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Abstract

This thesis deals with the determination of fluorine in various food and environmental samples. A method for fluoride determination is fluoride ion selective electrode (F-ISE). This method relies on the determination of free fluoride ions (F–), therefore the samples must undergo total sample decomposition prior to F– measurement. Three methods for total sample decomposition are used in our laboratory. Analyses of certified reference materials with matrix similar to analysed samples proved that alkaline carbonate fusion and pyrohydrolysis assure accurate results within the certified intervals for soil samples and that alkaline carbonate fusion and oxygen bomb combustion assure accurate results for plant samples. However, lower results were observed in the case of oxygen bomb combustion. By calculating Gibbs energies for reactions of some metal fluorides proceeding during these decomposition methods, it was demonstrated that reactions proceeding during alkaline carbonate fusion and pyrohydrolysis are thermodynamically favourable, while reactions proceeding during oxygen bomb combustion are not. Therefore, the results in the case of oxygen bomb combustion are probably lower because only the content of soluble fluorides is determined by F-ISE, while insoluble fluorides are not decomposed during the combustion.
The result of the analysis has to be reported with measurement uncertainty (MU). A procedure for evaluating MU based on the "Guide to the Expression of Uncertainty in Measurement" was developed for fluorine determination in plant samples. The calculated MUs represented 7–15 % of the result. The largest contribution was the uncertainty of bias (76–98 %), the contribution of the uncertainty of precision was 2–23 % and the other uncertainties (weighing, volumetric glassware, cell potential, concentration and density of standard F– solution) were negligible.
Accurate analytical methods for fluorine determination are of crucial importance because fluorine may be toxic to plants, animals and humans. Fluoride is taken up by plants from the soil via roots or from the air via leaf stomata and is then carried to the margins and tips of the leaves, where it accumulates and can cause necrosis and chlorosis. These symptoms were observed in some plant species in the vicinity of the Rogaška glassworks (Rogaška Slatina, Slovenia), where hydrofluoric acid is used for glass polishing. Fluorine content was determined in five plant species (Norway spruce, peach, hornbeam, bean and grape vine). High values (87–676 μg/g) in comparison to the unpolluted area (10 μg/g) confirmed the uncontrolled release of gaseous fluorides from the glassworks.
Fluoride is beneficial for humans at low intake as it prevents dental caries, but chronic excessive intake can cause dental fluorosis in children and skeletal fluorosis in children and adults. An important source of fluoride for humans is diet. In recent years, so-called superfoods have become popular due to their supposed extraordinary health benefits. There is practically no information on the fluoride content in superfoods in fluoride databases, in scientific papers or on packaging labels; therefore, it was determined in 18 plant-based superfoods available on the Slovenian market. In eight superfoods (matcha, gotu kola, ginkgo biloba, tulsi, neem, brahmi, gynostemma, moringa), it was surprisingly high (16–373 μg/g) in comparison to other commonly consumed foods.