The determination of metal-containing additives in plastic materials via acid digestion protocols has attracted growing interest to address potential environmental implications. However, the lack of protocol harmonization hinders data comparability within the literature. Here, six acid digestion protocols were employed to determine the metal(loid) content in plastics: these included three different acid mixtures (HNO3 combined with H2SO4, HCl or H2O2) for microwave-assisted digestion, with or without an additional room-temperature digestion step with H2O2. Each protocol was first validated for seven metal(loid)s (As, Cd, Cr, Pb, Sb, Sn and Zn) using a low-density polyethylene (LDPE) certified reference material (ERM®-EC681m). Then, validated protocols were applied on end-use materials, including conventional (i.e., LDPE) and compostable (i.e., PBAT/TPS) plastics. The combination of H2SO4 and HNO3 with a further digestion step with H2O2 was the most suitable protocol: it successfully passed validation thresholds for all metal(loid)s (recoveries in the range 98.6-101.0 %) and yielded the highest concentrations in end-use materials. All other protocols resulted in a less efficient digestion of the sample matrix, leading to lower recoveries and the formation of solid residues. Notably, end-use plastics showed a great variability in metal(loid) concentrations, likely due to their additive-rich composition, in contrast to the minimal content of acid-soluble additives of the reference material. This study represents an initial step towards the harmonization of acid digestion protocols and highlights new challenges in accurately analyzing end-use plastic materials, due to their complex additive composition.

Department of Human and Innovation for the Territory University of Insubria Via Valleggio 11 22100 Como Italy

Department of Science and High Technology University of Insubria Via Valleggio 11 22100 Como Italy

Department of Science and High Technology University of Insubria Via Valleggio 11 22100 Como Italy; Norwegian Institute for Water Research Økernveien 94 0579 Oslo Norway

Faculty of Chemistry and Chemical Technology University of Ljubljana 113 Večna pot 1000 Ljubljana Slovenia; Faculty of Mechanical Engineering Brno University of Technology Technická 2896 2 61669 Brno Czech Republic

Norwegian Institute for Water Research Økernveien 94 0579 Oslo Norway; RECETOX Masarik University Kamenice 753 5 625 00 Brno Czech Republic

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