Final report on CCQM-K85: Malachite green in fish tissue

Abstract

During a review of its activities, the organic analysis working group of the consultative committee for amount of substance noted that evidence supporting institute capabilities for providing services supporting the food sector was lacking. A request was made for a proposal of a study that would imply an institute's broader capability for the analysis of trace residues in food.Malachite green is a triphenylmethane dye with powerful antifungal properties. Its presence in food is not permitted and routine monitoring is performed in many regions at the µg/kg level. Its measurement is considered challenging for a number of reasons. Malachite green readily metabolises to leucomalachite green, with the sum of both forms being reported as 'total' malachite green. However, the two compounds have different stability and extraction requirements and often readily interconvert, proving a challenge for the successful quantification of both. The study material was a specially prepared homogenized salmon tissue from fish that had been administered with malachite green. Laboratories were requested to measure malachite green and leucomalachite green with the sum of both being reported and used for this key comparison. All laboratories used liquid chromatography–mass spectrometry with a compound-specific isotope dilution method.Six laboratories participated in the study, with all six providing results for malachite green, leucomalachite green and 'total' malachite green, reported as the sum of the two forms. A number of estimators were proposed for the calculation of the key comparison reference value (KCRV), with the working group deciding on the median and its uncertainty estimate as the most appropriate. This resulted in a KCRV of (9.77 ± 0.35) ng/g. The calculated degrees of equivalence for all laboratories were less than 1 ng/g. This was considered exceptional, especially when the complexity of the matrix, the differential partitioning of the two interconverting compounds and their sub 10 ng/g levels in the fish are considered. Participating labs demonstrated their capability in extracting trace levels of potentially unstable compounds from a solid matrix. Successful participation is considered indicative of an institute's ability to measure low levels (1 ng/g to 100 ng/g) of medium to high polarity residues in muscle tissue.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).