Rapid and precise analysis of rare earth elements in small volumes of seawater - Method and intercomparison

Rare earth elements (REEs) are used as tracers for oceanic trace element cycling. However, the low (picomolar level) dissolved REE concentrations and time-consuming methods have so far hindered their extensive use in marine studies. This study reports the first application of the automated seaFAST-pico system (Elemental Scientific Inc.) in offline mode and using multi-element isotope dilution inductively coupled plasma-mass spectrometry (ID ICP-MS) for the robust and rapid pre-concentration, purification, and analysis of dissolved REEs from small volumes of seawater (11-12 mL). Accuracy of our new method is checked with replicates of GEOTRACES intercalibration seawater from BATS (Bermuda Atlantic Time Series, North Atlantic) at 15 m and 2000 m water depths. Our results show excellent agreement (within the analytical uncertainty, 2σ SD) with the published intercalibrated values from the GEOTRACES intercalibration study. Replicates of GEOTRACES intercalibration seawater from SAFe at 3000 m water depth (Sampling and Analysis of Iron, North Pacific) indicate a procedural long-term error of ≤ 3.9% 2σ RSD for all REEs, except for Ce and Gd. An international intercomparison from 4 labs using SAFe 3000 m seawater aliquots, 2 of which also use the seaFAST-pico system in offline mode, and different pre-concentration, purification, and analytical methods shows excellent agreement between REE concentrations within 7% (2σ RSD) (except for Ce 71%, Gd 14%, Lu 12%). This is comparable to the agreement obtained for the BATS 15 m and 2000 m samples by six different labs for the international GEOTRACES intercalibration study (van de Flierdt et al., 2012). The REE intercomparison values of SAFe 3000 m (i.e., the average of the values from the 4 labs) in this study agree within the analytical uncertainties (2σ SD) with published REE values of deep seawater from nearby stations. Our international intercomparison provides the first REE intercomparison values for the GEOTRACES intercalibration station SAFe at 3000 m, and establishes the first reference seawater REE values for quality control of future REE studies in the Pacific Ocean as SAFe will remain a GEOTRACES baseline, and hence intercalibration station, for cruises in the North Pacific. Our method is easy to adopt and enables the extensive use of REEs, thereby opening the way to build a global seawater REE data set.