An efficient Cd two-stage column system for high-precision determination of Cd isotopic compositions by double spike MC-ICP-MS

Abstract

There has been a growing interest in using cadmium (Cd) isotopes to trace the source of Cd contamination and Cd cycling in the Earth surface. Yet it remains technically challenging to precisely and efficiently determine the Cd isotopic compositions of samples with extremely low Cd concentrations due to the imperfect separation of Cd from the matrix and the interference caused by resin-derived organics when TRU Spec resin was used. In this study, we present a two-stage column chromatography system (AG-MP-1M resin + UTEVA resin) for Cd purification. Here, a Prefilter resin combined with UTEVA resin effectively purifies Cd from Sn after the separation of Cd by AG-MP-1M column while keeping the phosphorous organic compounds to an acceptable level. In contrast to TRU Spec resin, which is prone to leaking through the Prefilter resin, we find that UTEVA resin is effectively retained by the Prefilter resin, thus avoiding the P-related anomalous shifts in the Cd isotope measurement (P/Cd < 1). UTEVA resin also exhibits a comparable capacity for purifying Cd from Sn and Mo when compared to TRU Spec resin. The recovery of the two-stage column system is generally higher than 99% and the procedural blank accounting for sample digestion and chemical purification is lower than 20 pg. The Cd isotopic compositions of several reference materials are determined using double spike MC-ICP-MS, with a ¹¹¹Cd–¹¹³Cd double spike pair optimized for the ¹¹¹Cd, ¹¹²Cd, ¹¹³Cd, and ¹¹⁴Cd inversion. The δ^114/110Cd values of these reference materials relative to NIST SRM 3108 agree within analytical uncertainty with previous studies. The long-term external precision of δ^114/110Cd determined based on replicated measurements of BCR-2 is 0.04‰ (n = 10, 2 SD) for a minimum sample size of 10 ng. Our study thus provides an efficient two-stage column system for determining the Cd isotopic compositions of geological samples with relatively low Cd concentrations.