Application of a “Continuous‐Acquisition‐Method” to potassium isotope measurements by multi‐collector inductively coupled plasma mass spectrometry

Abstract

Rationale The very small mass difference between 41K and 40ArH+ makes the flat, hydride interference‐free peak shoulders very narrow (0.002–0.003 m/z unit), bringing a number of analytical challenges when measuring K isotopic compositions by multi‐collector inductively coupled plasma mass spectrometry (MC‐ICP‐MS). In traditional Sequence Run mode, the parameters are loaded every line of the sequence which can introduce tiny drifts of tune parameters and mass peaks. This may occasionally lead to the failure of K isotope measurements when mass drifts exceed 0.002 m/z unit. It is thus essential to keep the tune parameters, especially the magnet current, very stable to achieve high‐precision K isotopic compositions. Method We developed a “Continuous‐Acquisition‐Method” (CAM) MC‐ICP‐MS Run mode to improve the stability when determining K isotopes. Two sets of experiments were designed: (a) Stability test: measuring a single pure K solution (viz. NIST‐999c) for ~3 h and comparing the stability of the two run modes; and (b) GSB‐K test: measuring our inhouse pure K standard solution (GSB‐K) in both run modes and comparing the accuracy and precision. Results The traditional Sequence Run mode only kept the MC‐ICP‐MS system stable for the first ~1.5 h during the ~3‐h test, with an offset of the mass peaks of ~0.003 m/z unit. The CAM Run mode yielded higher stability during the whole test (~3 h), with a peak shift <0.0004 m/z unit. Measurement of the GSB‐K standard solution in Sequence Run and CAM Run modes gives identical δ41K values when the magnet was kept stable, with the CAM Run mode offering a better precision and keeping the instrument stable for longer time. Conclusions The MC‐ICP‐MS CAM Run mode shows higher stability and better precision. It is, therefore, good for high‐precision K isotope measurements.

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

“…All data on this figure are normalized to NIST SRM 3141a with 2 standard deviation error bars. Data sources: references 6, 15, 16, 25, 34, 35, 37–41, and 43 [Color figure can be viewed at wileyonlinelibrary.com]…”

“…This issue is particularly critical for the instrumental setting in this study, because the mass spectrometer ran on a conventional hot plasma setting without any measures to suppress 40 Ar + generation. Four methods have been reported in literature to address the issue of 40 Ar + during K isotope analysis: (a) using a baffle plate anterior to the collector array to block 40 Ar + , 2,41 (b) using a grounded dummy bucket to collect 40 Ar + , 3,39 (c) moving the Faraday cup away from the beam path of 40 Ar + , 4,29,36,37,40 and (d) using collision cell to suppress 40 Ar + 6,26,35,38,42 . In this study, the 40 Ar + beam was treated by a new method, that is, introduced into an enclosed space of the ion guide rail for Daly detector, where 40 Ar + ions are neutralized (Figure 2).…”

“…Starting from the beginning of the 21st century, applications of multicollector inductively coupled plasma mass spectrometer (MC‐ICP‐MS) led to dramatic improvement in analytical precision of many metal stable isotopes 33,34 . However, the development of high‐precision analysis of K isotopes by MC‐ICP‐MS lagged behind for a decade 26,29,35–42 . The main challenge in K analysis by MC‐ICP‐MS is that Ar‐based ICP generates intense signals of argon hydrides (e.g., 40 Ar 1 H + ) that interfere with K ions (e.g., 41 K + ).…”

“…Two main strategies have been adapted to overcome this obstacle, with the core concept to suppress 40 Ar 1 H + interference. One is to run MC-ICP-MS under "cold plasma," by reducing the forward power of ICP to lower 40 Ar 1 H + production and resolving 40 Ar 1 H + from 41 K + mass spectrometrically 29,36,37,[39][40][41] or correcting 40 Ar 1 H + carefully offline 43 The other one is to perform K isotope analysis on MC-ICP-MS that is equipped with collision cell, in which 40 Ar + and Ar-based interference are removed by collision and reaction with gas molecules such as H 2 and He. 6,26,35,38,42 Although the new-generation collision-cell MC-ICP-MS enables high-precision K isotope analysis, such instruments are not available at many research laboratories, where conventional MC-ICP-MS is commonly equipped.…”

“…Therefore, developments in analytical methods for K isotopes based on MC-ICP-MS without collision cell could be found useful as this brings research capability on K isotope stable geochemistry to many existing MC-ICP-MS laboratories. 36,37,43 In this study, we report the details of the high-precision K isotope analytical method using a high-resolution MC-ICP-MS under conventional operation conditions. In particular, we demonstrate that when mass resolution is sufficiently high to resolve 41 K + from 40 Ar 1 H + , precise and accurate analysis of stable K isotopes can be achieved without applications of "cold plasma.…”