The purpose of this project is to calibrate an internationally available certified reference material for molybdenum isotopes. This project will, for the first time, provide the vast molybdenum isotope user community with a reliable δ-zero anchor for molybdenum isotopic data that is fully traceable to the SI norm and as such allow reliable interlaboratory comparison of Mo isotopic results.
Over the past decade the multiple collector inductively coupled plasma mass spectrometer has enabled the determination of isotopic composition of molybdenum with unprecedented levels of precision. Molybdenum is redoxsensitive and abundant in rocks, seawater, and an important metal in many biological processes. Many working groups in geochemistry, the environmental and life sciences are employing isotope abundance data to investigate processes involving this element. However, the intercomparison of results among research groups is impeded because there are no molybdenum isotopic reference materials and no agreed-upon delta zero material. To enable interlaboratory data comparison, a commercially available material has to be calibrated and suggested as the delta zero reference material.
The purpose of this project is to bring together key laboratories to participate in the full calibration of a suitable reference material. In order to achieve this goal, it is necessary to first identify suitable candidates. Criteria will include a material with isotopic abundances that fall in the mid range of terrestrial isotopic composition. The material must be distributed by recognized international organizations (i.e. IAEA, NIST, IRMM). The material must be available in reasonable quantities and preferably in liquid form. The calibration exercise will be carried out in three different laboratories using agreed upon analytical protocols that fulfill the requirements of the Commission on Isotopic Abundances and Atomic Weights definition of a best measurement. Finally, participants will identify and calibrate commonly used reference materials (e.g. certified seawater and USGS geochemical reference materials) to serve as secondary standards and the laboratory and represent important terrestrial molybdenum reservoirs.
Face-to-face meetings will be organized to discuss the selection of appropriate materials, establish measurement protocols, and discuss and evaluate measurement results. Outcomes will be disseminated as oral or poster presentations at major international conferences where significant numbers of users meet. Data will be published in high impact peer-reviewed journals to reach as broad an audience as possible to encourage adoption of the new calibrated reference materials.
July 2014 update – An isotopic composition and atomic weight analysis of Mo in the NIST standard solution SRM 3134 was completed and published in the Journal of Analytical Atomic Spectrometry 2014, 29, 85-94 (DOI: 10.1039/C3JA50164G). This report was evaluated by the CIAAW in 2013 and is the basis for the new recommended atomic weight of Mo. In addition, the relative isotopic composition of a laboratory Mo solution and Johnson-Matthey metal rod were also reported. Work is now ongoing to incorporate similar determinations in two additional laboratories, as described in the project proposal.
July 2015 update – The group published in the topical collection celebrating Anal. Bioanal. Chem 13th Anniversary (January 2015, Vol 407, Issue 3): Malinovsky, D., Dunn, P.J., Petrov, P., and Goenagga-Infante, H. Investigation of mass dependence effects for the accurate determination of molybdenum isotope amount ratios by MC-ICP-MS using synthetic isotope mixtures, Anal. Bioanal. Chem., 407, 869; https://dx.doi.org/10.1007/s00216-014-8112-1.
The results to date indicate that the NIST SRM 3134 now has a reliable absolute isotopic composition and atomic weight and it was demonstrated that, for the materials investigated so far, isotopic variability is the result of mass dependent fractionation processes. The focus of the project will now shift to the second stated goal, that is to identify and calibrate commonly used reference materials (e.g. certified seawater and USGS geochemical reference materials) to serve as secondary standards and the laboratory and represent important terrestrial molybdenum reservoirs.
Sep 2017 update – The relative Mo isotopic composition (relative to NIST SRM 3134) has been measured for additional reference materials as well as natural samples from freshwater and seawater covering the known natural abundance variation of Mo. The results will be implemented into future SNIF diagrams as well as the updated publication of stable isotope reference materials.
June 2019 update – The paper “The absolute isotopic composition and atomic weight of Mo in SRM 3134” was published as major outcome of this project. In addition, data evaluation of SRM 3134 was accomplished within this project. As a further effort, Arizona State University connected the Mo standard with all previous isotopic standards. RM 8334 is currently being measured against the original SRM 3134 used in the scale off-set study at Arizona State University and NIST. It is intended to become the de facto zero-point reference of the Mo delta-scale. It is important that any Mo iRMs being calibrated for absolute isotope abundance measurements be tied to RM 8334, or better yet, that this iRM be used for the absolute isotope abundance measurements. The project provides the basis for the comparison of the Mo data.
Page last updated: 8 Jan 2021