Report of the sixth interlaboratory comparison : total Cd, Pb and As, and Hg in food supplements

The Institute for Reference Materials and Measurements (IRMM) of the Joint Research Centre (JRC), a Directorate-General of the European Commission, operates the Community Reference Laboratory for Heavy Metals in Feed and Food (CRL-HM). One of its core tasks is to organise interlaboratory comparisons (ILCs) among appointed National Reference Laboratories (NRLs). This report presents the results of the sixth ILC o the CRL-HM which focused on the determination of total Cd, Pb, As and Hg, (related to dry mass) in food supplements. The test material used in this exercise was the standard reference material (SRM) SRM 3256, of the National Institute of Standards and Technology (NIST), the matrix being green tea of Chinese origin. The material was labelled at IRMM and dispatched to the participants on the first week of June 2009. Each participant received two sachets containing approximately 2.5 g of test material each. Thirty-three participants from twenty-five countries registered to the exercise of which 32 sets of results were reported for total Cd and for total Pb, and 29 for total As and for total Hg. One laboratory reported two sets of values for total Cd, Pb, As and Hg obtained with two different techniques, respectively. The assigned values were the certified values as provided by NIST. The uncertainties of the respective assigned values, uref, were also provided by NIST. Participants were invited to report the uncertainty on their measurements. This was done by the majority of laboratories taking part in this exercise. Laboratory results were rated with z and zeta scores in accordance with ISO 13528. Standard deviation for proficiency assessment (also called target standard deviation) was fixed to 15 % for total Cd, Pb and As by the advisory board of this ILC, on the basis of the outcome of previous ILC's organised by the CRL-HM for the same population. For total Hg the standard deviation for proficiency assessment was set to 22 % as derived by the modified Horwitz equation.