Efficient Cd separation protocols for high-precision cadmium isotope analyses of diverse samples by double spike MC-ICP-MS

Abstract

Cadmium isotope analyses are applied for research in planetary, Earth, environmental and life sciences. However, there is still a lack of efficient methods for the separation of the trace element Cd from the different types of samples that are of interest for isotopic analyses. This study presents new and improved Cd separation and purification techniques for meteorite, diverse terrestrial and seawater samples prior to Cd isotope measurements by multiple collector ICP-MS using the double spike approach for mass bias correction. The first separation stage for meteorites and terrestrial samples employs an existing anion exchange chromatography method that uses AG MP-1M resin, whilst AG 1-X8 and Nobias Chelate PA-1 resins were applied for the initial isolation of Cd from small and large seawater samples, respectively. Teflon microcolumns and AG MP-1M anion exchange resin was then employed for further Cd purification of all sample types. The methods consistently isolate Cd from matrix elements more efficiently than previous protocols, whilst achieving consistently high Cd recoveries, of >95% for meteorite and terrestrial samples and >80% for seawater samples, combined with low procedural blanks of <0.08 ng. The performance of the new protocols is demonstrated by repeated Cd isotope analyses of well-characterized geological and biological reference materials, which produced data that are in excellent agreement with results that were obtained with previous, more laborious separation methods. Furthermore, precise Cd isotope compositions (δ114Cd) are presented for the Murchison meteorite and, for the first time, the ordinary chondrite GRO 95504. Finally, analyses of seven seawater samples, with Cd concentrations between 0.25 and 0.91 nmol kg-1 using sample volumes of up to 2 L, generated results consistent with those of previous studies. Overall, the new methods thus enable unbiased δ114Cd measurements for diverse sample types with a 2SD precision of better than ±0.07‰, even for samples with 10 ng or less of natural Cd.