Determination of trace impurity rare earth elements in high-purity rare earth element samples using high-resolution inductively coupled plasma mass spectrometry

Abstract

Trace impurity rare earth elements in high-purity Y₂O₃ and Gd₂O₃ were determined using high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS). Interference problems from GdO, GdH and GdOH on Tb, Yb, Tm and Lu were overcome by using doubly charged ions for detection. The mass resolution was set to 400, which is much higher than the normal resolution (80–90) of a conventional quadrupole mass filter and much lower than the standard setting for ICP-MS in the high-resolution mode. As a result, the transmission decrease that occurs with a quadrupole mass filter if increased resolution is used was not a problem. The only controlling factor for the sensitivity was the production rate of doubly charged species. Hence optimization was applied to maximize the proportion of doubly charged species. The detection limits obtained were 0.05–3 ng 1^–1 in solution. If singly charged ions were used, the detection limits were 5–30 pg 1^–1. Determinations of the trace rare earth elements in three Y₂O₃ and three Gd₂O₃ test samples were performed. The concentrations of the trace rare earth elements were found to be variable between the samples.