**4.1.4 Biological fluids and tissues**

Due to the noninvasive way of sampling, urine is the most commonly analysed biological fluid. Efficient management of national salt iodization programmes depends on quality data on iodine concentrations in the urine and salt samples. These data are crucial in the evaluation of iodine interventions. Most of the analytical methods for urinary iodine concentration are based on the manual spectrophotometric measurement of Sandell-Kolthoff reduction reaction catalyzed by iodine using different oxidising reagents in the initial digestion step (Jooste & Strydom, 2010). Bilek et al. (Bilek et al., 2005) used a method which was based on alkaline ashing of urine specimens preceding the Sandell-Kolthoff reaction using brucine as a colorimetric marker. The detection limit was 2.6 µg/l and the limit of quantification was 11.7 µg/l, with intra-assay precision of 4% and inter-assay precision of 4.9%.

Another study described simple photometric determination of the iodine concentration in the thyroid tissue of small animals. Again, the method was based on the well-known catalytic Sandell-Kolthoff reaction. Prior to the analysis, the tissue was digested in a mixture of sodium chlorate and perchloric acid at 100 degrees C. Using this manner of digestion between 94 and 110% of iodine in the sample was recovered. Comparison with the neutron activation analysis showed excellent agreement of the obtained values (Tiran et al., 1991).

#### **4.2 Chromatographic methods**

While the main advantage of catalytic spectrophotometric methods is low cost of the needed equipment, chromatography is arguably the most widely used separation technique in the modern analytical laboratory. Fast, simple, reliable and sensitive chromatographic systems coupled with various detectors became the basic tool in many analytical laboratories. Routine analysis of iodine compounds can be carried out by means of gas chromatography (GC) and high performance liquid chromatography (HPLC). Analysis of inorganic iodine species in waters is mainly carried out with the use of ion chromatography (IC) or IC inductively coupled-mass spectrometry (IC-MS). Separation methods enable direct determination of various species of iodine in the presence of various kinds of complex components with the detection limit in the range of sub μg/l or µg/l (Hu et al., 1999; Schwer & Santschi, 2003). The IC method can separate I directly by using anion-exchange column, while HPLC method usually uses the reverse phase column modified by an ionpairing reagent in the mobile phase. Both spectrophotometric and electrochemical detectors are commonly used. Pulsed amperometric detection (PAD) typically utilizes gold, silver, platinum and glass carbon electrodes.

It has to be emphasized that both spectrophotometric and chromatographic methods are not applicable to a wide range of matrices. As far as complex matrices are concerned (e.g. seawater with high content of Cl and Br- and relatively small of I-) IC and HPLC are useful tools for iodine determination. Unfortunately, none of these methods are flexible enough to measure all iodine species, including organo-iodine in biological samples. The oxidative pretreatment of biological materials limits the application of the described methods (IO3 - and I- can be converted to I2 under acidic conditions). In order to analyse IO3- , I-, and organic forms of iodine in the same sample, IC is often coupled with ICP-MS. What is more, the coupling of highly efficient IC to multi-dimensional detectors such as MS or ICP/MS significantly increases sensitivity, while simultaneously reducing possible matrix interference to the absolute minimum.
