Stable potassium (K) isotopes (41K/39K) have shown great promise as novel chemical tracers for a wide range of bio-, geo-, and cosmo-chemical processes, but high precision stable K isotope analysis remains a challenge for plasma source mass spectrometry due to intense argon-related interferences produced directly from argon plasma. Here we provide an assessment on the analytical figures of merit of a new generation collision-cell equipped multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), Sapphire from Nu Instruments, for K isotope analysis based on our extensive tests over a duration of ∼8 months. Because use of helium and hydrogen as collision/reaction gases can reduce argon-related interferences to negligible levels at optimal flow rates, the collision-cell mode can operate at low mass resolution during K isotope analysis, providing >2 orders of magnitude higher K sensitivity (>1000 V per μg mL−1 K), as compared to the widely used “cold plasma” method, and the capability for direct 40K measurement. One challenge of the collision/reaction cell analysis on Sapphire is its higher susceptibility to matrix effects, requiring effective sample purification prior to analysis. Also, the collision-cell mode on Sapphire shows a pronounced effect associated with concentration (or ion intensity) mismatch between the sample and the bracketing standard during analysis, and this effect may not be fully eliminated through conventional concentration matching practice. Instead, we developed a correction method for this concentration/ion intensity mismatch effect. Our method reduces the burden to the operator and increases sample throughput. This method allows for accurate K isotope analysis with an intermediate precision of ≤0.05‰ (2SD) to be routinely achieved using the collision cell on Sapphire, representing a major advance to stable K isotope analysis.
Bibliographical noteFunding Information:
This work is supported by the National Science Foundation under Grant No. 1741048 and a start-up fund from the University of Minnesota to X-Y Zheng. Acquisition of iCAP TQ-ICP-MS was supported by National Science Foundation under Grant No. 1946945 to X-Y Zheng. We would like to thank Michael Jones and Lee Griffiths from Nu Instruments for their technical support in bringing SP006 online during a global pandemic. We also thank Prof. Fang-Zhen Teng at the University of Washington for sharing a suite of NIST solutions used in this study, including NIST 3141a, 193, 918b, and 999c. We thank two anonymous reviewers for their constructive comments and the editor for efficient editorial handling.
© 2022 The Royal Society of Chemistry