Conclusions from 13 years of stability testing of CRMs for determination of metal species

We present results of the systematic stability monitoring of certified reference materials (CRMs) for the mass fractions of metal species. Materials are tested regularly for the mass fractions of organotin, organomercury, organolead and organoarsenic species. Evaluation is based on ratios of results on samples stored under normal conditions and under even safer conditions (“reference stock”) to eliminate the effects of laboratory-to-laboratory variation. Of all results in this study, the results of organotin measurements show the highest relative standard deviations (up to 45%). Determination of methylmercury in sediment and fish is possible with much better precision (standard deviations of reproducibility below 5%). The accuracies of determinations of trimethyllead and organoarsenic species are slightly worse but still below 10%. The data confirmed stability since production (i.e. up to 13 years). However, shelf lives cannot be extended into the future based on these data, as uncertainty contributions for stability need to be included in the uncertainty budgets of CRMs to allow positive demonstration of the validity of certificates. Estimation of typical measurement uncertainties based on the data obtained from stability monitoring shows that potential degradation is negligible compared to uncertainties typical for this kind of measurement, demonstrating that the materials are still sufficiently reliable to demonstrate laboratory proficiency.