Certification of the lead mass fraction in wine for comparison 16 of the International Measurement Evaluation Programme

Comparison 16 of the International Measurement Evaluation Programme (IMEP) focused on the evaluation of measurement performance for the determination or the Ph mass fraction in a commercial red wine, resulting in a demonstrated degree of equivalence for laboratories internationally. IMEP-16 wine test samples were tested for homogeneity and stability to pressure and temperature variations (isochronous comparison). Pb concentration in the average test sample was certified using the combination of a straightforward microwave sample digestion protocol and inductively coupled plasma isotope dilution quadrupole mass spectrometry applied as a primary method of measurement. Certification of SI traceable reference values was achieved to 1.2% relative expanded uncertainty (k = 2). Nearly half of the uncertainty was due to the measured sample homogeneity correction factor (1 +/- 0.4%), followed by the contributions from the corrections for instrumental background and the dead time effects (respectively about 5%, and 23% of the calculated combined uncertainty), and the contribution from the measurements performed to correct for mass discrimination effects (12.3% in total). 129 laboratories from 33 countries participated in IMEP-16. The results are spread over a broad range of values (from <1 to >2900 mug L-1), and one third ties below -50% or above +50% deviation from the certified Pb concentration. About 40% of all participants reported accurate results to within +/- 10% uncertainty. For the vast majority, measurement uncertainty was largely underestimated. The quality of results does not seem to be correlated either to the self-declared experience in this type of measurements, or to the type of analytical technique applied. Identical samples were distributed to 14 National Measurement Institutes (from 12 countries and the IRMM) within the frame or the CCQM-P12 pilot study. All their results were in agreement within +/- 10% of the IMEP-16 certified value, thus offering a striking contrast with the spread of IMEP-16 results.