High-precision Cd isotope analysis of soil and rock by double spike MC-ICP-MS

Cadmium (Cd) is a trace element on Earth and causes health problems to humans. A monitoring of the Cd level of, for example, arable soil is therefore of utmost importance. Even though Cd isotopes are a valuable proxy for such a monitoring, the low concentration of Cd in soil and rock challenged its determination hitherto. Thus, here we developed a novel high-precision Cd isotope measurement method for soil and rock samples. First, we developed a one-step anion chromatography method using micro-columns. Second, we developed a MC-ICP-MS method using double spike approach to correct for instrumental mass-dependent isotope fractionation. Parameters affecting the precision and accuracy of the Cd isotope composition (δ114/110Cd) were systematically tested. These tests included molarity and concentration mismatches between purified samples and the bracketing standard NIST SRM 3108, and the variation in double spike and sample ratio. We also evaluated potential mass biases caused by molecular and isobaric interferences. The long-term external precision for δ114/110Cd is better than ± 0.05‰ (2 SD). Cd isotope compositions of fifteen international reference materials were determined, including soils and igneous rocks of different Cd concentrations and a manganese nodule. The δ114/110Cd values of the reference materials ranged from -0.25 ~ 0.63‰ for soil samples, and from -0.20 to 0.10‰ for rock samples. The large variations in δ114/110Cd in soils and igneous rocks suggest the application of Cd isotopes to gain more detailed insights into magmatic processes and the biogeochemical cycle of Cd in terrestrial ecosystems.