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Potassium isotopic composition of various samples using a dual-path collision cell-capable multiple-collector inductively coupled plasma mass spectrometer, Nu instruments Sapphire

Abstract : Mass-dependent K isotopic fractionations can be used to trace cosmochemical, geological, and biological processes such as evaporation/condensation, core formation, magmatic processes, weathering, and cellular metabolism. However, the application of stable K isotopes has been limited by major isobaric interferences from Ar, common on conventional multi-collector inductively-coupled-plasma mass-spectrometer (MC-ICP-MS), particularly for the low-K samples. Here, we present a set of high-precision K isotopic data acquired on terrestrial rocks, seawater, as well as a lunar meteorite using the recently released Nu Sapphire™ MC-ICP-MS that utilizes a collision cell to minimize Ar based interferences while maintaining remarkably high K sensitivity (≈ 2000 V/ppm). The influence of several parameters on the precision and accuracy of the K isotopic data has been evaluated, including total K concentration, K intensity mismatch between sample and standard, HNO3 molarity mismatch between sample and standard, and the presence of matrix elements. We found that the Nu Sapphire™ can be used to acquire precise and accurate data using as little as 125 ng of K, which represents an improvement by a factor 10 compared to what has been done on previous instruments. We present data for 23 previously analyzed samples; these data are highly consistent with literature values. On the other hand, accurate measurements are conditioned 1) to the close matching of sample and standard K intensities (a 1% mismatch creates a 0.02‰ offset on the 41K/39K ratio), and 2) to the absence of Ca (a Ca/K ratio of 1% creates a 0.069‰ offset on the 41K/39K ratio). In addition, Rb/K, Na/K, Ti/K and Cr/K ratio should also be maintained under 2.5% to avoid isotopic offset. We confirm the existence of significant mass-dependent K isotopic variations in terrestrial samples and that lunar rocks are isotopically heavier than terrestrial rocks. The incomparable sensitivity offered by the Nu Sapphire opens the possibility for high-precision K isotopic measurements across a wide range of samples for diverse applications.
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Submitted on : Thursday, September 23, 2021 - 5:02:59 PM
Last modification on : Wednesday, October 27, 2021 - 4:15:40 PM

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Frédéric Moynier, Yan Hu, Kun Wang, Ye Zhao, Yvan Gérard, et al.. Potassium isotopic composition of various samples using a dual-path collision cell-capable multiple-collector inductively coupled plasma mass spectrometer, Nu instruments Sapphire. Chemical Geology, 2021, 571, pp.120144. ⟨10.1016/j.chemgeo.2021.120144⟩. ⟨insu-03353073⟩

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