Reliability of proficiency test results for metals and phthalates in plastics

In a recent article it was concluded that the Horwitz curve could not be used for evaluation of results from analysis of plastics. This conclusion was based on 15 different proficiency tests of which 11 were organized by the Institute for Interlaboratory Studies. The authors suggest that part of the poor interlaboratory reproducibility can be explained by the inhomogeneity of consumer plastic products. Also, the lack of standard methods has been mentioned as problematic. The value of the iis data sets used by the authors is discussed in this paper. More recent data are also investigated to determine whether the situation with regard to analysis of plastics is as bad as suggested by Ritter and Meyer.