Evaluation of testing capabilities for the determination of melamine in milk through an interlaboratory proficiency test programme during the melamine crisis

An interlaboratory proficiency testing programme for melamine in milk was organized for field laboratories in Hong Kong, China, during the melamine crisis in late September 2008. One blank test sample and three homogenous samples prepared by gravimetric spiking of melamine at the concentration range of zero to 4.5 mg kg-1 were given to participants in this programme. A total of 13 participants returned the results to the organizer and they used either liquid chromatography-tandem mass spectrometry (LC-MS/MS) or gas chromatography-mass spectrometry (GC-MS) for their determinations. The performance of the participants was assessed by determining z-scores, calculated from the bias from the assigned reference values and Horwitz standard deviation. The median values of pooled data were found to be in good agreement with the reference values and the majority of the participants demonstrated their capabilities in the quantitative measurement of melamine in milk samples. However, four participants gave false-positive results for the blank test sample, probably due to cross-contamination from other samples, and they were requested to investigate the actual causes. In summary, eight participants (or 62%) demonstrated their competence for all the four test samples.