High-precision zinc isotopic characterization of twenty soil reference materials from China determined by MC-ICP-MS

Zinc isotopic ratios serve as powerful tools for tracing biochemical cycling of metals at Earth's surface, including the distribution, transportation, and enrichment of zinc (Zn) in soil. To conduct such studies and enable inter-laboratory comparisons, high-precision Zn isotopic measurements require the use of soil reference materials (RMs). However, there have been limited reports on the high-precision Zn isotope ratios of soil RMs thus far. In this study, we have developed a two-step Zn chemical separation protocol utilizing Bio-Rad AG MP-1M resin columns. This method has demonstrated excellent reproducibility for measuring the external δ⁶⁶Zn values (relative to JMC-Lyon) of standard soil reference materials over an extended time period, with a better than 0.06‰ (2SD) precision. Remarkably, this study is the first to report the Zn isotopic compositions of 20 soil reference materials from various soil types in China. With the exception of one sample obtained from a mining area, the Zn isotopic compositions of all the analyzed soil reference materials exhibit remarkable similarity, with an average δ⁶⁶Zn value of 0.31 ± 0.12‰, which aligns closely with the values observed in igneous rocks. The exceptional sample, with a higher δ⁶⁶Zn value of 0.61 ± 0.02‰, indicates potential contamination during mining activities.