High-precision measurement of Cd isotopes in ultra-trace Cd samples using double spike-standard addition MC-ICP-MS

Abstract

The double spike-standard addition (DSSA) is a powerful method proposed for isotope measurement of ultra-trace elements in samples. Here, we test the method's practicality and robustness for obtaining high-precision δ^114/110Cd data in samples with complex matrices and ultra-trace cadmium (Cd) concentrations as low as 0.004 μg g⁻¹. Through a DS data reduction routine and isotope binary mixing model, the δ^114/110Cd values obtained for numeral geological and biological reference materials (GRMs/BRMs) agree well with certified or previous measurements, even for ∼2.1 ng Cd of BCR-2. The overall precision of both single- and multi-standard addition depends on the sample fraction (f_spl) owing to the error propagation. And 0.041 ± 0.022‰ (2SD, n = 46) of precision for δ^114/110Cd can be achieved when the sample fraction (f_spl) is ≥20%, comparable to 0.056 ± 0.039‰ (2SD, n = 23) obtained by the traditional DS method. However, it becomes greater than 0.110‰ when f_spl is <20%, indicating that 20% (1/5) −50% (1/2) of f_spl is the optimal mixing range to obtain high-precision data if the minimal sample sizes are required. These further confirm that accurate and precise Cd isotope ratios can be determined by DSSA. Correspondingly, the purification scheme can be simplified to a single column due to the added standard solution boosting analyte and diluting ratios of matrix/Cd. Animal organs, with 0.004–0.106 μg g⁻¹ Cd, yield large variations of δ^114/110Cd (−0.054 ± 0.030‰ (2SD, n = 4) and 0.681 ± 0.022‰ (2SD, n = 4) for ovine liver and kidney, respectively), suggesting that Cd isotopes can be fractionated significantly during biological metabolic processes and may be a potential use in medical diagnosis. Robust measurement of Cd isotope composition in ultra-trace Cd samples with complex matrices by DSSA broadens the scope of measurable samples by the traditional DS method, thus potentially opening a range of new opportunities in life, agricultural, environmental, and earth/planetary sciences.