A two-stage Cd purification method with anion exchange resin and BPHA extraction resin for high precision determination of Cd isotopic compositions by double spike MC-ICP-MS

Abstract

TRU Spec resin is commonly used to purify Cd from residual Sn, however, this resin can induce anomalous interference on Cd isotopic measurement which may be attributed to phosphor bound organics that are eluted along with Cd fractions. In this study, progressive δ^114/110Cd_{NIST 3108} isotopic positive shifts are observed with the increase of the concentration ratio of [P/Cd] (e.g., ≥100), which further confirms the interference of P on Cd isotopic measurement. Furthermore, mass scans of the P solution reveals no interfering ion beams across the Cd mass range (masses 106 to 116). These results indicate that anomalous shifts in the Cd isotope measurement may be caused by P-related matrix effects. Here, BPHA extraction resin instead of TRU Spec resin is employed to further purify Cd from residual Sn after the separation of Cd using an anion exchange resin column (AG-MP-1M). The BPHA extraction resin is made of N-benzoyl-N-phenylhydroxylamine (BPHA) impregnation into a microporous acrylic ester polymeric resin (CG-71) without phosphorous organic compounds. This resin exhibits a good ability to purify Cd fractions from interference elements such as Sn, Mo, Zr, and Nb. Cd isotope ratios are determined using double spike MC-ICP-MS. The δ^114/110Cd_{NIST 3108} values of geological reference materials determined by the presented method are in agreement with previous results within analytical uncertainties. The results confirm that our proposed method is suitable for accurate and precise determination of Cd isotopic compositions in low-Cd geological samples.