Abstract:
The geology and mining industries rely heavily on reliable quantitative data on the elemental composition of various rocks and minerals. The analytical accuracy of the best measurement systems are, however, limited to the accuracy with which the composition of a generally accepted reference sample is known, which serves to calibrate an instrument or validate a method. Thus, there is an established market for well-characterised reference materials which trade internationally as value added commodities. International organisations prepare strict procedures for the preparation and certification of these reference materials. ISO/REMCO is the ISO Committee on Reference Materials that carries out and encourages a broad international effort for the harmonisation and promotion of reference materials, their production and their application. In view of recent developments in ISO/REMCO related to updated guides and changes in definitions, SARM 1 to 6 (the NIMROCs) are in danger of losing their status as certified reference materials (CRMs). Due to the continued popularity of the NIMROCs and the availability of bulk sample, the National Metrology Institute of South Africa (NMISA) embarked on a project to re-certify three materials from the suite of six igneous rock samples, i.e. SARM 2 (Syenite), SARM 3 (Lujavrite) and SARM 4 (Norite). The aim of the re-certification project was to re-certify the materials with a single primary reference measurement procedure in a single laboratory to comply with one of the possible routes to the establishment of metrological traceability as described in ISO Guide 34. Eight elements (Ba, Sr, Cd, Mo, Cu, Ni, Pb and Zn) of SARM 2, SARM 3 and SARM 4 were value assigned during this study to ensure that they comply with the ISO Guide 30 and ISO GUM requirements for traceability and measurement uncertainty. Traceability to the SI units of mass and amount of substance at the highest level of accuracy was established by employing the definitive technique of double isotope dilution ICP-MS, with primary assay standards traceable to the NIST SRM 3100 series and gravimetrically prepared samples. For the first time assigned values with uncertainty statements for these elements in these reference materials were calculated according to the ISO GUM and CITAC/EURACHEM guidelines for expression of the uncertainty of measurement. The uncertainties of the assigned values for all elements were within the target of 5% relative set during the development of the methodology, except for elements in concentrations below 10 mg.kg-1. The ID-ICP-MS results are supported by good comparability with results reported in the literature since 1978, as well as the original certified values published in 1978. The results from this study for all elements in these reference materials have the potential to become new certified values in accordance with the requirements of ISO Guides 34 and 35. This will improve the usefulness of these reference materials to the South African and international geology and mining communities for the purposes of analytical quality control and method validation in exploration and geochemical analysis. Confirmation of the homogeneity and stability of the remaining units of the reference materials is considered necessary to complete the re-certification project.
