Analytical Artifacts Associated with the Chelating Resin Extraction of Dissolved Rare Earth Elements in Natural Water Samples

In order to determine rare earth elements (REE) in coastal seawater and groundwater samples using an inductively coupled plasma mass spectrometer, a time-efficient chelating resin column method has been widely used for small water volumes (<100 mL). We obtained improved, quantitative (>95%) results for extracting REE from most of the seawater (including the certified reference materials) and groundwater samples at extraction conditions of pH 5.8–6.0, flow rates <1.2 mL min⁻¹, and 1.2 g of wet resin weight (Chelex 100, Bio-Rad, 7 mm diameter). However, we often encountered large fractionations of HREE (heavy REE) relative to LREE (light REE) associated with chelating resin extraction for some organic-rich coastal seawater samples. This fractionation could be due to unidentified complexation of HREE with natural organic substances in water samples. Our results imply that many previous observations reporting the natural fractionation of HREE using this method could have been misleading. Thus, we suggest that the quantitative (>95%) extraction of REE should be confirmed by using either one of the HREE (i.e., Tm) as a spike or isotope dilution for natural aqueous samples which have large matrix effects.