Development of methods for the determination of rare earth elements as trace impurities in high-purity rare earth oxides and rare earth elements by ICP-MS and HPLC-ICP-MS

Rare earth elements (REEs) are greatly demanded in modern technology industries and are widely used in several technological applications. The ability of the REEs to carry out their unique properties in their specific applications is dependent on the purity of the respective element, as the presence of impurities can affect both their physical and chemical properties. The main metal-impurities in high-purity REOs are basically trace amounts of REEs. This study focuses on the development of methods in order to determine the purity of standard solutions of Yb and Eu, and high-purity Yb2O3 by the impurity approach, utilizing inductively coupled plasma mass spectrometry (ICP-MS) hyphenated with high performance liquid chromatography (HPLC) for online separation of the REEs. Due to the highly concentrated matrix element, certain spectral and non-spectral interferences are commonly associated with the determination of REEs as trace impurities, such as formation of polyatomic ions causing mass overlaps with REEs of interest, and signal suppression effects which reduce the signal intensity of the REEs. In this study, non-spectral interferences were handled by dilution and were found to be not observable when the matrix element concentration was ≤ 10 µg mL-1. Several attempts were made in order to eliminate the contribution of polyatomic ions to the signal intensities observed at masses of interest. Formation of 174YbH and 153EuO hindered the determination of Lu and Tm in their respective sample solutions of Yb and Eu. By altering the plasma conditions from its standard operational conditions, the signal intensity of 174YbH was reduced, but the reduction was due to an overall decrease in instrumental sensitivity. Separation of the matrix elements from their respective interfered analytes were performed with various separation systems, such as ion pair chromatography (IPC) and extraction chromatography (EXC). IPC utilizing the OS-HIBA and HFBA-HIBA systems were found to be either unsuitable or unsuccessful for online separation of Lu from Yb and Eu from Tm. EXC with the HNO3-HDEHP system did successfully separate Lu from Yb and Eu from Tm. Methods developed with the HNO3-HDEHP system can be used for the determination of spectrally interfered REEs and as a diagnostic tool to investigate the contribution of polyatomic ions to the signal intensities of the interfered analytes. The methods developed were able to determine the purity of the standard solutions of Yb and Eu, and high-purity Yb2O3, which all were found to be > 99.99 % pure