Rare earth element analysis in natural waters by multiple isotope dilution – sector field ICP-MS†
The rare earth elements (REEs) are valuable tracers in the earth, ocean and environmental sciences. Ten out of fourteen stable REEs have two or more isotopes, making them suitable for quantification by isotope dilution. We present a plasma mass spectrometry based multiple isotope dilution method for high precision REE concentration analysis in aqueous media. Key aspects of the method are: (i) flexible spiking of ten REEs via two LREE and HREE mixed spike solutions. (ii) Offline pre-concentration and matrix removal, by ion chromatography for freshwater samples and by iron co-precipitation or ion chromatography with the Nobias™ resin for seawater samples. (iii) High sensitivity detection by sector field-inductively coupled plasma mass spectrometry (SF-ICP-MS). (vi) The use of a desolvation micro-nebulization introduction system to lower polyatomic Ba and LREE-oxide interferences on HREEs. The method is suitable for a range of freshwater to seawater type samples, and was validated against SLRS-4, SLRS-5, and CASS-5 reference materials and two GEOTRACES marine inter-comparison samples. Long-term external precision on all REEs was <2% RSD, except La and Ce. Minimum sample volumes are 1 ml for freshwater and 50 ml for seawater. The multispike SF-ICP-MS method should be of particular interest in exploring subtle variations in aqueous REE fractionation patterns and anomalies in large numbers of samples.