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Potassium stable isotope compositions (41K/39K ratio) exhibit a wide range of natural variation (∼3‰), whose measurement has been recently achievable thanks to the development of various methods, including collision cell, XHR mode, cold and dry plasma, or dummy bucket MC-ICP-MS. Here, we report the capabilities of a Thermo Scientific new generation MC-ICP-MS, Neoma, to routinely measure the 41K/39K ratio. We found that the effect of potassium concentration mismatch between the sample and standard is comparable to other reported methods. However, the effect of acid concentration mismatch is not perceptible from 0.05 M to 0.5 M HNO3, and the effect of matrix elements on the accuracy of sample-standard bracketing measurements is lower compared to other reported methods up to the element-K normalized ratio of >15%. This relative insensibility to non-spectral matrix effects most probably results from the new design of the introduction system that integrates an iCAP Qnova Series ICP-MS torch and injector assembly. The resulting repeatability (or short-term external precision) and long-term intermediate precision (or long-term reproducibility) are 0.06‰ (2SD, n = 55) and 0.07‰ (2SD, n = 12), respectively. We then use a very simple one step chemical purification of K by ion exchange chromatography and apply the method to the measurement of the δ41K value of geological and biological reference materials, and blood samples of cancer and control patients. The results for reference materials show very good agreement with previously reported values and extend the available data for future interlaboratory comparison studies. Blood samples of cancer and control patients exhibit similar δ41K values despite a suspected isotopic difference between tumour and normal tissues. The overall instrumental robustness observed using the XHR mode remains to be challenged with the precell mass filter and hexapole collision/reaction cell upgrade. [ABSTRACT FROM AUTHOR]

We report novel methods for the chemical purification of Mg from silicate rocks by ion-exchange chromatography, and high-precision analysis of Mg-isotopes by high-resolution multiple collector inductively coupled plasma source mass spectrometry (HR-MC-ICPMS). Using these methods, we have measured the relative and absolute Mg-isotope composition of a number of terrestrial and extraterrestrial materials, including international reference rock standards as well as pure Mg standards, olivine crystals separated from a mantle-derived spinel lherzolite (J12 olivine), one enstatite chondrite, a martian shergottite and sea water samples. Repeated analyses of terrestrial and extraterrestrial samples demonstrate that it is possible to routinely measure the relative Mg-isotope composition of silicate materials with an external reproducibility of 2.5 and 20 ppm for the μ26Mg* and μ25Mg values, respectively (μ notation is the per 106deviation from a reference material). Analyses of bulk mantle-derived rocks as well as a martian shergottite and an enstatite chondrite define a restricted range in μ25Mg of −120 ± 28 ppm (2sd) relative to the DSM-3 reference standard (μ25,26Mg = 0), suggesting that the Mg-isotope composition of inner solar system bulk planetary materials is uniform within the resolution of our analyses. We have determined the absolute Mg-isotope composition of the J12 olivine, two CI chondrites as well as the DSM-3 and Cambridge-1 reference standards using a mixed 26Mg-24Mg double-spike. The differences between the absolute 25Mg/24Mg ratios of the various materials analyzed relative to the DSM-3 standard are in excellent agreement with results obtained by the sample-standard bracketing method. Based on the averages obtained for the J12 olivine separates, we estimate the absolute Mg-isotope composition for Earth''s mantle – and hence that of the bulk silicate Earth – to be 25Mg/24Mg = 0.126896 ± 0.000025 and 26Mg/24Mg = 0.139652 ± 0.000033. Given the restricted range of μ25Mg obtained for bulk planetary material by the sample-standard bracketing technique and the excellent agreement between the data obtained by the relative and absolute methods, we propose that these new values represent the absolute Mg-isotope composition of the bulk inner solar system. Using the absolute Mg-isotope composition of the J12 olivine, we calculate the isotopic abundances of Mg as 24Mg = 0.789548 ± 0.000026, 25Mg = 0.100190 ± 0.000018, and 26Mg = 0.110261 ± 0.000023. Based on this result, we have calculated an atomic weight for Mg of 24.305565 ± 0.000045, which is marginally heavier than previous estimates but a factor of 10 more precise. [ABSTRACT FROM AUTHOR]