Development of REE-doped CaWO4 single crystals as reference materials for in situ microanalysis of scheelite via LA-ICP-MS

Abstract

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been widely used for in situ microanalysis of elemental concentrations in solid samples, but its application is hampered by a lack of matrix-matched and homogeneous reference materials (RMs). In this work, a new method for preparing matrix-matched and homogeneous LA-ICP-MS reference materials has been developed. A multiple rare earth element (REE) doped CaWO₄ single crystal (CaW-1) was successfully prepared by the Czochralski technique. The matrix composition and crystal structure were confirmed by X-ray diffraction (XRD) and electron microprobe analysis (EMPA), which indicates that these crystals have a matrix composition of CaWO₄ and are matrix-matched with natural scheelite. The homogeneity of REE distribution was systematically studied by seven methods and the results demonstrated that REEs are distributed homogeneously in the CaWO₄ single crystal with slight fluctuation (RSDs: ca. 4%). It can be concluded that the synthesized CaWO₄ single crystal meets both crucial requirements, matrix-matched and homogeneous distribution, for an LA-ICP-MS reference material, and therefore it was used to quantitatively calibrate REE concentrations in natural scheelite. The results showed that acceptable analytical errors (<8%) were obtained, which were more desirable than those obtained with non-matrix-matched NIST SRM 610 as the RM. It robustly validates the ability of the developed method for preparing CaWO₄ single crystals as possible LA-ICP-MS reference materials to calibrate REE concentrations in natural scheelite.