A Robust Procedure for High-Precision Determination of Rare Earth Element Concentrations in Seawater

This study reports a robust procedure that permits precise measurement of all fourteen naturally occurring rare earth element (REE) concentrations, present at ng kg^-1 to sub ng kg^-1 levels, in ~ 100 ml seawater. This procedure is simple and can be routinely applied to measure seawater REEs with relatively high sample throughput. The procedure involves addition of a ¹⁴²Ce-¹⁴⁵Nd-¹⁷¹Yb-enriched spike mixture, iron co-precipitation, REE purification with chromatographic separation and the use of a magnetic-sector-field ICP-MS (Element 2) coupled with a desolvating sample introduction system (Aridus 1). Critical steps of the procedure, including co-precipitation pH and matrix removal, have been optimised through a set of experiments described here. The accuracy of the new procedure was assessed against a gravimetric mixture of REEs, and the precision was demonstrated by repeated measurement of two well-mixed natural seawaters. Repeated analyses of these seawater reference materials (RMs), using ~ 100 ml seawater for each aliquot, indicate precision of 3% (1s) for the REEs. Measured REE concentrations of two uncertified seawater RMs (CASS-4 and NASS-5) are consistent with published values, and REE concentrations of the GEOTRACES intercalibration samples show good agreement with those reported by other participant laboratories. REE concentrations for other intercalibration samples (SAFe and Arctic PS70) are also reported.