Abstract
Globally it has been identified that most countries are struggling with micronutrient deficiencies that affect both humans and animals. The primary cause of micronutrient deficiency is the lack of diversity in diets, especially those containing the required nutrients for human health. Different strategies have been used to combat micronutrient deficiency, and biofortification seems feasible and has been applied. Obtaining information about biofortification strategies that can help overcome micronutrient deficiencies is essential. Assessing these micronutrients' nutrient concentration and chemical speciation within the biofortified products is also necessary. It has been proven to be a challenge to measure selenium (Se) levels globally, and it has been identified as one of the essential micronutrients to human health. The challenges include a variety of methodologies used for Se detection and different regulations applied for assessment reports. This study aimed to develop analytical methodologies for selenium quantification and speciation in fortified and biofortified food samples sourced locally and others imported from Poland. Also, water samples collected from various Provinces within South Africa were analysed. The quantification of Se in food and water samples was done using inductively coupled plasma optical emission/ mass spectrometry (ICP-OES and ICP-MS) and high-pressure liquid chromatography coupled with a diode array detector (HPLC-DAD).
The first objective was to develop a dispersive magnetic solid phase extraction method to quantify Se in water and food samples. Firstly, a magnetic adsorbent material composed of zeolite, iron oxide and alumina were synthesised and characterised using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM), Zeta potential analysis and Brunauer-Emmett-Teller. The adsorbent material was used to develop a reliable magnetic phase extraction method to extract and preconcentrate Se in water and food samples. To examine the method's optimum conditions, central composite design (CCD) was used to investigate the experimental conditions influencing the extraction process. At optimum conditions, the newly developed method offered limit of detection (LOD) and limit of quantification (LOQ) 0.018 and 0.060 μg/L, respectively. The linear range obtained was 0.05- 500 μg/L with an enhancement factor of 112 and a precision of less than 5%. The accuracy of both methods in the first and second objectives was validated using certified reference materials (IRMM 804 rice flour and NIST 1567b wheat flour), and the results obtained were in close agreement with those reported in certified values.
v
This study's second objective was to determine Se in food samples (fortified and biofortified) after digestion using an alcohol-based deep eutectic solvent (DES). The DES mixture was synthesised using choline chloride and phenol. The DES solvent was characterised using FTIR. Factors affecting the extraction efficiency of the method were optimised using surface response methodology based on central composite design (CCD). Under optimised conditions, the limit of detection (LOD) and quantification (LOQ) were 0.01 and 3.8 μg/g with a linearity of 0.01- 0.2 μg/g Se.
Lastly, a simple and fast extraction method that is environmentally friendly was developed for the determination of Se in food samples using HPLC-DAD and ICP-MS while carrying out speciation studies from the results obtained. Factors that affect the efficiency of HPLC method were optimised, including mobile phase, wavelength selection, analysis mode of HPLC, extraction time and extraction solvent. Under optimum conditions, the technique achieved LOD and LOQ of 0.58 and 1.9 μg/L, respectively. The obtained linear range was 1.9- 250 μg/L with a precision of less than 5%.
All the objectives set up in this study were achieved, three different methods for analysis of organic and inorganic Se species were developed and applied in fortified and biofortified food samples. These methods include the application of solid phase microextraction and liquid phase microextraction techniques that use non- chromatographic and chromatographic techniques for analysis and detection of Se species. Additionally, the results obtained from these different methods has enabled the speciation study of Se present within the food samples.