Comparison of Microwave-Assisted Digestion and Consensus Open-Vessel Digestion Procedures for Evaluation of Metalliferous Airborne Particulate Matter

Abstract

Metal occupational exposure limits mainly focus on total content of the respective metals of interest. The methods applied for trace metal analysis in occupational health and safety laboratories are usually standardized to pragmatic consensus digestion schemes, ensuring comparability of results. The objective of the present study entailed the evaluation of a recently developed HNO3-only microwave-assisted digestion procedure by comparison with the German consensus hot-block digestion and other national digestion schemes. An inter-laboratory comparison test with participation of nine national occupational health and safety laboratories from Europe and North America was organized. For adequate emulation of what workers are at risk of inhaling four different industrial metal processing workplace dusts (electronic recycling, high-speed steel grinding, cylinder head cleaning, and battery combustion ash) were homogenized and sieved to the particle size < 100 μm diameter at IFA. The participants were asked to process air sample-typical amounts according to the German hot-plate technique, the IFA microwave-assisted digestion scheme as well as their national or in-house conventional digestion method for airborne dust and analyze for Cd, Co, Cr, Co, Fe, Mg, Ni, Pb, and Zn. Recoveries (relative to consensus open-vessel digestion) obtained for the new IFA microwave-assisted digestion were between 88 and 114% and relative reproducibility standard deviations were <10% for most metals of interest. The in-house digestion procedures applied varied widely but (whether microwave, hot block, or open vessel) yielded comparable results for the predominantly elemental alloy type dusts supplied. Results become more diverse for the combustion dust, especially if a combination of microwave-assisted digestion procedures with high temperatures and hydrofluoric acid is applied. ISO 15202-2 is currently being revised; this digestion procedure will be included as a possible variant in annex 2.