Anal. Chem. 2003, 75, 3396-3403
A Preconcentration/Matrix Reduction Method for the Analysis of Rare Earth Elements in Seawater and Groundwaters by Isotope Dilution ICPMS Timothy J. Shaw*
Department of Chemistry and Biochemistry, Marine Science Program, University of South Carolina, Columbia, South Carolina 29208 Thomas Duncan
Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208 Bernhard Schnetger
Institute for the Chemistry and Biology of the Marine Environment, University of Oldenburg, Oldenburg, Germany
The study of rare earth elements (REEs) in seawater and groundwaters is driven by their utility as process tracers,1-4 contaminant proxies,5-7 and geochemical proxies.8-12 REEs are released into natural waters by weathering of source rock and
remobilization from particles and sediment. The lanthanide contraction, a gradual decrease in ionic radius from La to Lu, leads to changes in the strength of complexation across the REE series. Heavy REEs (HREEs) typically form stronger complexes than light REEs (LREEs) and remain in solution; whereas, the LREEs are preferentially scavenged and adsorbed onto settling particles and surfaces. The change in ratio between the LREEs and HREEs is useful in determining the dominant ligands in solution and the nature of the source rock to which the water mass was exposed. Further, the anomalous behavior of Ce in natural waters makes it useful as a tracer of oxidizing and reducing conditions. The REEs typically occur in the 3+ oxidation state in natural waters; however, Ce is readily oxidized to Ce4+. In this oxidization state, Ce is more particle reactive and tends to be easily removed from solution. By comparing Ce to its neighboring REEs, a Ce anomaly can be determined, revealing the oxidizing and reducing chemistry of the water mass. Unfortunately, the analysis of the REEs in natural waters is complicated by low concentrations (low pM to nM), and a variety of matrix interferences (e.g., Ba2+, seawater salts). Separation of Ba is critical in REE analysis because the oxides of the five most abundant Ba isotopes have masses overlapping isotopes of Sm, Eu, and Gd. Ba is highly enriched in natural waters with respect to the REEs (e.g., ∼5 ng/g in surface seawater compared to
