Precise/small sample size determination of stable Cd isotope ratios of geological samples with double spike MC-ICP-MS

Abstract

The precise determination of Cd isotope ratios via multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has made the widespread application of Cd isotopes possible. However, previous studies are complicated by the chemical purification method and molecular interferences from Zn (e.g.⁷⁰Zn³⁶Ar⁺, ⁶⁸Zn³⁸Ar⁺, and ⁶⁶Zn⁴⁰Ar⁺ on ¹⁰⁶Cd⁺, ⁷⁰Zn³⁸Ar⁺ and ⁶⁸Zn⁴⁰Ar⁺ on ¹⁰⁸Cd⁺, ⁷⁰Zn⁴⁰Ar⁺ on ¹¹⁰Cd⁺). Separation of Zn from Cd requires the usage of a toxic reagent (HBr) and is also complicated by the relatively high procedural Zn blank especially when only a very small amount of Cd is available. Here we present a high-precision Cd isotope measurement method using double spike MC-ICP-MS, which avoids the molecular interferences from Zn introduced by both sample and chemical purification. A ¹¹¹Cd–¹¹³Cd double spike pair optimized for the ¹¹¹Cd, ¹¹²Cd, ¹¹³Cd, and ¹¹⁴Cd inversion was used to correct for the mass bias. Elemental doping experiments suggest that the measured Cd isotope data were not significantly affected when the measured solution has, Zn/Cd < 10³, Pd/Cd < 10², Fe/Cd < 10⁻², In/Cd < 10⁻³, Sn/Cd < 10⁻¹, Zr/Cd < 10⁻², Nb/Cd < 10⁻¹, Mo/Cd < 10⁻², Ru/Cd < 10⁻², Ga/Cd < 10⁻², Ge/Cd < 10⁻², Se/Cd < 10⁻¹, and As/Cd < 10⁻¹. We thus modified the chemical purification method by removing the HBr step and eluting Zn and Cd together. The minimum requirement of sample size for precise measurement of stable Cd isotope ratios was tested to be as low as 10 ng. The measurements of pure Cd solutions (BAM I012 Cd and HPS Cd) give a long-term intermediate precision for δ^114/110Cd relative to NIST SRM 3108 of 0.05‰. We further validated our method by measuring different reference materials covering a relatively wide range of Zn/Cd (∼9 to 1239), the results of which agree within uncertainty with previous studies. Our study thus provides an efficient and precise way to determine stable Cd isotope ratios of geological samples with relatively low Cd concentrations.