IMEP-31 : total arsenic, cadmium, copper, lead and mercury, as well as extractable cadmium and lead in mineral feed : interlaboratory comparison report

The Institute for Reference Materials and Measurements (IRMM) of the Joint Research Centre (JRC), a Directorate-General of the European Commission, operates the International Measurement Evaluation Programme® IMEP. It organises interlaboratory comparisons (ILC's) in support to EU policies. This report presents the results of an ILC which focussed on the determination of total As, Cd, Cu, Pb, and Hg, as well as extractable Cd and Pb in mineral feed according to Directive 2002/32/EC of the European Parliament and of the Council on undesirable substances in animal feed. The test material used in this exercise was the Certified Reference Material (CRM) BCR-032 (Moroccan phosphate rock) from the IRMM. The material was relabelled and each participant received one bottle containing approximately 100 g of test material. Fifty-six laboratories from 26 countries registered to the exercise and 51 of them reported results. Total As, Cd, Cu and Hg were certified in BCR-032 in 1979. The material was re-analysed by two expert laboratories and As an Cd values could be confirmed. Copper could not be analysed in time by an expert laboratory, and thus it was decided to use the indicative value from the certificate as assigned value. The assigned values for total Hg and total Pb were determined at IRMM by a primary method. The same method was used to determine extractable Cd and Pb, whose mass fractions appeared to be identical to the respective total mass fractions and thus the same assigned values were used. The standard deviation for proficiency assessment was set at 11 % for total As, 10 % for total and extractable Cd, 9 % for total Cu, and at 15 % for total Hg based on the modified Horwitz equation and/or the outcome of previous ILCs organised by IMEP. For total and extractable Pb, was set at 25 %. The majority of the laboratories reported uncertainties with their results and were rated with z- and ζ-scores (zeta-scores) in accordance with ISO 13528. Performances appear to be good for total & extractable Cd and total & extractable Pb, the percentage of satisfying z-scores ranging between 85 % and 89 %. Share of satisfactory z-scores are significantly lower for total As (61 %), Cu (67 %) and in particular for Hg (47 %). No distinct reason could be given, but it seems altogether that the analytical methods were not always adjusted to the inorganic test material, reflected by some influence of applied technique and inappropriate choice of reference material.