Robust Potassium Isotopic Analysis of Geological and Biological Samples via Multicollector ICP-Mass Spectrometry Using the “Extra-High Resolution Mode”

Abstract

Potassium isotopic analysis is arousing increasing interest, not only in geochemistry, but also in biomedicine. However, real-life applications are still hindered by the lack of robustness of the methods used. In this work, a novel and robust method for high-precision K isotopic analysis of geological and biological samples was developed, based on the use of a multicollector ICP-mass spectrometer providing a mass resolving power of 15,000 (extra-high resolution mode, XHR). After evaluation of different measurement conditions, i.e., hot vs cold plasma conditions, standard-type vs jet-type sampling cone, and high resolution (HR) vs XHR, a combination of hot plasma conditions, use of the high-transmission jet-type sampling cone, and the XHR mode allowed for high-precision and interference-free K isotopic analysis. Potassium signal monitoring was performed in the ArH+ interference-free 0.006–0.007 amu wide peak shoulder using the XHR mode. The within-run, short-term external, and long-term external precisions for the δ41K value were 0.02‰ (2se, N = 50), 0.03‰ (2SD, N = 7), and 0.06‰ (2SD, N = 163), respectively. A two-stage chromatographic procedure was developed for the isolation of K from both geological and biological samples, and potential matrix effects affecting the K isotope ratio were systematically evaluated. The method was first applied to geological reference materials (RMs) for validation purposes, and the K isotope ratio results were in good agreement with those previously reported. Subsequently, a series of biological RMs, including serum, whole blood, cerebrospinal fluid, bovine muscle, and lobster hepatopancreas, were characterized for their K isotopic composition.

Steady analyses of potassium stable isotopes using a Thermo Scientific Neoma MC-ICP-MS

“…IAPSO seawater gives a δ 41 K value of 0.03 ± 0.02‰ (2SD, n = 2), slightly lower than the accepted average seawater δ 41 K value of 0.12 ± 0.07‰ (2SD, n = 46), 25 but still in the range of isotopic variability. The tuna fish ERM-CE 464 reference material yields a δ 41 K value of 0.38 ± 0.01‰ (2SD, n = 2), consistent with the value of 0.31 ± 0.05‰ (2SD, n = 2) reported by Moynier et al , 15 and the lobster hepatopancreas TORT-3 reference material gives a δ 41 K value of −0.29 ± 0.03‰ (2SD, n = 2), consistent with the value of −0.31 ± 0.03‰ (2SD, n = 3) reported by Hobin et al 18 The δ 41 K value of the DORM-4 fish protein is −0.24 ± 0.03‰ (2SD, n = 2). The bovine liver SRM-1577c gives a δ 41 K value of 0.15 ± 0.004‰ (2SD, n = 2), different from the bovine liver ERM-BB185 reported by Moynier et al 15 at −0.22 ± 0.03‰ (2SD, n = 2), showing a metabolic influence of the hepatic K isotope composition that remains to be elucidated.…”

“…The results generally show a decreasing d 41 K value with increasing K concentration mismatch, except for the study of Gu et al 4 in which the 41 K/ 39 K ratio is measured using a Neptune + under cold plasma and low-resolution conditions. Our results show a weaker dependency to K concentration mismatch compared to CC-MC-ICP-MS, 13,15 but a higher dependency compared to a dummy bucket-or XHR-equipped MC-ICP-MS 8,18 (Fig. 2).…”

“…When using the sample-standard bracketing method, the mismatch in the acid concentration between samples and standards also has a potential effect on the δ 41 K value of the sample. 4,8,13,15,18 Under the working conditions of the present study, the molarity of HNO 3 is 0.05 M and we have evaluated the influence of the mismatch of the acid concentration from 0.025 M to 0.5 M and compared the results with the literature values (Fig. 3).…”

“…Given this set of parameters, a K solution at 1 mg L −1 gives a typical 39 K voltage of ∼40 V, which is about two-thirds more sensitive than with a Neptune XT. 18 The sample-standard bracketing approach with a solution of SRM-3141a (NIST) as the external standard is then used for correcting instrumental mass discrimination. Analyses are carried out using 0.05 M HNO 3 with blank correction (on-peak zero) before measurement and the K isotopic composition is expressed as a delta value ( δ 41 K, per mil, ‰) relative to SRM-3141a according to:…”

“…Regarding K isotopes, the resolution increases from 9000 to 15 000, leading to precise and accurate measurements of the 41 K/ 39 K ratio. 18 A table summarizing all the compared setups and the corresponding general performances is provided as Table S1. †…”