Determination of trace elements in food contact polymers by semi-quantitative inductively coupled plasma mass spectrometry. Performance evaluation using alternative multi-element techniques and in-house polymer reference materials

The determination of trace elements in a wide range of polymers intended for food contact use was carried out by inductively coupled plasma mass spectrometry (ICP-MS). Microwave digestion in concentrated nitric and sulfuric acids enabled rapid wet oxidation of all the polymers, to give samples suitable for introduction into the plasma. Semi-quantitative multi-element screening of the polymer digests could then be carried out using a single indium internal standard. Limits of detection for the polymers were generally in the order of 1 mg kg–1. Owing to the paucity of suitable standard reference polymer materials required for validation of the procedures used, a series of in-house reference materials was prepared by incorporating a cocktail of metal stearates into a range of polymer materials via the melt. Levels of elements in these reference materials were established by two reference methods: direct neutron activation analysis (NAA) and a quantitative ICP-MS method. Results for the two reference methods were encouragingly close to nominal added values and replicate analyses indicated that homogeneity was good. Further analysis of the materials using laser ablation ICP-MS confirmed that the distribution of elements within the polymer matrix was satisfactory. In addition, NAA was carried out on the great majority of the polymer samples as a further aid to validation of the semi-quantitative multi-element ICP-MS method.