The IRMM International Measurement Evaluation Programme (IMEP) IMEP-4: Trace elements (Li, Cu, Zn) in serum

The aim of the International Measurement Evaluation Programme (IMEP) is to give an objective picture of state-of-the-practice analytical measurements by comparing them to a reference value obtained by a primary method of measurement. The referencevalue is therefore as traceable to the SI system of measurements as can presently be achieved and is independent of human or political decisions. Thus a large scale field test is made to achieve (international) comparability of measurements by comparing them with an independent metrologically established value. In the fourth measurement round, IMEP-4, three trace elements, Li, Cu and Zn, at three different levels in (bovine) serum were measured by about 12 participating laboratories using their routine methods. The (coded) results are graphically reported and compared to certified reference values established by an isotope-specific method, isotope dilution mass spectrometry, defined as a primary method of measurement by the Consultative Committee on Amount of Substance. Results indicate a spread of more than ±50%, asymmetrically distributed around the reference value with its own uncertainty range, although the self-declared accuracy of the laboratories was 5–10%. Self-assessment by participants of their analytical capabilities does not show a high correlation between self-rating ("more experienced" or "less experienced") and actual performance. In the way they have been applied, all methods seem to produce results of approximately the same quality.