Development of an eco-friendly sample preparation protocol for metal determination in food samples: an oxygen pressurized single reaction chamber using diluted nitric acid

Abstract

Many efforts have been recently made to improve the digestion efficiency by using powerful equipment or by using an auxiliary reagent. In this work, an alternative method is reported, which explores a digestion system based on a single reaction chamber (SRC) technology pressurized with O₂ for reducing the amount of acid, without impairing the digestion efficiency. Before digestion, the system was pressurized with compressed air (20 bar, 20% O₂) while the temperature was evaluated from 180 up to 270 °C. The procedure was also carried out under O₂ pressure (20 bar). For each temperature several acid concentrations were evaluated (0.1 to 3 mol L⁻¹ HNO₃), being possible to correlate the effectiveness of each acid concentration with temperature. The proposed method was applied to the simultaneous digestion of several organic matrices with variable content of fat, protein, and carbohydrate (whole milk powder, bovine liver, parsley, and linseed). The residual carbon content was lower than 4% (C lower than 200 mg L⁻¹ in digests), showing the high digestion efficiency of the proposed approach. Up to 250 mg of all food matrices were digested using a sub-stoichiometric amount of HNO₃ (1 mol L⁻¹ solution), which was only achieved due to the use of O₂ as an auxiliary reagent. Barium, Ca, Cu, Fe, K, Mg, Mn, Na, Sr, and Zn were determined by ICP-OES, and the accuracy was better than 95% for standard reference materials of corn bran, whole milk powder, and bovine liver. It is an important feature, being in agreement with green chemistry recommendations because very low amounts of reagents are required for sample digestion, as well as low amounts of residues are generated.