Investigation of Sample Preparation Methods of Dietary Supplements for Total Selenium Determination

This study investigated three sample preparation methods for digestion of dietary supplements prior to selenium (Se) determination by graphite furnace atomic absorption spectrometry (GF AAS), such as conductive heating digestion, microwave-assisted wet digestion (MWAD), and microwave-induced combustion (MIC). In wet digestion methods, variable nitric acid and hydrogen peroxide concentrations were evaluated. For MIC, a diluted nitric acid concentration was also investigated as absorbing solution, while microcrystalline cellulose was employed as a combustion aid. The optimized conditions were: 250 mg of the sample with 5 mol L^-1 nitric acid and 30% w/w hydrogen peroxide (160 °C, 5 h) for conductive heating; 350 mg with 2.5 mol L^-1 nitric acid and 30% w/w hydrogen peroxide (180 °C, 40 min) for MWAD; and 200 mg of sample, 300 mg of microcrystalline cellulose, and 0.25 mol L^-1 nitric acid as the absorbing solution (29.2 bar, > 1000 °C during the combustion reaction and 180 °C, 20 min in the reflux step) for MIC. Digestion efficiency was evaluated by residual carbon content (RCC) and residual acidity (RA) in final solutions. The greenness of each method was evaluated by using the AGREEprep software. Selenium was quantified by GF AAS using Pd as the chemical modifier using pyrolysis and atomization temperatures of 1200 and 2000 °C, respectively. The limits of detection (LOD) obtained for Se were 1.71, 0.84, and 0.08 μg g^-1 for conductive heating, MWAD, and MIC, respectively, with agreement of 96.7-99.5% for the selenium-enriched yeast certified reference material. Although all methods showed good agreement (ANOVA, 95% confidence level), MIC was the most efficient, providing the lowest RCC (0.1%) and RA (0.5%) values, better LOD, and a greenness score of 0.49.