ISO Best Practices in Reference Material Certification and Use in Geoanalysis

Abstract

The International Organisation for Standardisation (ISO) has published many guides, or technical standards, of great value to analytical geochemists. Two of particular importance are Guide 33 (Uses of Certified Reference Materials) and Guide 35 (Certification of Reference Materials). Both were first developed in the 1980s and undergo regular review and updating by the Reference Materials Committee (REMCO) that operates within ISO. Recent revisions have focused on adding statistical rigour to both guides. Although this offers significant advantages for use by professional metrologists, there are consequent issues of comprehension by the analytical chemists who in fact have the greatest need of them. A major focus of Guide 35 is the development of reference material uncertainties that are in full compliance with the Guide on Measurement Uncertainty (GUM), jointly issued by ISO, IUPAC and others. Guide 35 details handling of uncertainty due to (1) degradation on the shelf and in transport, (2) sample heterogeneity and (3) inter‐method and inter‐laboratory bias, as well as within‐laboratory repeatability. The International Association of Geoanalysts has developed a protocol for reference material certification that applies Guide 35 to the specific needs of the geoanalytical community. The approach being taken by the IAG in developing GUM‐compliant uncertainties for its certified values is presented. Recommendations made in Guide 33 for how a laboratory should compare its own results with certified values in assessing laboratory accuracy are outlined. Additionally, the subject of misusing reference materials is discussed. The apparent misuse occurs because so few CRMs exist that meet critical measurement needs of geoanalytical laboratories and that also meet the rigorous metrological demands of the latest editions of the ISO Guides. All of the focus of the IAG certification programme has been to undertake certifications that would fill gaps in CRM availability and thus serve to limit this misuse.

Determination of Cr, Cu, Ni, Sn, Sr and Zn Mass Fractions in Geochemical Reference Materials by Isotope Dilution ICP‐MS

“…Reference materials (RM) play a crucial role in geochemical analysis because their use enables the establishment of metrological traceability, that is, comparability between measurement results (Kane 2001, Kane and Potts 2002, 2007, Barwick and Wood 2010). Metrological traceability is a pre‐requisite for evaluating measurement uncertainty, which is a quantitative indication of the quality of a result (GUM 2008) according to its fitness‐for‐intended‐use (Kane 2002, Kane and Potts 2007, de Bièvre 2010).…”

Experience of the International Association of Geoanalysts as a Certifying Body

“…2009b, c). This certificate includes all of the information identified in ISO Guide 31 (ISO 2000) as well as some additional information required for user laboratories to evaluate their data for CRM analysis against the CVs and their U values according to ISO Guide 33 (ISO 2000, Kane and Potts 2007). The IAG also prepared more detailed reports of the certification, to make the full certification data set and the data for traceability available to user laboratories (Kane 2004, 2005, Kane et al.…”

Certified Reference Materials of Molybdenum Ore and Molybdenum Concentrate (GBW07141, GBW07142, GBW07143 and GBW07144)

“…The expanded uncertainty of certified value U was evaluated using Equation :where t 0.05( n −1) is Student's t distribution for n −1 degrees of freedom at the 95% confidence limit (Kane and Potts ).…”