Report of IMEP-27 : total Cd, Pb and As, and extractable Cd and Pb in mineral feed

This report presents the results of an Interlaboratory Comparison (ILC) which focused on the determination of total Cd, Pb and As and extractable Cd and Pb in 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 a commercial mineral feed provided by AGES, Zentrum Analytik und Mikrobiologie, in Austria. The material, naturally contaminated, was processed, bottled, labelled and dispatched by the Reference Materials Unit of the IRMM. The samples were dispatched on the second half of November 2008. Each participant received one bottle containing approximately 30 g of test material. Twenty-four participants from 11 countries registered to the exercise of which 19 submitted results for extractable Pb and Cd and 20 submitted results for total Pb, Cd and As. Three laboratories did not submit results. The assigned values (Xref) for total and extractable Cd and Pb were provided by IRMM using isotope dilution-inductively coupled plasma-mass spectrometry (ID-ICP-MS). The analytical uncertainty of Xref, uchar, was calculated according to the ISO Guide to the Expression of Uncertainty in Measurement (GUM). The assigned value for total As was provided by the Studiecentrum voor Kernenergie (SCK-CEN) using neutron activation analysis. The analytical uncertainty of Xref, uchar, for total As was calculated according to GUM. Homogeneity and stability studies were subcontracted to Bayer Antwerpen. The uncertainties of the respective assigned values, uref, were calculated combining the analytical uncertainty, uchar, with a contribution for the between-bottle homogeneity, ubb, and for the short term stability of the test material, usts. Participants were invited to report the uncertainty of their measurements. This was done by 17 laboratories for extractable Pb and Cd and by 18 laboratories for total As, Cd and Pb. The laboratory performance was evaluated using z and zeta scores in accordance with ISO 13528. The standard deviations for proficiency assessment (also called target standard deviation) were calculated using the modified Horwitz equation and were between 15 and 16 % for all the measurands.