Solvent Extraction ICP-MS/MS Method for the Determination of REE Impurities in Ultra-high Purity Ce Chelates

This paper describes a new method for the determination of trace rare earth elements (REEs) in cerium chelates of extremely high purity by triple quadruple inductively coupled plasma mass spectrometry (ICP-MS/MS) after solvent extraction. Based on the outstanding performance of ICP-MS/MS, interferences on La, Pr, Tb, Gd, and Yb were reduced by 2 similar to 3 orders of magnitude in comparison with ICP-QMS. Further, chemical separation employing solvent extraction using Cyanex (R) 272 (bis(2,4,4-trimethylpentyl) phosphinic acid) was carried out, and about 99.95% of the Ce matrix, after being digested and oxidized to Ce4+ with KMnO4, could be removed, with recoveries for all of the 14 REEs ranging from 92.8% to 103%. Key factors that influenced the separation effects, such as the extraction acidity and time, the amount of KMnO4, the back-extraction acidity and time, were systematically studied and optimized. The established new separation procedure showed great superiority over the reported EHEHP (2-ethylhexyl hydrogen-2-ethylhexylphosphonate) based method in terms of conciseness in operation, low acidity for back-extraction, and high matrix separation efficiency. Thus, with the combination of chemical and physical high efficiency separation of the matrix-induced interferences, truly interference-free determination was realized, together with high analytical sensitivity and detection limits (3 sigma) for the 14 REEs ranging from 0.013 similar to 0.44 ng g(-1). The proposed new method was also verified by using three real samples for batch analysis of cerium chelates up to 7N purity.