Synergic solvent extraction of rare earth elements using mixed ligand complexes of hexafluoroacetylacetone and tri-n-butylphosphate and their determination in environmental waters by low temperature ETV-ICP-MS
Abstract: A synergic solvent extraction system has been developed for the rapid extraction of rare earth elements (REEs) from aqueous media with cyclohexane containing hexafluoroacetylacetone (HHFA) and tri-n-butylphosphate (TBP). The resulting mixed ligand complexes of REEs-HHFA-TBP were introduced into a graphite furnace for the determination of REEs by low temperature electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS). In the absence of TBP or HHFA, the REEs were not efficiently extracted with cyclohexane-HHFA or TBP, and also could not be vaporized at low vaporization temperature, possibly because these metal chelates contain coordinated water. Various factors influencing the extraction of REEs, including the pH values of the solution, the type of extraction solvent, and the phase ratio, have been systematically investigated. It was found that by using HHFA and TBP as chelating reagents, REE-HHFA-TBP complexes could be quantitatively extracted into cyclohexane at pH 6.0 with a phase ratio (aqueous phase to organic phase) of no more than 10. The factors affecting the vaporization and transportation of analytes, such as the concentration of HHFA and TBP, the drying temperature and time, and the vaporization temperature, were also studied; the optimal experimental conditions were established for ETV-ICP-MS. Under the optimized conditions, the detection limits of the method for 15 REEs were in the range of 0.017 ng L-1 [Eu(III)] 0.277 ng L-1 [Nd(III)], and the relative standard deviations (RSDs) for the determination of REEs at 0.05 ng mL(-1) were between 4.5% [Ce(III)] and 9.9% [La(III)]. The proposed method was applied to the analysis of environmental samples, and the recoveries for the spiked samples were in the range of 84-119%.