A Preconcentration/Matrix Reduction Method for the Analysis of Rare Earth Elements in Seawater and Groundwaters by Isotope Dilution ICPMS

A simple method of simultaneous preconcentration and matrix reduction was developed for the analysis of rare earth elements (REEs) in seawater and groundwater by ID ICPMS. The method utilizes partitioning of the REEs with solid hydroxides to separate them from soluble matrix species (e.g., Ba2+, NOM, seawater salts). Acidified samples were spiked and equilibrated with an enriched isotope cocktail (142Ce, 145Nd, 161Dy, 171Yb). Aqueous NH3 was then added to the spiked samples to induce the coprecipitation of the REEs with a small fraction of the natural Mg2+ as Mg(OH)2. The samples were centrifuged and the precipitate was rinsed to remove more than 99.8% of the Ba2+ along with the matrix salts. The precipitate was dissolved in 400 microL of 10% HNO3 for ICPMS analysis. The four spiked elements, determined by isotope dilution, served as internal standards for the remaining REEs. Analysis of NASS-4 and NASS-5 seawater reference materials showed good agreement with published values. Calculated limits of detection for a 1.65-g sample ranged from 0.1 pg/g for the light REEs to 0.02 pg/g for the heavy REEs. The reagent blanks ranged from a high of 0.28 pg/sample for Ce to a low of 0.0036 pg/sample for Tb.