The determination of the isotopic composition of Cu and Zn in seawater

Abstract

The stable isotope compositions of Zn and Cu in natural materials are newly available for measurement with the advent of multiple-collector inductively coupled mass spectrometry (MC–ICPMS). Although the oceans are prime scientific targets, no progress has been made as yet because of considerable analytical challenges involving the low concentrations of these elements in seawater. We present a procedure which allows isotopic analysis of Zn and Cu isotope compositions of seawater samples at least 1 L in size by MC ICPMS. The main difficulty in analysing Zn and Cu isotopes from seawater samples is their low concentrations (nM/kg) in a matrix with much higher concentrations of other elements. We have surmounted these problems using a procedure that involves pre-concentration using either a BioRad Chelex®100 column or co-precipitation, followed by further purification and separation of Cu and Zn on an anion column. The major advantage of co-precipitation is that the blank is low and allows the precise isotopic analysis of even the most Zn-depleted surface ocean samples. Zn samples have been analysed using a double spike in order to correct for analytical mass discrimination. This approach has been tested using standard-doped seawater samples that had previously been stripped of their metal contents using the Chelex column. The approach is highly successful and yields δ 66Zn = − 0.02 ± 0.05‰ ( n = 5) relative to the standard dopant. For multiple analyses of a large sample of the English Channel we obtain a result for δ 66Zn of 0.35 ± 0.08‰ ( n = 11) for Chelex extraction and 0.31 ± 0.04‰ ( n = 5) for co-precipitation. For both Cu and Zn, correction for instrumental mass discrimination using standard-bracketing requires some care to overcome artefacts-related sample introduction into the mass spectrometer. A depth profile from the NE Pacific is also presented. Measured concentrations are very similar to those reported elsewhere. Cu and Zn isotopes in the upper 400 m of the water column are anti-correlated and may reflect the dominance of scavenging for Cu and biological recycling for Zn.