A two-stage column protocol for the separation of Cu from geological materials and high-precision Cu isotopic analysis on a MC-ICP-MS

Abstract

This study presents a simple and efficient two-stage tandem protocol for separating Cu from geological samples for accurate and precise measurement of Cu isotopic ratios employing multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The first column utilizes Cu-selective resin to separate Cu from the matrix by employing 0.001 mol L⁻¹ and 0.5 mol L⁻¹ HNO₃ to elute the majority of matrix elements and Cu from the resin, respectively. The second column utilizes AG50W-X12 cation exchange resin (100–200 mesh, Bio-Rad) to further purify Cu from the remaining trace elements such as Ti, V, and Fe. A two-stage tandem column setup is utilized, with the Cu eluent from the Cu-selective resin column being allowed to directly pass onto the cation exchange column without an evaporation step. The presented column protocol only requires a total of 38 mL of dilute acid and takes ∼3 h for Cu separation, generating a low analytical blank level (0.37 ± 0.05 ng; 2SD, n = 8). The instrumental mass bias of Cu isotopic measurement induced by MC-ICP-MS is corrected by applying a sample-standard bracketing method. In light of repeated measurements of NIST SRM 976, NWU-Cu-A, and NWU-Cu-B, the long-term external reproducibility is better than ±0.07‰ (2SD, n = 277) for δ⁶⁵Cu. The validation of the presented protocol is confirmed by analysis of twelve geological reference materials with accurate and precise results, which are identical to the published values within uncertainty. Therefore, the presented protocol can be employed efficiently for obtaining pure Cu fractions and determining Cu stable isotopic ratios in geological samples.