The Geo<i>PT</i> Proficiency Testing Programme as a Scheme for the Certification of Geological Reference Materials

ISO Guide 35:2017 provides, for the first time, an alternative way of characterising certified reference materials using proficiency testing. In this paper, the properties of assigned values derived from the well-established GeoPT proficiency testing scheme are examined. This scheme, designed for laboratories that undertake the routine analysis of silicate rocks and related materials, routinely has over 100 participants contributing results. Following a detailed assessment of the metrological properties of GeoPT assigned values in relation to Guide 35 recommendations, it is demonstrated that these values may be regarded as certified values, provided a number of criteria are met. These criteria include: a demonstration of sufficient homogeneity of the candidate CRM; circulation, when judged to be appropriate of an established matrix-matched CRM for co-analysis in that round; the robust statistical analysis of data sets using GeoPT established-criteria to decide whether assigning a value is justified (based on a minimum of fifteen valid results);an expert group to manage the certification and the maintenance of appropriate records.In summary, the GeoPT proficiency testing scheme, subject to the arrangements summarised above, is considered to be competent for the certification of geological reference materials

A properly developed consensus from a proficiency test is, for all practical purposes, interchangeable with a certified value for a matrix reference material derived from an interlaboratory comparison

A properly developed consensus from a proficiency test is, for all practical purposes, interchangeable with a certified value derived from an interlaboratory comparison. But some scientists have doubts about the metrological status of such values. In particular, the materials are not regarded as equivalent to certified reference materials (CRM) because of a lack of traceability of the consensus value. However, the detailed considerations in this study show that such a distinction is without foundation. The following issues are addressed. (a) Misgivings about the traceability of an interlaboratory consensus are found to be insubstantial because chemical measurements are hardly ever fully traceable. (b) Reproducibility (i.e., interlaboratory) conditions account for all sources of variation in results other than long-term stability, so lead to a valid estimate of uncertainty. (c) Certification of a reference material according to ISO procedures is homologous with a proficiency test and no more secure against bias in the characterised value and its standard error. In particular, residual (common) bias cannot be fully addressed in either approach. (d) A consensus in itself attests to a valid outcome. (e) Proficiency test providers have access to a powerful method of monitoring long-term stability that is denied to CRM producers