Optimizing a Method for the Direct Determination of Rare Earth Elements in Refractory Geological Materials Using Electrothermal Vaporization Coupled to Inductively Coupled Plasma Optical Emission Spectrometry

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Kienast, Sophia
ETV-ICPOES , Analytical Chemistry , Rare Earth Elements
Rare earth elements (REEs) are a set of 17 elements on the periodic table that have low naturally occurring abundances. They have many essential uses, for example in catalysis and the manufacturing of batteries and permanent magnets. Their determination is made challenging by the refractory ores in which they typically exist. Traditional analysis methods require aqueous samples; however, preparing aqueous solutions from refractory materials is difficult, time-consuming, and costly. For this reason, a direct solid sample analysis method would be preferred. Electrothermal vaporization coupled to inductively coupled plasma optical emission spectrometry (ETV-ICPOES) enables the direct analysis of solid samples without extensive sample preparation. This analytical technique has been applied successfully to many different sample types, matrixes, and analytes in the past. In this thesis, ETV-ICPOES is used to determine the concentrations of REEs in refractory geological certified reference materials (CRMs). To do so, extensive method optimization was required to ensure precise and accurate results. To optimize a method for the determination of REEs, many different instrument parameters were varied throughout this project, two different chemical modifiers were evaluated, and a plasma sheathing gas (N2) was used. Several figures of merit, including sensitivity, limit of detection (LOD), and robustness were considered. Chemical modifiers are typically used to enhance analyte volatility. In this thesis, polytetrafluoroethylene (PTFE) powder was evaluated as a greener alternative to the commonly used CF4, and the LODs were compared between the two chemical modifiers. Although satisfactory LODs were achieved in this experiment, the robustness was low, so a plasma sheathing gas, N2, was used to improve the robustness of the plasma in a subsequent experiment. The results obtained with a CRM indicate that the method is likely suitable for use with refractory materials such as slag. By building on past optimization work, a set of instrument conditions that resulted in a robust plasma and low LODs for the determination of 16 REEs was identified. To the best of the author’s knowledge, this is the first time that a method for ETV-ICPOES has been developed for the direct determination of REEs in refractory geological materials.
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