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 utiliz ing isotope dilution and inductively coupled plasma mass spectrometry (ICP/MS) to determine the picomolar and femtomolar concentrations of d issolved Zr and Hf found in seawater. Data acquired using this techniq ue 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, wit h concentrations increasing smoothly with depth. The chelating ion-exc hange resin, Chelex-100, was found to be suitable for the extraction/c oncentration step required for the determination of dissolved Zr and H f in seawater using isotope dilution analysis. The extraction was opti mized for pH, flow rate, resin volume, elution volume, and the time re quired for isotope equilibration. The isotope ratios Zr-91/Zr-90 and H f-178/Hf-177 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 (Is) 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. (C) 1998 Elsevier Science B.V.