Abstract

This report describes the results of an interlaboratory comparison study on the analysis of dioxins in cow's milk held between the laboratory Okometric GmbH, Bayreuth, F.R.G. and the Laboratory of Organic Analytical Chemistry (LOC), National Institute of Public Health and Environmental Protection (RIVM), Bilthoven, The Netherlands. The study included the duplicate analysis of three cow milk samples using in house methods and standards. The comparison showed considerable differences in the level of individual congeners. In general, values found by Okometric were higher than that of RIVM/LOC, with a ratio between values ranging from 0.95 to 6.9 for the majority of congeners except for octa CDF and CDD for which larger differences were observed. Best correspondence was obtained for the toxicological major congeners in cow's milk, i.e. 2,3,7,8-TCDD, 1,2,3,7,8-PeCDD and 2,3,4,7,8-PeCDF, for which ratio's have been obtained of 2.7, 1.2 and 1.6, respectively. As a result, when expressed in toxicological equivalences (TEQ), the mean level as determined by Okometric was by 80% higher than the mean TEQ value determined by RIVM/LOC. From an analysis of methods used, it is concluded that the major part of occurring differences can be ascribed to the performance of the combined gas chromatography-mass spectrometry (GCMS) instrument, in particular to the separation efficiency of the chromatographic column and the sensitivity of the mass spectrometer. Both features were found inadequate for the GCMS facility of Okometric for performing reliable analysis of dioxines at the low ppt level in cow's milk.