A high-sensitivity loading method using tantalum (Ta) gel for high-precision Ca isotope measurement by TIMS

Abstract

This study proposed a new loading technique using a tantalum (Ta) gel activator for high-precision Ca isotope (δ^44/40Ca) measurement by double-spike thermal ionization mass spectrometry (DS-TIMS). This method significantly reduces the required sample size of an individual analysis to 25–50 ng and yields a similar level of analytical precision, using double rhenium (Re) filaments, compared with conventional high-precision Ca isotope measurements, which consume microgram-level samples (3–5 µg) using TIMS. We show that the assistance of the Ta gel significantly enhances thermal ionization to a total ion efficiency of up to ∼0.8% compared with the typical rates of ∼0.01–0.05% reported in previous studies, representing an order of magnitude improvement in sensitivity. With a greatly reduced sample size, the potential domain-mixing effect would be avoided, and the mass fractionation is shown to fit well with the exponential law typically observed for TIMS. The δ^44/40Ca values of a suite of standards and geologic reference materials are reported to validate the accuracy and precision of this new method. Repeated measurements of primary standard NIST SRM 915a yielded an external reproducibility better than 0.06‰ (2SD, n = 9, 50 ng sample size). The analyses of USGS standards BHVO-2, BCR-2, and AGV-2 (25–50 ng Ca for each individual measurement) yielded mean δ^44/40Ca values of 0.82 ± 0.09 (2SD, n = 7), 0.79 ± 0.07 (2SD, n = 6), and 0.76 ± 0.05 (2SD, n = 4), respectively. These results suggest the utility of the Ta gel activator in Ca isotope measurements of small samples for potential sample-limited applications in geochemistry and cosmochemistry.