High‐Precision Cd Isotope Measurements of Soil and Rock Reference Materials by MC‐ICP‐MS with Double Spike Correction

Abstract

This study presents a high‐precision Cd isotope measurement method for soil and rock reference materials using MC‐ICP‐MS with double spike correction. The effects of molecular interferences (e.g., 109Ag1H+, 94Zr16O+, 94Mo16O+ and 70Zn40Ar+) and isobaric interferences (e.g., Pd, In and Sn) to Cd isotope measurements were quantitatively evaluated. When the measured solution has Ag/Cd ≤ 5, Zn/Cd ≤ 0.02, Mo/Cd ≤ 0.4, Zr/Cd ≤ 0.001, Pd/Cd ≤ 5 × 10−5 and In/Cd ≤ 10−3, the measured Cd isotope data were not significantly affected. The intermediate measurement precision of pure Cd solutions (BAM I012 Cd, Münster Cd and AAS Cd) was better than ± 0.05‰ (2s) for δ114/110Cd. The δ114/110Cd values of soil reference materials (NIST SRM 2709, 2709a, 2710, 2710a, 2711, 2711a and GSS‐1) relative to NIST SRM 3108 were in the range of −0.251 to 0.632‰, the δ114/110Cd values of rock reference materials (BCR‐2, BIR‐1, BHVO‐2, W‐2, AGV‐2, GSP‐2 and COQ‐1) varied from −0.196‰ to 0.098‰, and that of the manganese nodule (NOD‐P‐1) was 0.163 ± 0.040‰ (2s, n = 8). The large variation in Cd isotopes in soils and igneous rocks indicates that they can be more widely used to study magmatic and supergene processes.

Effect of the Dry Ashing Method on Cadmium Isotope Measurements in Soil and Plant Samples

“…Previous studies have suggested that high‐pressure bomb digestion for high‐sensitivity determination of Cd isotopes can be adapted to a wide range of geological and environmental samples, especially samples with low Cd content and complex matrices (Liu et al . 2020, Tan et al . 2020).…”

“…In addition, the δ 114/110 Cd values from high‐pressure bomb digestion of NIST SRM 2710, NIST SRM 2710a, NIST SRM 2711 and NIST SRM 2711a in this study are −0.16 ± 0.03‰ (2 s , n = 14), −0.15 ± 0.07‰ (2 s , n = 13), 0.66 ± 0.06‰ (2 s , n = 6) and 0.57 ± 0.04‰ (2 s , n = 6), respectively, which are consistent with the values of −0.18 ± 0.06‰, −0.20 ± 0.05‰, 0.63 ± 0.04‰ and 0.55 ± 0.05‰ obtained by Liu et al . (2020). The δ 114/110 Cd value by high‐pressure bomb digestion of NIST SRM 1570a was 0.47 ± 0.03‰ (2 s , n = 8), which is consistent with the value of 0.54 ± 0.07‰ (non‐Ag‐doping) and 0.51 ± 0.03‰ (Ag‐doping) reported by Scott et al .…”

“…Cadmium purification followed the same procedure as Liu et al . (2020). Two millilitres of anion exchange resin (AG1‐X8, 100–200 mesh; Bio‐Rad, Hercules, CA, USA) was subsequently cleaned with 5 ml HPW, 10 ml 1 mol l −1 HNO 3 and 2 ml 6 mol l −1 HCl and conditioned with 2 ml 6 mol l −1 HCl.…”

“…111 Cd‐ 113 Cd was employed as the double‐spike pair according to Liu et al . (2020). The instrumental parameters and cup configurations are summarised in Table 1.…”

“…Although the high‐pressure bomb is more complicated than the ashing method, it would not affect Cd isotope measurement results (Liu et al . 2020). We analysed Cd isotope ratios of different types of soil and plant samples, including seven certified reference materials, two natural soils (R01 and R02) and four sedum shoot samples (SS01, SS02, SS03 and SS04).…”