of iodide in seawater collected around the Osaka Bay. In addition, an improved transient isotachophoresis procedure was developed for the preconcentration of iodide from highly saline matrices (Hirokawa et al., 2003). The procedure took advantage of introducing cetyltrimethylammonium chloride into the high sodium chloride background electrolyte, which was due to a specific interaction with iodide-amended placement of the analyte at a large distance from the matrix chloride. Computer simulation showed that 2-(N-morpholino)ethanesulfonate could be adopted as a suitable terminating ion to enable isotachophoretic focusing at the beginning of the capillary electrophoretic run. The sensitivity response of iodide was improved by a factor of 140 over normal capillary electrophoretic mode. This allowed direct ultraviolet (UV) detection of as low as 0.6 µg · l-1 iodide, and made capillary electrophoretic analysis of undiluted surface seawater samples feasible. The proposed method could be extended to the determination of other trace anions (e.g., iodate) in seawater. Furthermore, iodide and iodate in seawater could be determined simultaneously using capillary zone electrophoresis with transient isotachophoresis as an online concentration procedure (Yokota et al., 2004). The effective mobility of iodide was decreased by an addition of 20 mmol · l-1 cetyltrimethylammonium chloride to an artificial seawater background electrolyte so that transient isotachophoresis functioned for both iodide and iodate. The detection limits of iodide and iodate were 4.0 and 5.0 µg · l-1 (as iodine), respectively. The relative standard deviations of the peak area, peak height, and migration times for iodide and iodate were 2.9, 1.3, 1.0, 2.3, 2.1, and 1.0%, respectively. The electropherogram of iodide and iodate in surface seawater is shown in Figure 1.2. A similar technique was developed by Huang et al. (2004b) for simultaneous determination of iodide and iodate in seawater. The proposed method was based on the on-capillary preconcentration of iodide and iodate using the principle of transient isotachophoresis stacking, and direct UV detection of the separated species at 226 and 210 nm, respectively. The preconcentration procedure took advantage of the electrokinetic introduction of the terminating ion 2-(N-morpholino)ethanesulfonate into the capillary, which enabled a longer transient isotachophoresis state. The valid calibration was demonstrated in the range of 3-60 µg · l-1 for iodide and 40–800 µg · l-1 for iodate. The detection limits were 0.23 µg · l-1 (2 nmol · l-1) for iodide and 10 µg · l-1 (57 nmol · l-1) for iodate. The method could be applied to direct speciation analysis of surface and seabed seawater. The comparison of capillary electrophoretic results with those of an IC proved that the method had acceptable accuracy.
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