Optimization of a 48Ca–43Ca double-spike MC-TIMS method for measuring Ca isotope ratios (δ44/40Ca and δ44/42Ca): limitations from filament reservoir mixing

Abstract

We used a Monte Carlo error model to optimize a ⁴⁸Ca–⁴³Ca double-spike technique for measuring Ca isotope ratios (δ^44/40Ca and δ^44/42Ca) by Multi-Collector Thermal Ionization Mass Spectrometry (MC-TIMS). The model considers errors for counting statistics and Johnson noise, as well as changes in collector cup efficiency (drift). For a 20 V ⁴⁰Ca ion-beam implemented in a three-hop, dynamic multi-collection routine, the model predicts that a wide range of ⁴⁸Ca/⁴³Ca and spike/sample ratios should yield internal precisions (2σ_SEM) of 0.015–0.020‰ for δ^44/40Ca and 0.025–0.030‰ for δ^44/42Ca. Using a Thermo Fisher MC-TIMS (Triton), we tested ⁴⁸Ca/⁴³Ca = 1.5 [⁴³Ca/(⁴⁸Ca + ⁴³Ca) = 0.40 mol mol⁻¹] and spike/sample = 0.66 [Ca_dsp/(Ca_dsp + Ca_smp) = 0.40 mol mol⁻¹] by repeatedly analyzing OSIL Atlantic seawater, NIST SRM 915a, NIST SRM 915b, USGS BHVO-1, and CaF₂ over 4 sessions spanning 1 month. While the measured internal precisions generally agreed with model predictions, external reproducibility (2σ_SD) was much worse than expected. For the 81 measurements made, the average external reproducibility was 0.223‰ for δ^44/40Ca and 0.126‰ for δ^44/42Ca. After processing raw data through the double-spike equations, nearly all fractionation-corrected ratios showed remnant fractionation patterns. Such patterns reflect deviation from ideal exponential mass-fractionation due to mixing of multiple, independently fractionating reservoirs on the filament. Additional model simulations, as well as comparison against δ^44/40Ca values determined with a ⁴³Ca–⁴²Ca double-spike, support the concept of an “average mass rule”, which states that inaccuracies in fractionation-corrected data are greater for isotope ratios having an average mass further away from the average mass of the normalizing ratio. Until advancements are made to eliminate filament reservoir effects, ⁴³Ca–⁴²Ca and ⁴⁶Ca–⁴³Ca double-spikes should yield the most precise δ^44/40Ca and δ^44/42Ca values, respectively, when using MC-TIMS. Within the limits of the ⁴⁸Ca–⁴³Ca double-spike technique, we observed no evidence for ⁴⁰Ca enrichments among the standards analyzed. Finally, we found that sample matrix effects do not influence the quality of Ca isotope measurements by MC-TIMS, and we tentatively propose that the external reproducibility determined from the repeated analysis of standards can represent the uncertainty of a single sample analysis.