Determination of Inorganic Selenium and Organic Selenium in Selenium Fortified Infant Formula Foods by Inductively Coupled Plasma Mass Spectrometry with Solid Phase Extraction

A method for the determination of inorganic selenium and total selenium in selenium fortified infant formula foods by inductively coupled plasma mass spectrometry （ICP-MS） with solid phase extraction was proposed, and the determination of organic selenium was calculated by the subtraction method. For the determination of total selenium in the sample, 0.5000 g of sample was taken and placed in a digestion tank, and 5 mL of nitric acid was added. The mixture was digested at 185 ℃ for 20 min, the acid was driven off at 120 ℃ until the solution was evaporated to near dryness. After cooling, the mixture was made its volume up to 50 mL with 0.3 mol·L⁻¹ hydrochloric acid solution. For the determination of inorganic selenium in the sample, 1.000 0 g of sample was taken and placed in a 50 mL-centrifuge tube, and 40.0 mL of water was added. The mixture was vortexed for 10 min and sonicated for 10 min, and 0.4 mL of 3% （volume fraction） acetic acid solution was added. The solution was shaken well and settled, and after precipitation, the mixture was centrifugated for 10 min. 5.0 mL of supernatant was taken, and the pH was adjusted to 5‒7 with 5 g·L⁻¹ sodium hydroxide solution, and the solution was transferred to an activated Oasis WAX solid phase extraction column. It was rinsed with 5 mL of water, elution was performed with 5 mL of 0.3 mol·L⁻¹ hydrochloric acid solution, and the eluate was collected. Determination was performed by ICP-MS in the condition of oxygen reaction mode, and 1 mg·L⁻¹ germanium internal standard solution was added online. As shown by the results, linear relationship between the ratios of the corresponding signal response intensity to the internal standard signal response intensity and mass concentrations of the inorganic selenium was kept in the range of 0.5‒10 μg·L⁻¹, and the detection limit （3s） was 0.1 μg·（100 g）⁻¹. Test for recovery was made by the standard addition method, giving results in the range of 91.0%‒97.0%, with RSDs （n=6） of the determined values in the range of 2.0%‒2.4%. This method was applied to the analysis of 14 selenium fortified infant formula food samples, and selenium was mainly presented in organic form. Among them, the proportion of inorganic selenium in selenium fortified infant formula food for special medical purposes was higher than 10.00% and lower than 14.00%, while the proportion of inorganic selenium in ordinary selenium fortified infant formula food was lower than 6.00%.