The future demand for geological reference materials

Geological RMs (G-1 and W-1) were introduced in 1951 for the purpose of validating the accuracy of silicate rock analysis by dc arc spectrography. Since then the introduction of an array of other spectrographic methods has greatly enhanced research into geological processes. The range of elements that could be determined was expanded, and the detection limits for measurement was lowered repeatedly through the years. The development and use of reference materials was critically important in supporting this rapid expansion of geological research. Essentially, all RMs are of importance to the geosciences community since G-1 and W-1 have been prepared and distributed by national geological institutions, first by the USGS or the CRPG, rather than by national metrology institutions. These geological institutions are not yet certifying their RMs according to ISO Guides. The International Association of Geoanalysts (IAG) is seeking to meet this higher metrological requirement. Since the inception of the IAG certification program in 2003, five powdered silicate rock materials have been issued to meet the demand with respect to calibration, method validation, traceability, etc. for whole rock major and trace element analysis. The introduction of microanalytical techniques nearly decades ago and the more recent advent of MC-ICP-MS have become new driving forces in geochemical research. The first opened the possibility of performing in situ elemental composition studies at the μm scale. The second led to the discovery of small isotope composition variations of mass- and non-mass-dependent processes in “non-traditional” stable isotopes (e.g., Fe, Cu, Zn, Mo, W, and Hg) through cosmo- and geochemical processes. Coupling the two techniques expands in situ analysis to isotopic studies. These developments have created great demand for (certified) RMs for both isotope ratio and microanalytical measurements for the geochemical community that is not yet being met. Homogeneity at a μm scale and unmatched matrices of the natural minerals or synthetic doped glasses hamper the progress in certification of RMs for the microanalytical measurement community. A challenge for the production of isotope RMs is to prepare an RM solution with an isotopic composition similar to the natural systems under investigation. Refined cadmium and nickel metals, for example, have fractionated isotopic compositions far above the range observed in natural systems of interest. Yet, the calibration RM cannot fulfill its purpose when the uncertainty of its isotopic composition exceeds that of the unknowns being measured against it. In this regard, the IAG has recently certified a calibration solution for the determination of Os isotopic ratios. It is also working through member organizations, USGS, and MPI for Geochemistry (Mainz) to develop appropriate microanalytical standards. In addition to these current and future challenges, establishing metrological traceability of geological reference materials in the absence of starting points developed by national metrology institutions is a major issue that needs attention in all future certifications.