Inductively Coupled Plasma Mass Spectrometry For The Trace Analysis Of Rare Earth Elements And Their Speciation

Acting as a key element in the development of high technology, rare earth elements (REEs) has been widely used in various research fields, including materials, information, energy, ocean and space technology. With the increasing exploration and application of REEs resources, REEs analysis is of great importance in both theory and practical application. By taking advantages of multi-elemental detection capabilities along with high selectivity and sensitivity, inductively coupled plasma mass spectrometry (ICP-MS) has been regarded as one of the most powerful analytical techniques for REEs than other conventional techniques. However, the main drawbacks associated with this analytical technique arise from the spectral interference and the matrix effect. In order to overcome the above problems and to further improve the analytical performance of ICP-MS in trace REEs analysis, researchers pay much attention to the study of sample introduction techniques, spectral interferences, matrix effect and solid sample direct analysis.It should be pointed out that a hidden pollution trouble came to being during the migration and accumulation of REEs in environmental and ecological circulatory. Accordingly, speciation analysis of REEs, which will express their chemical behavior, physiological activity and toxicity, has attracted great interest of analyst and other people. As well known now, the key points to get the accurate speciation are to identify and accurately determine the species of a given element. ICP-MS is unable to show information of certain species, although it serves as a standout analytical technique. However, the combination of ICP-MS with appropriate chromatographic and non-chromatographic techniques has been proven to be one of the most powerful techniques for the speciation analysis of REEs.The aim of this dissertation is to systematically study the spectral interference, matrix effect, sample introduction techniques (including pneumatic nebulization (PN) and electrothermal vaporization (ETV)) in REEs analysis by ICP-MS, and to develop new methods for the determination of trace REEs in high purity rare earth oxides and environmental samples. Additionally, high performance liquidchromatography (HPLC) was hyphenated with ICP-MS to investigate the retention behaviors of Ce and its chelates on different stationary phase preliminarily. The major contents of this dissertation are described as follows:1. The effect of ICP-MS operating parameters on the REO(H)+/RE+ production ratio was studied in detail, and a optimal ICP operating conditions were established. At the same time, the relationship between REO(H)+/RE+ production ratio and the bond strength of the rare earth oxides was also discussed briefly. For the correction of the spectral interference induced by the matrix (neodymium), a simple correction equation was used for correcting interference of polyatomic ions NdO+ and NdOH+ with 159Tb and l65Ho. The proposed method was applied to the determination of trace rare earth impurities in high purity Nd2C>3, and the analytical results were in good agreement with the recommended reference values.2. A new method of stepwise dilution was proposed to study the matrix effects in the determination of REEs by ICP-MS. The highest tolerant level of matrix obtained by this method coincided with that obtained by the conventional method. Trace REEs in different samples were determined directly by ICP-MS without the use of internal standard and matrix matching and the determined values are in a good agreement with the certified values. Stepwise dilution method is a potential method in practical analysis with advantages such as simplicity for operation, low cost, no need for higher purity matrix, without the use of internal standard and suitable to any other samples with complicated matrix.3. An ICP-MS analysis method with just one standard series for direct determination of trace rare earth impurities in various rare earth oxides was developed. The proposed method was applied to the analysis of trace rare earth impurities in different high purity rare earth oxides (Y2O3, PreOn, Nd2C>3, Dy2O3, Er2O3), and the analytical results obtained were in good agreement with the recommended values. This method was characterized with simplicity, rapidity, sensitivity, small sample amount required, and no internal standard/matrix matching requirements.4. PN-ICP-MS and ETV-ICP-MS were employed for the determination of trace REEs in soil samples, and their analytical characteristics, spectral interference and matrix effect were evaluated and critically compared. Under the optimizedoperation conditions, the relative detection limits of Y, La, Ce, Ho, Lu for PN-ICP-MS and ETV-ICP-MS were 1.1-21 ng-L1, and 0.8-4.1 ng-L"1, respectively; the absolute detection limits for ETV-ICP-MS were 8-41 fg, which were improved by 1-2 orders of magnitude than PN-ICP-MS. While the analysis precision of ETV-ICP-MS is worse than that of PN-ICP-MS with the RSD (%) of 4.4-10 % for the former and 2.9-5.7 % for the latter. Regarding to the matrix effect, a stepwise dilution method was employed to observe the effect of matrix and it was found that the highest tolerance concentration of the matrix is 800 mg-L"1 and 1000 mg-L"1 for PN-ICP-MS and ETV-ICP-MS, respectively. To validate the accuracy, both methods were applied to the determination of trace REEs in a soil standard reference material and the determined values for both methods are in a good agreement with the certified values.5. A method of slurry sampling ETV-ICP-MS has been developed for the direct determination of trace REEs in soil samples, in which polytetrafluoroethylene (PTFE) emulsion was used as fluorinating reagent to improve the vaporization of REEs. The fluorinating vaporization behaviors of REEs and matrix elements, especially silicon, have been studied systematically. It was found that the main matrix element Si in soil resulting in severe matrix effect could be effectively in-situ removed by selecting the suitable ETV heating program. A study of the effect of particle size indicated that no obvious signal fluctuation was found with the selected particle size range of 0.105-0.054 mm. Under the optimal operating conditions, the developed method has been applied to the direct analysis of soil standard material (GBW07406) and other soil samples with satisfactory results and no chemical pretreatment was necessary.6. High performance liquid chromatography (HPLC) was hyphenated with ICP-MS to investigate the retention behavior of Ce and its chelates (Ce-EDTA, Ce-citrate acid, Ce-humic acid) on different stationary phase (cation exchange resin, PMBP resin, P507 resin and Cg silica gel) preliminarily. The obtained experimental results provide a valuable reference for the further speciation analysis of REEs.