Advances in Calcium Isotope Purification and Analysis Using Cutting-Edge Signal Amplifiers for Matrix-Diverse Reference Materials

Abstract

Stable calcium (Ca) isotopes are increasingly applied across geosciences, medical sciences, ecology, paleontology, and archaeology. However, the deployment speed of Ca isotope applications worldwide is hampered by three major challenges: 1) the necessity for complex Ca purification procedures prior to analysis; 2) expensive instrumentation (typically TIMS or ICP-MS) requiring specific configurations and fine-tuning to generate reliable data; and 3) the exhaustion of some of the most widely used reference materials for cross-laboratory comparisons. This study presents methodological advances aimed at lifting some of these barriers. First, existing chromatography methods for purifying Ca were refined by developing a branching procedure based on commercially available labware to allow faster method transfer and to minimize resin and reagent consumption for a variety of sample matrices. These adjustments drastically improved strontium (Sr) separation from Ca, including for Sr-rich samples such as seawater. Second, the potential of 10¹³Ω Faraday cup amplifiers for improving Ca isotope measurements was explored. Results show improved precision in ⁴³Ca measurements under low ionic transmission configurations with δ^43/42Ca standard deviation value reduced by half. This expands the list of ICP-MS configurations capable of producing reliable Ca isotope measurements and delineates a path for less sample-destructive methods (i.e., lower Ca analytical requirements). These amplifiers also markedly enhanced the correction of Sr²⁺ interferences typically affecting Ca ion beams. In this configuration, accurate and precise Ca isotopic measurements were obtained without prior Sr removal for Sr/Ca concentration ratios up to 10^-2. Lastly, using these technical advancements, existing and new international certified reference materials (SRM1486, SRM1400, IAPSO, CACB-1, DOLT-5, DORM-5, TORT-3) were analyzed, complementing existing and out-of-stock standards of the Ca isotope toolbox, notably for Ca carbonate and marine soft tissues. Together, these advances open the door of Ca isotope research to more laboratories and pave the way for future developments and applications.