High-precision MC-ICP-MS measurements of Cd isotopes using a novel double spike method without Sn isobaric interference†

Abstract

Previous studies have reported that even low concentrations of Sn can lead to biased Cd isotopic measurements using MC-ICP-MS. In this paper, we propose a novel method of Cd isotopic analysis involving use of a ¹⁰⁶Cd–¹¹¹Cd double spike. To eliminate isobaric interference from ¹¹⁴Sn, we use ¹¹³Cd together with ¹⁰⁶Cd, ¹¹⁰Cd, and ¹¹¹Cd to obtain δ^113/110Cd, and then calculate δ^114/110Cd as 1.33 × δ^113/110Cd. We find that when Sn/Cd is ≤2.5 in sample solutions, δ^114/110Cd values are not affected by Sn, which behaves as a matrix element rather than causing isobaric interference. Cd isotopic measurements are not sensitive to the molarities of diluted HNO₃ or the Cd concentration. Additionally, when Mo/Cd, Ni/Cd, and Se/Cd are ≤1 in sample solutions, Cd isotopic measurements are not significantly affected. Instead, when Zn/Cd is >0.1, In/Cd > 0.05, and Pd/Cd > 5 × 10⁻³, the measured δ^114/110Cd values deviate significantly from zero. However, Zn and In can be eliminated completely, and Pd was not detected in any Cd eluents. The δ^114/110Cd values of three standard solutions (Spex-CUGB, BAM I012, and Münster) and four geochemical reference materials (SGR-1b, BCR-2, GSD-7a, and NIST 2711a) were measured and found to be in close agreement with published results (with 2SD and ranges for all data of less than 0.090 and 0.120, respectively). This indicates that the data obtained by our double spike method are precise and reliable. Additionally, our new technique can help to simplify separation procedures, thus saving time and reducing the quantities of acid required.