RESUMO
Vessel-inside-vessel microwave-assisted acid digestion was developed for the analysis of samples with high-unsaturated fat content. For the first time, thermal decomposition of (NH4)2S2O8 solutions was evidenced for SO3 generation in situ and gas-phase modification in pressurized digestion flasks. NMR analysis demonstrated the oxidative effect of SO3 on olefin double bonds despite incomplete mineralization of oil samples. In this context, (NH4)2S2O8 decomposition was used in association with HNO3 solutions for sample digestion and mineral determination in edible oils (safflower, coconut, flaxseed, and chia). For all oils, dissolved organic carbon (DOC) contents lower than 5% m m-1 were obtained under optimum conditions: 210 °C with an irradiation time of 40 min, 7.0 mol L-1 HNO3 and 2.0 mol L-1 (NH4)2S2O8 in 0.9 mol L-1 H2SO4. Thus, a DOC reduction of about 70% was reached compared to digestions using only HNO3 at the same conditions. Additionally, a time reduction of up to three-fold was achieved compared to typically demanding edible oil digestions. The proposed method allowed the determination of As, Cd, Cr, Mn, Ni, and Pb in edible vegetable oil samples by ICP-MS. Accuracy was evaluated against the reference method, and no significant difference was observed (p = 0.05), with wide linear ranges and good linearity (r ≥ 0.999) and LOD ranging from 0.48 (As) to 2.41 (Cd) µg L-1.