High speed-low volume automated ICP-QMS method for determination of Mg/Ca in biogenic calcite

Abstract

The Mg/Ca molar ratio in foraminiferal calcite accumulated in marine sea sediments is a well-established proxy of ancient ocean temperatures. Magnesium is three orders of magnitude less abundant than Ca (0.5–10 mmol per mol), and the relationship between their ratio and sea surface temperature is exponential. Consequently, the reliability of the calculated temperature depends dramatically on the accuracy and precision of measurements, and the comparability of the results is heavily impacted by standardisation of the analytical methodology. Here, we extended the applicability of inductively coupled plasma-mass spectrometry for the determination of Mg/Ca in foraminifera, by implementing an automated on-line dual-dilution manifold combined with mixed instrumental acquisition modes (no gas-H₂ reaction), and the use of on-line Sc addition as an internal standard. This allowed the independent acquisition of Mg and Ca at their optimised working concentrations under instrumental conditions that were free of significant spectral interferences. Matrix effects and instrumental drift are effectively mitigated, as supported by comparison with results from on-line isotope dilution analysis. Memory effects were minimized, and only 20 μg of calcitic material was required for analysis. Automation of the setup allowed dual dilution analysis of samples without any significant increase in preparation or analysis time. The system is easily upgradable to multiple on-line dilutions and multi-elemental ratios. The method was found to be accurate over the wide range of Mg/Ca values (from 0.8 to 5.7 mmol mol⁻¹) present in the certified reference materials BAM-RS3, ECRM-752-1 and CMSI-1767, with repeatability in the range of 0.3–0.7% and an instrumental LOD of 0.6 nmol mol⁻¹. Test application to five samples from the Integrated Ocean Drilling Program Site U1313 gave results consistent with literature values.