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

In this study, we develop a method for high precision Cd isotopic analysis by double spike MC-ICP-MS. A two-step Cd purification by 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 double spike-sample mixture (p) on the measured δ114/110Cd is evaluated, which broadened the appropriate p range from 0.3~0.6 to 0.1~0.8. In addition, the observed shift/deviation of the measured δ114/110CdNIST 3108 value from zero-delta due to the change of experimental condition is discussed on theoretical basis and a correction approach was developed. Potential Mo and Zr based polyatomic ions had no significant effect on δ114/110Cd 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 δ114/110Cd 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 nodule, the δ114/110Cd 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 reported values, and δ114/110Cd value of -0.08±0.04‰ (GSD-3a) was reported first time.