Precise analysis of copper and zinc isotopic compositions by plasma-source mass spectrometry

Abstract

The stable isotope geochemistry of Cu and Zn is poorly known because of the lack of a suitable analytical technique. We present a procedure for the analysis of Cu and Zn isotope compositions by plasma-source mass spectrometry (Plasma 54) together with a method to purify Cu and Zn from natural samples of silicates, ores, and biological material. A plasma-source mass spectrometer equipped with a magnetic filter and multiple collection can make up for the instability of the ICP source and provide precise Cu and Zn isotope compositions. Instrumental mass fractionation is corrected with respect to the isotopic composition of a standard of a different element added to the sample (Zn for a Cu sample, Cu for a Zn sample) previously purified by anion-exchange chemistry. We have adopted the NIST Cu standard (SRM 976) and a Johnson-Mattey Zn solution as references. This external normalization leads to an internal precision of 20 ppm and an external reproducibility of 40 ppm or 0.04 per mil (95% confidence level). The isotopic compositions can be obtained on as little as 200 ng of element. Isobaric interferences are small enough to be neglected. Isotopic fractionation is observed for Cu on the anion-exchange resin, which requires a full yield to be achieved upon purification. The exponential law of mass fractionation is shown to provide a more consistent correction than the linear and the power law. It is shown that in the mass spectrometer Cu and Zn isotopes do not fractionate to the same extent. The ratio of instrumental mass biases remains constant over one measurement session. This ratio is determined from the analysis of mixed standard solutions, then is used to correct the isotopic composition measured for Cu and Zn samples. Some preliminary results show the existence of isotopic variations of up to several per mil amongst natural samples of silicates, ores, sediments, and biological material, which paves the way for the use of Cu and Zn isotopes as geochemical and biochemical tracers.