Precise measurement of ⁴¹K/³⁹K ratios by high‐resolution multicollector inductively coupled plasma mass spectrometry under a dry and hot plasma setting

Abstract

Rationale: Stable K isotope geochemistry is becoming an important tool for various applications. Developments in analytical methods for K isotopes based on multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) without collision cell will bring research capability of K isotopes to many existing MC-ICP-MS labs.Methods: Stable K isotopes were analyzed without applications of "cold plasma" and collision cell on a Nu 1700 Sapphire high-resolution multicollector inductively coupled plasma mass spectrometer. A conventional dry and hot plasma setting is used for analysis to maintain high K sensitivity and signal stability, and high mass resolution was applied to provide interference-free shoulders of 39 K + for isotopic measurement of 41 K/ 39 K ratios. 40 Ar + ion beam generated in ICP was neutralized in the ion guide rail for the Daly detector.Results: Under such operating conditions, an external reproducibility of <±0.1‰(2 standard deviation) for 41 K/ 39 K is achieved for K solutions of 1 ppm or above.Tests were carried out to evaluate the influence of total K loading, K concentration and acid molarity mismatch, matrix effects, and 40 Ar + and 40 Ar 1 H + tailing on K isotope analysis. We found that the accuracy of K isotope analysis can be compromised by concentration mismatch of sample and standard K, by 0.007‰ in δ 41 K per 1% mismatch of K content. By contrast, mismatch of HNO 3 molarity or existence of HCl in HNO 3 exerts negligible influences on the analytical precision and accuracy of K isotope analysis. Furthermore, K isotope analytical results remain accurate when Na/K, Mg/K, Ca/K, Rb/K, V/K, and Cr/K ratios are below 3%. Conclusions:The high-precision K isotope analytical method reported here is robust for studies on K isotopic variations in geological, cosmochemical, and biological samples. The f 41 K values of six international geostandards measured using our method are consistent with data measured using different analytical methods from other laboratories. | INTRODUCTIONPotassium (K) is a major element that plays important roles in a plethora of cosmochemical, geological, geochemical, and biological processes due to its moderately volatile, lithophile, water-soluble, and biophile properties. 1 Potassium has two stable isotopes, 39 K (93.3%) and 41 K (6.7%), and there is a large (5%) relative mass difference between the two isotopes. 41 K/ 39 K ratios are remarkably variable in nature, and significant K isotope fractionations have been documented in various processes, 1 including planetary evaporation

High-precision double-spike Sn isotope analysis of geological materials by MC-ICP-MS

“…Previous studies reported analytical inaccuracy when there was a mismatch in the concentration of the sample and standard during the double spike analysis of Cd and Se isotopes and sample-standard-bracketing analysis of K and Ba stable isotopes. 80–83 The effect of intensity mismatch on Sn isotope analysis was evaluated by analyzing spiked NIST 3161a solutions of concentrations varying from 20 to 150 ng g −1 against the spiked NIST 3161a bracketing standard with a constant concentration of 100 ng g −1 Sn. Both the bracketing standards and samples were prepared from a 1 μg g −1 spiked NIST 3161a stock solution.…”

“…Elements other than the target element may affect the transport of the analyte in the ICP and the ion extraction in the interface, causing matrix effects. 81,83 Abnormal isotopic results could also arise due to molecular and isobaric interferences caused by matrix elements. 80 Although column chemistry was developed to purify the target elements from matrix elements, elements like Mg, Fe, and Ca are common and major elements in geological matrices.…”

Significantly Enhanced Robustness of K Isotope Analysis by Collision Cell MC-ICP-MS and Its Application to the Returned Lunar Samples by China’s Chang’e-5 Project

“…High sensitivity is the main advantage of the collision cell approach over the high mass resolution approach in K isotope analysis by MC-ICP-MS. Sensitivity of 1000–2000 V per μg/g for K can be achieved using a new generation collision cell MC-ICP-MS (CC-MC-ICP-MS), ,,, compared to the typically sensitivity of <30 V per μg/g for K on HR-MC-ICP-MS without a collision cell. ,,, The >1000 V per μg/g sensitivity means that, from a counting statistics point of view, precise K isotope ratio measurements can be achieved for a diluted K solution with a concentration as low as 10 ng/g (i.e., 10–20 V total K signal intensity) on CC-MC-ICP-MS. However, the accuracy of K isotope analysis by CC-MC-ICP-MS can be compromised by Ca in the sample solutions, where a [Ca]/[K] of 0.01 can cause a 0.03–0.10‰ offset in measured 41 K/ 39 K ratios. ,,, Because Ca is a major contaminant that ubiquitously exists in reagents, acids, water, containers, reaction vessels, pipet tips, and air dusts, ng/g to sub-ng/g level background of Ca is common in K analyte solutions, even after careful chemical procedures of purification.…”

“…Sensitivity of 1000−2000 V per μg/g for K can be achieved using a new generation collision cell MC-ICP-MS (CC-MC-ICP-MS), 8,10,13,18 compared to the typically sensitivity of <30 V per μg/g for K on HR-MC-ICP-MS without a collision cell. 11,17,19,27 The >1000 V per μg/g sensitivity means that, from a counting statistics point of view, precise K isotope ratio measurements can be achieved for a diluted K solution with a concentration as low as 10 ng/g (i.e., 10−20 V total K signal intensity) on CC-MC-ICP-MS. However, the accuracy of K isotope analysis by CC-MC-ICP-MS can be compromised by Ca in the sample solutions, where a [Ca]/[K] of 0.01 can cause a 0.03−0.10‰ offset in measured 41 K/ 39 K ratios.…”

“…Researchers continued to use the state-of-the-art mass spectrometry of their times to analyze K isotopes, and the precision of K isotope analysis was improved to ±1‰ in the 1970s by using thermo ionization mass spectrometry (e.g., refs and ) and to ±0.5‰ in the 1990s by using secondary ionization mass spectrometry (e.g., ref ), but the precision was still not sufficient to discern most natural K isotope variability from terrestrial samples. Since 2016, multicollector ICP mass spectrometry (MC-ICP-MS) has become the mainstream method to measure 41 K/ 39 K ratios with analytical precision pushed to a level of <0.1‰, which finally opened the door to the full application of stable K isotope ratios to study various natural processes, and an explosion in K isotope research is ongoing. ,− …”