The determination of dissolved zirconium and hafnium from seawater using isotope dilution inductively coupled plasma mass spectrometry

This paper describes the development of an analytical technique utilizing isotope dilution and inductively coupled plasma mass spectrometry (ICP/MS) to determine the picomolar and femtomolar concentrations of dissolved Zr and Hf found in seawater. Data acquired using this technique are presented to verify the method; in the subarctic North Pacific Zr ranges from 25 to 366 pmol/kg and Hf from 0.20 to 1.02 pmol/kg, with concentrations increasing smoothly with depth. The chelating ion-exchange resin, Chelex-100, was found to be suitable for the extraction/concentration step required for the determination of dissolved Zr and Hf in seawater using isotope dilution analysis. The extraction was optimized for pH, flow rate, resin volume, elution volume, and the time required for isotope equilibration. The isotope ratios 91Zr/90Zr and 178Hf/177Hf were measured using flow injection ICP/MS. The detection limits for a one litre sample are 0.21 and 0.03 pmol/kg for Zr and Hf, respectively. The analytical precision (1s) of the technique improves with increasing concentration and varies from 7% at 42 pmol/kg to 2.5% at 280 pmol/kg for Zr and from 22% at 0.28 pmol/kg to 9% at 1.4 pmol/kg for Hf.