Automated separation of Sr from natural water samples or carbonate rocks by high performance ion chromatography

Abstract

An automated separation of strontium prior to isotopic analysis from liquid samples (river water, seawater or solutions resulting from natural carbonate dissolution) was developed using High Performance Ion Chromatography (HPIC) technique. The experimental set up consists of an automatic sampler, an ion chromatograph and a fraction collector. To evaluate the performance of the set up, we compare the separation of strontium performed by HPIC with routine techniques which are manually operated using Sr-Spec resin ([Birck, J.-L.,1986. Precision K–Rb–Sr isotopic analysis; application to Rb–Sr chronology. Chemical Geology, 56, 73–83; Horwitch et al., 1990; Pin, C. and Bassin, C., 1992. Evaluation of a strontium-specific extraction chromatographic method for isotopic analysis in geological materials. Analytica Chemica Acta, 269, 249–255]) for 18 river-waters with very different Sr concentrations and Ca / Sr ratios, 5 seawater samples, and two solutions obtained by partial dissolution of Cenozoic high carbonate content marine sediments. The 87Sr / 86Sr isotopic ratios were measured by TIMS. Results agree within error at acceptable statistical scatter that can mostly be accounted for by the uncertainty of the mass spectrometry. Blank values average at 127 pg, but contamination is more than 90% introduced by the eluent and can be reduced if necessary. We show also that the conductivity cell detector after eluent suppression is sensitive enough to accurately measure these very low concentrations and thus that blank values can be evaluate directly by the HPIC set up. The major advantage of this technique is the saved handling time. Moreover, this technique benefits also from a number of on-line controls. During the procedure, Sr and Ca concentrations in the sample are determined. Within the same analytical procedure, others major elements such as Na, K, or Mg concentration can also be evaluated. In addition, the efficiency and the purity of the separation can easily be monitored by determination of Rb, Ca, and Sr content of the collected fraction. We show that the HPIC-separation technique has a great potential for dating marine carbonate sediments and for other high-resolutions studies. Extension to rare earth and lead separations is foreseen in the near future as well as an extension to Mg isotopic analysis. Silicate analysis is anticipated to be a more difficult step.