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Abstract

Iodine is an essential element for humans and other mammals, important for normal growth and development. It is a component of thyroid hormones, which regulate metabolism and thus many important biochemical reactions. The main natural iodine source is seafood but the majority of iodine comes into the body by iodized salt, which has been introduced to reduce iodine deficiency disorders. However, the consumption of salt must be reduced and new/additional sources of iodine for human nutrition must therefore be found, one of which could be iodine enriched plants.
However, before such plants can be introduced into nutrition, it is important to know the iodine content, in order to control iodine intake. In the first part of the thesis, therefore, a method for iodine determination by ICP-MS after alkaline microwave extraction was optimized. Firstly, Schöniger combustion was compared to microwave extraction, which was chosen for further work. The temperature program and ratio between water and tetramethylammonium hydroxide (TMAH) were optimized in the sample preparation step. For iodine determination, the concentration of TMAH in the measured solution was optimized, external calibration was compared to the standard addition technique for quantitative iodine determination and the need to use an internal standard was evaluated. In addition to evaluation of LODs, linearity and repeatability, the independent method k0-INAA was used to confirm the trueness of the obtained results, because of the lack of biological materials with certified iodine value.
Due to possible differences in the bioavailability, mobility and toxicity of different species of the elements, speciation studies are also important. Thus, a method for iodine speciation by HPLC-ICP-MS was also optimized, with special interest in the extraction procedure and stability of the species. In the first step, the suitability of an ultrasonic probe for extraction of iodine species was evaluated and, because of some difficulties using the ultrasonic probe, a second step – optimization of the enzymatic extraction for iodine speciation analysis – was performed with a water bath with longitudinal shaking. Iodate interconversion into iodide was observed in the presence of the enzyme or plant material and conditions to prevent this transformation could not be found, so the method is appropriate for determination of MIT, DIT and inorganic iodine. The best enzyme for extraction of iodine species was selected, dissolving enzyme in water or in buffer was tested and the optimal temperature and time were selected. Column recovery, limits of detection, linearity, repeatability and procedure of recovery were also evaluated.
Iodine content was determined in pea, pumpkin and buckwheat sprouts, grown from seeds soaked in solutions of different iodine forms and combinations of iodine and selenium forms. Furthermore, iodine content was determined in different parts of pea plants, pumpkin seeds, buckwheat achenes and chicory leaves, foliar sprayed with the same solutions as used for seed soaking. In all of the investigated plants or parts of plants, elevated iodine concentrations were observed, compared to controls, except in pumpkin seeds. Additionally, in some of the investigated parts of plants, only treatment with one of the forms resulted in higher iodine content. Comparing all selected plants, each of them responded differently to the addition of iodide and iodate and there was no general trend of better uptake of one or another of the forms.
Selenium also had various effects on iodine uptake; in some cases it did not affect iodine uptake, in some cases it had a synergistic effect, while in others an antagonistic effect, which could not be explained.
Iodine compounds were determined in iodine enriched pea, pumpkin and buckwheat sprouts. The results showed that iodine was mostly present in inorganic form and small amounts of MIT were also observed. DIT was not present in the analyzed plant samples or it was in concentrations lower than the detection limit. Some unknown iodine species were also observed in traces. Good results in terms of extractible iodine (around 75% on average) and identified species (around 90% on average) were observed. Buckwheat sprouts grown from seeds soaked in a solution of iodine and selenium combinations were also analyzed and the results showed no influence of selenium presence on iodine compounds.