High-precision cadmium stable isotope measurements by double spike thermal ionisation mass spectrometry

Natural mass-dependent fractionation (MDF) of cadmium isotopes is a promising new tool for investigating Cd pathways and cycling in geological and biological materials. One interesting new application is as chemical tracer of deep water circulation and nutrient distribution in the oceans. But since natural isotope fractionation of Cd appears to be extremely limited and Cd abundances low, excellent external precision and sensitivity are needed to make full use of its potential. Here, we describe a newly-developed double spike (DS) method for determining the MDF of Cd isotopes. For inorganic matrices, samples are spiked with a mixed Cd-106-Cd-108 tracer prior to high-purity anion exchange-based separation of Cd. Isotope measurements are performed by TIMS using a silica gel activator. Overall, the DS-TIMS technique offers benefits in terms of superior precision and sensitivity compared to MC-ICPMS methods currently in use. External precisions on 100-ng-sized Cd standard loads, double spiked as unknowns, are +/- 14 ppm on the Cd-112/Cd-110 ratio (2SD), while ionization efficiencies (ions per atom loaded) are similar to 0.3%. Using this technique, we calibrated the relative difference in MDF between several Cd standard materials used in various laboratories as "zero reference standards''. We also show that the Cd-112/C-110 d ratio can be fractionated by as much as similar to 0.2% by ion-exchange elution, which is potentially problematic for Cd isotope studies not using a double spike.