PRECONCENTRATION OF TRACE RARE-EARTH ELEMENTS IN SEAWATER BY COMPLEXATION WITH BIS(2-ETHYLHEXYL) HYDROGEN PHOSPHATE AND 2-ETHYLHEXYL DIHYDROGEN PHOSPHATE ADSORBED ON A C18 CARTRIDGE AND DETERMINATION BY INDUCTIVELY COUPLED PLASMA MASS-SPECTROMETRY

A rapid and reliable sample preconcentration method with minimum time-consumption and labor has been developed to separate rare-earth elements (REEs) from seawater for the measurement by inductively coupled plasma mass spectrometry (ICPMS). The method involves adsorption of a mixture of bis(2-ethylhexyl) hydrogen phosphate (HDEHP) and 2-ethylhexyl dihydrogen phosphate (H2MEHP) onto a C18 cartridge. By passage of seawater samples through the C18 cartridge, quantitative complexation of REEs with adsorbed complexing agent took place and almost all matrix elements passed through to the drain. The REEs are then removed from the adsorbed complexing agent with an acid eluent. This technique used an eight-channel peristaltic pump for sample processing. Eight 1-L samples of seawater at flow rates of 20 mL/min were preconcentrated within 1 h. This method permitted large enrichment factors of 200-1000-fold to be attained while providing almost matrix-free concentrate suitable for ICPMS analysis. The total contamination determined for the whole process was much less than 1% of the amount of REEs in 1 L of seawater. The average precisions for all REEs after four replicate preconcentrations of 1000 and 5000 mL of the same seawater to final measurement solutions of 5 mL were calculated to be 2.72% and 1.04%, respectively. The parameters which were evaluated for the optimum experimental conditions include pH of the sample, amount of complexing agent loaded on the C18 cartridge, flow rate of the sample, volume of seawater, and concentration and volume of the acid for elution. Since there is no standard seawater sample for REEs, in order to evaluate the precision and accuracy of this novel technique, the analytical results obtained by this method were compared with those obtained by ICPMS coupled with solvent extraction concentrations and neutron activation analysis (NAA) after Chelex 100 concentrations.