A new methodology for precise cadmium isotope analyses of seawater

Xue, Zichen, Rehkämper, Mark, Schönbächler, Maria, Statham, Peter J. and Coles, Barry J. (2012) A new methodology for precise cadmium isotope analyses of seawater. Analytical and Bioanalytical Chemistry, 402 (2), 883-893. (doi:10.1007/s00216-011-5487-0).

Abstract

Previous studies have revealed considerable Cd
isotope fractionations in seawater, which can be used to
study the marine cycling of this micronutrient element. The
low Cd concentrations that are commonly encountered in
nutrient-depleted surface seawater, however, pose a particular
challenge for precise Cd stable isotope analyses. In this
study, we have developed a new procedure for Cd isotope
analyses of seawater, which is suitable for samples as large
as 20 L and Cd concentrations as low as 1 pmol/L. The
procedure involves the use of a 111Cd–113Cd double spike,
co-precipitation of Cd from seawater using Al(OH)3, and
subsequent Cd purification by column chromatography. To
save time, seawater samples with higher Cd contents can be
processed without co-precipitation. The Cd isotope analyses
are carried out by multiple collector inductively coupled
plasma mass spectrometry (MC-ICP-MS). The performance
of this technique was verified by analyzing multiple
aliquots of a large seawater sample that was collected from
the English Channel, the SAFe D1 seawater reference
material, and several samples from the GEOTRACES
Atlantic intercalibration exercise. The overall Cd yield of
the procedure is consistently better than 85% and the
methodology can routinely provide ?114/110Cd data with a
precision of about ±0.5 ? (2sd, standard deviation) when at
least 20–30 ng of natural Cd is available for analysis.
However, even seawater samples with Cd contents of only
1–3 ng can be analyzed with a reproducibility of about ±3
to ±5 ?. A number of experiments were furthermore
conducted to verify that the isotopic results are accurate to within the quoted uncertainty.

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