Simultaneous determination of six rare earth elements in geological and spent catalyst samples using electrothermal vaporization inductively coupled plasma mass spectrometry following dispersive liquid–liquid microextraction

Abstract

The determination of Gd, La, Tb, Tm, Yb and Y in a variety of digested geological and catalyst samples was carried out using electrothermal vaporization inductively coupled plasma mass spectrometry. The analytes were extracted into CHCl₃ using dispersive liquid–liquid microextraction (DLLME) with 8-hydroxyquinoline as the chelating agent and ethanol as the dispersing solvent. Multivariate optimization of the experimental parameters associated with DLLME was employed to reduce the number of experiments and to evaluate the interaction between variables. Aerosol carrier effects associated with Pd, Zr or a poly(tetrafluoroethylene) (PTFE) slurry, as well as their combinations, were evaluated as to their ability to increase the transportation efficiency of the analytes from the graphite tube to the inductively coupled plasma. Palladium (1.0 μg) was efficient in equalizing the transportation efficiency of the analytes vaporized from aqueous standards and digested samples, both submitted to DLLME, which allowed calibration against aqueous standards to be carried out. Detection limits as low as 0.08 ng g⁻¹ and enhancement factors of up to 323 were obtained. Two spent catalyst samples from the petrochemical industry and three certified reference materials (CRMs) of geological origin (granite, gabbro rock and diabase rock) were analyzed. Three distinct microwave-assisted digestion procedures were evaluated, consisting of mixtures of HNO₃, HCl, H₂O₂, HF and H₃BO₃, and the joint use of the first three reagents was found to be sufficient to provide accurate results for the investigated rare earth elements. Good agreement between determined and certified values, at a 95% statistical confidence level, was achieved for the CRMs, whereas the concentrations of REEs obtained for the catalyst samples were compared to the results obtained from the direct analysis of digested samples using pneumatic nebulization, also with adequate statistical agreement.