Effective separation of zinc from geological samples for high-precision zinc isotope measurement using MC-ICP-MS

Abstract

We present two efficient chemical procedures to separate Zn from geological samples using a single-stage column with AG1-X8 (100–200 mesh) for high-precision Zn isotope ratio determination. These two methods take advantage of 3 M HCl–1 M HF (procedure A) and 3 M HCl–0.1% H₂O₂ (procedure B) to quantitatively and effectively remove Ti and also most of the matrix from anion exchange resin. Then mainly Nb and Ta in the procedure A and mainly U and Fe in the procedure B can be further eluted from anion exchange resin using 0.5 M HCl–0.5% H₂O₂, respectively. The Zn cut is finally eluted from anion exchange resin using 0.5 M HNO₃ for both procedures. The results show that all the studied samples, even the sample with Ti/Zn ratios as high as ∼400 (e.g., GSR-10), can achieve very low levels (⁴⁸Ti/⁶⁴Zn < 0.0035) in the Zn cut after processing with our proposed procedures. Zinc isotope ratios were determined with a Neptune Plus MC-ICP-MS, utilizing Cu external normalization combined with sample-standard bracketing for instrumental mass discrimination correction. The long-term measurements of international standards yielded δ^66/64Zn_{JMC Lyon} values of −1.43 ± 0.04‰ (2SD, n = 362) for NIST SRM 3168a, 0.29 ± 0.04‰ (2SD, n = 61) for IRMM 3702 and 0.29 ± 0.04‰ (2SD, n = 61) for AA-ETH-Zn, which align well with published values. The long-term external reproducibility for these three pure Zn standard solutions was ±0.04‰ (2SD). Geological reference materials (GRMs) processed by both procedures produced consistent δ^66/64Zn_{JMC Lyon} composition, which agreed well with previously reported data. These results confirm that the proposed procedures are suitable for purifying Zn for high-precision isotope determinations using MC-ICP-MS.