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

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 Cd-106-Cd-111 double spike. To eliminate isobaric interference from Sn-114, we use Cd-113 together with Cd-106, Cd-110, and Cd-111 to obtain delta Cd-113/110, and then calculate delta Cd-114/110 as 1.33 x delta Cd-113/110. We find that when Sn/Cd is <= 2.5 in sample solutions, delta Cd-114/110 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 x 10(-3), the measured delta Cd-114/110 values deviate significantly from zero. However, Zn and In can be eliminated completely, and Pd was not detected in any Cd eluents. The delta Cd-114/110 values of three standard solutions (Spex-CUGB, BAM I012, and M & uuml;nster) 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.