Review of techniques, challenges, and new developments for calcium isotope ratio measurements

Calcium isotope meaurements apply to problems spanning the ‘cosmos to benthos.’ Fractionation, source mixing, radioactive decay, and nucleosynthetic processes cause variations in Ca isotope ratios, which can be measured using different types of mass spectrometers. Early measurements employed thermal ionization mass spectrometry (TIMS), which continues to be the most common and precise reference technique. An increasing number of studies use multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Both TIMS and MC-ICPMS can yield high-quality data for δ^44/40Ca and δ^44/42Ca, respectively, but wide ranges in precision exist. Sample size requirements are lower for TIMS, but MC-ICPMS offers faster throughput. Both techniques require purifying Ca by ion-exchange chromatography when analyzing samples with complex matrices. In-situ techniques, like laser ablation MC-ICPMS (LA-MC-ICPMS) and secondary ion mass spectrometry (SIMS), do not require purification. In-situ techniques provide high spatial resolution, but LA-MC-ICPMS and (nano)-SIMS suffer from relatively poor precision compared to TIMS and MC-ICPMS, and the methods lack well-characterized standards, which are essential for comparing datasets. The past few years have seen advances in Ca purification techniques, including automated methods that accelerate sample throughput. The next generation of Ca purification techniques should focus on complete purification and quantitative separation of low-Ca samples. Application of collision-cell technology for suppressing Ar-related molecular isobaric interferences will improve quantification of ⁴⁰Ca using certain MC-ICPMS models. Similarly, development of new amplifier technology will foster high-precision Faraday cup measurements of low abundance Ca isotopes. Adopting common reference materials, including newly developed standards with complex matrices, would signficantly aid inter-laboratory comparisons, method development, and data compilation efforts. In general, at least two widely-used standards should be measured along with unknown samples to confirm data quality, as well as improve utilization of published Ca isotope datasets by a broader community.