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

Abstract

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.