High-Precision Calcium Isotope Analysis On TIMS Using A Double Spike Technique: The Instrumental Drift And Its Correction

We report Ca isotopic measurement results of five widely used reference materials (SRM915a, SRM915b, Atlantic seawater, BHVO-2 and BCR-2) on a Triton Plus thermal ionization mass spectrometer (TIMS) over the past eight years. Significant instrument drift was observed, and can be attributed to change in collector efficiency, which has been demonstrated by jumps in measured values after replacement of graphite Faraday cup liners. The drift on delta 44/42Ca, delta 44/40Ca and epsilon 40/44Ca is consistent among all the five reference materials, and thus it can be corrected based on measurements on certain reference materials. Being reported relative to SRM915a measured in each analysis session, SRM915b, Atlantic seawater, BHVO-2 and BCR-2 over eight years yield mean values consistent with updated results reported by other laboratories and previously from this lab. The long-term reproducibility (2SD) was 0.048%o, 0.085%o and 0.92 for delta 44/42Ca, delta 44/40Ca, and epsilon 40/44Ca, respectively, given that each purified sample was typically measured at least six times. To minimize the uncertainty from drift correction based on few measurements on SRM915a, we suggest that unknown samples can be reported with correction of drift defined by all well-characterized reference materials (compared to their recommended values) in each analytical session. According to simulations, long-term reproducibility (2SD) of delta 44/42Ca, delta 44/40Ca, and epsilon 40/44Ca can be improved to 0.033%o, 0.044%o and 0.64, respectively, if each sample is measured eight times.