High-precision Cd isotope determination in low-Cd organic-rich biological samples by MC-ICP-MS with UV-assisted high-pressure bomb digestion

Abstract

Cadmium (Cd) isotopes serve as powerful tracers for biogeochemical cycling processes. However, significant interference from organic matter exists in the Cd isotope measurements of organic-rich samples with low Cd contents. Even when the residual organic carbon content is extremely low after complete digestion, inaccuracies in the Cd isotope ratios persist. In this study, the effects of organic matter on Cd isotope measurements were investigated using NIST SRM 3108 Cd standard solutions doped with varying proportions of glucose. Results showed that Cd signal intensities and isotope ratios were influenced by glucose, even at extremely low concentrations. LC-MS chromatogram and ICP-MS analyses indicated that the effect of extremely low organic matter on Cd isotope measurements was caused by polyatomic ion interferences resulting from the incomplete ionization of the ring-structured glucose molecules rather than matrix effects. A UV/HNO₃–H₂O₂ digestion procedure was developed and validated to eliminate these interferences. After UV digestion, accurate and high-precision Cd isotope ratios were obtained even for 10 ng mL⁻¹ Cd solutions containing up to 5000 µg mL⁻¹ glucose. The proposed method was applied to seven certified biological reference materials. The measured δ¹¹⁴Cd values for NIST SRM 1573a, GSB-14, and GSB-16 were consistent with previously reported values, while the δ¹¹⁴Cd values for NIST SRM 1568b, IAEA-450, GSB-1, and GSB-6 are reported for the first time.