Performance assessment for determining malachite green and leucomalachite green in swamp eel (Monopterus albus) muscle using assigned reference values in a proficiency test.

This paper presents the results of a proficiency test (APLAC T058) for malachite green (MG) and leucomalachite green (LMG) in swamp eel (Monopterus albus). The programme was organized by the Hong Kong Government Laboratory and Hong Kong Accreditation Service (HKAS), under the auspices of the Asia-Pacific Laboratory Accreditation Co-operation (APLAC) in 2007. Results submitted by participants were compared with the assigned reference values, which were determined by an accurate liquid chromatography-isotope dilution mass spectrometry (LC-IDMS) technique, and their performance was evaluated on the basis of z-score index. The distribution of data was very wide and discrepancy from the assigned reference values was found to be method dependent. Only 48% and 62% of participants achieved satisfactory z-scores (i.e. |z| <or=2) for MG and LMG, respectively. With the provision of reference values, the programme served to establish an external quality control mechanism for assessing the measurement capabilities of participating laboratories in the determination of MG and LMG in eel samples.