High precision cadmium isotope analysis of geological reference materials by double spike MC-ICP-MS

Abstract

In this study, we develop a method for high precision Cd isotope analysis by double spike MC-ICP-MS. Two-step Cd purification using AG-MP-1M resin and TRU resin was used to separate Cd from samples with a recovery of more than 95% Cd and a blank less than 0.1 ng. A practical double spike design and optimization procedure is proposed and the effect of the double spike ratio in the double spike-sample mixture (p) on the measured δ^114/110Cd is evaluated, which broadens the appropriate p range from 0.3–0.6 to 0.1–0.8. In addition, the observed shift/deviation of the measured δ^114/110Cd_{NIST 3108} value from zero-delta due to the change of experimental conditions is discussed on a theoretical basis and a correction approach is developed. Potential Mo and Zr based polyatomic ions have no significant effect on δ^114/110Cd in typical geological samples, while isobaric ions such as Sn, Pd and In can seriously affect the accuracy of measurements even at low concentration. It is shown that the tolerances of isobaric interference can be improved to 0.1, 0.05 and 0.02 for Sn/Cd, Pd/Cd and In/Cd, respectively, after mass discrimination correction. The precision and accuracy of the proposed method were verified by analyzing mixtures of NIST 3108 and a secondary reference material BGEG-Cd of different composition. Long term monitoring of the measured NIST 3108 and BGEG-Cd showed repeatability for δ^114/110Cd of 0.00 ± 0.04‰ and −1.00 ± 0.04‰ (2sd, n = 45), respectively. Finally, we measured Cd isotope compositions of several geological standard materials including soil sediments and manganese nodules, and the δ^114/110Cd values of 0.01 ± 0.02‰ (GSD-5a), 0.04 ± 0.02‰ (GSD-7a), −0.06 ± 0.05‰ (GSD-10), −0.34 ± 0.06‰ (GSD-11), −0.10 ± 0.06‰ (GSD-12), 0.10 ± 0.04‰ (GSD-17), 0.05 ± 0.03‰ (GSD-21), and 0.10 ± 0.07‰ (GSD-23) were in good agreement with the reported values, and a δ^114/110Cd value of −0.08 ± 0.04‰ (GSD-3a) was reported for the first time.