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

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 applications were 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 was developed. A multiple rare earth elements (REEs) 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) 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 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, 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 was more desirable than those obtained with non-matrix-matched NIST SRM 610 as RM. It robustly validates the availability of the developed method for preparing CaWO4 single crystals as possible LA-ICP-MS reference materials to calibrate REE concentrations in natural scheelite.