Accurate measurement of K/Ca in low-[K] carbonate samples using laser-ablation sector-field inductively coupled plasma mass spectrometry

Romi Nambiar; Jorit F Kniest; Alexander Schmidt; Jacek Raddatz; Wolfgang Müller; David Evans

doi:10.1002/rcm.9692

Accurate measurement of K/Ca in low-[K] carbonate samples using laser-ablation sector-field inductively coupled plasma mass spectrometry

Rapid Commun Mass Spectrom. 2024 Mar 15;38(5):e9692. doi: 10.1002/rcm.9692.

Authors

Romi Nambiar^{1

2}, Jorit F Kniest^{1

2}, Alexander Schmidt^{1

2}, Jacek Raddatz^{1

2}, Wolfgang Müller^{1

2}, David Evans^{1

2}

Affiliations

¹ Institute of Geosciences, Goethe University Frankfurt, Frankfurt am Main, Germany.
² Frankfurt Isotope and Element Research Center (FIERCE), Goethe University Frankfurt, Frankfurt am Main, Germany.

PMID: 38355885
DOI: 10.1002/rcm.9692

Abstract

Rationale: Potassium (K) is a major component of several silicate minerals and seawater, and, therefore, constraining past changes in the potassium cycle is a promising way of tracing large-scale geological processes on Earth. However, [K] measurement using inductively coupled plasma mass spectrometry (ICP-MS) is challenging due to an ArH⁺ interference, which may be of a similar magnitude to the K⁺ ion beam in samples with <0.1% m/m [K].

Methods: In this work, we investigated the effect of the ArH⁺ interference on K/Ca data quality by comparing results from laser-ablation (LA)-ICP-MS measured in medium and high mass resolution modes and validating our LA results via solution ICP-optical emission spectroscopy (OES) and solution ICP-MS measurements. To do so, we used a wide range of geological reference materials, with a particular focus on marine carbonates, which are potential archives of past changes in the K cycle but are typically characterised by [K] < 200 μg/g. In addition, we examine the degree to which trace-element data quality is driven by downhole fractionation during LA-ICP-MS measurements.

Results: Our results show that medium mass resolution (MR) mode is sufficiently capable of minimising the effect of the ArH⁺ interference on K⁺ . However, the rate of downhole fractionation for Na and K varies between different samples as a result of their differing bulk composition, resulting in matrix-specific inaccuracy. We show how this can be accounted for via downhole fractionation corrections, resulting in an accuracy of better than 1% and a long-term reproducibility (intermediate precision) of <6% (relative standard deviation) in JCp-1NP using LA-ICP-MS in MR mode.

Conclusion: Our [K] measurement protocol is demonstrably precise and accurate and applicable to a wide range of materials. The measurement of K/Ca in relatively low-[K] marine carbonates is presented here as a key example of a new application opened up by these advances.

Abstract

Grants and funding