Improved Sampling Design for Depth Profile Analysis of Marine Sediments Using Sector-Field – Inductively Coupled Plasma – Mass Spectrometry (SF-ICP-MS)

How Feasible is Direct Determination of Rare Earth Elements in Seawater by ICP-MS?

With the increasingly wide industrial use of rare earth elements (REEs), their release into marine systems makes it important to understand in what quantities they occur and to what geochemical processes they contribute, preferably using direct analytical methodology. In this study, analytical performance of high-resolution ICP-MS was assessed with regard to quantification of REEs in seawater without matrix separation and analyte preconcentration. With optimized sample dilution, precise and interference-free quantifications of most of the REEs in samples taken from Kara Sea were obtained, with an accuracy of 3 to 9% (against the independently asserted values), repeatability of 3 to 5%, and intermediate precision and reproducibility, averaging 4 and 11%, respectively. The method was further validated by using a certified reference material for nearshore seawater. However, the limits of detection obtained (0.04 - 0.38 ng L^-1), while not significantly inferior to those obtained after sample enrichment, appear to be not low enough to analyze high salinity sea samples (over 30 parts per thousand) or open-ocean water samples, which require higher dilution factors or contain (much) lower REE concentrations, respectively. Therefore, it was concluded that the direct determination of REEs is only possible from samples with moderate salinity such as estuarine or shallow-sea water. In the latter case, the longitudinal REE profiling assayed by ICP-MS allowed us to assume that the export of the contaminated material from land areas into estuaries and then to the sea by rivers may substantially contribute to the seawater pool of REEs.