A novel sample loading method and protocol for monitoring sample fractionation for high precision Ca stable isotope ratio measurements using double-spike TIMS

Abstract

The external reproducibility (2σ_SD) of Ca stable isotope ratio measurements (δ^44/40Ca) using double-spike thermal ionization mass spectrometry (TIMS) shows a large range from <0.1‰ to 0.5‰. We demonstrate that using a ⁴³Ca–⁴⁸Ca double spike, which allows simultaneous measurements of δ^44/40Ca and δ^44/42Ca, and analyses at moderate signal strengths (⁴⁰Ca ranging from 6 to 10 V), high precision δ^44/40Ca (external reproducibility better than ±0.08‰, 2σ_SD) as well as δ^44/42Ca can be obtained if samples and standards are analyzed under similar fractionation conditions. To monitor the fractionation conditions, the usage of a parameter β is proposed, which measures the deviation of the ⁴³Ca/⁴⁸Ca ratio of the sample–double spike mixture from that of the pure double spike. A novel, low-cost sample loading technique using a combination of Re and Ta filaments and a Ta₂O₅ activator is presented which results in a steady signal. We report the δ^44/40Ca and δ^44/42Ca values, calculated w.r.t. NIST SRM 915a and reported as deviations in parts per mil (‰), of NIST standards SRM 915a (0.01, −0.02) and SRM 915b (0.73, 0.32), NASS 6 (seawater, 1.80, 0.89), USGS silicate rock standards BHVO-2 (0.86, 0.47) and BCR-2 (0.89, 0.48), Geological Survey of Japan carbonate standards JCp-1 (coral, 0.85, 0.45) and JCt-1 (clam shell, 0.83, 0.46) and ECRM 752-1 (limestone, alternative name BCS-CRM 393) (0.83, 0.46) from the Bureau of Analyzed Samples Ltd. UK. The Ca stable isotopic compositions of JCt-1 and ECRM 752-1 are reported for the first time.