Preparation of REE-doped CaF₂ single crystals for accurate determination of REE concentrations in CaF₂ crystals via UV-LA-ICP-MS

Accurate determination of rare earth element (REE) concentrations in CaF₂ crystals using UV-LA-ICP-MS is important but challenging due to the lack of matrix-matched reference materials. In this study, a REE-doped CaF₂ single crystal (CaF₂-05), prepared via the Bridgman-Stockbarger technique, was systematically characterized to assess its optical properties, homogeneity, and stability. Analyses using portable laser irradiation, UV-vis spectroscopy, back scatter electron (BSE) imaging, and cathodoluminescence (CL) demonstrated that the crystal possesses high optical quality and exhibits distinct optical adsorption properties compared to the silicate reference material NIST SRM 610. The within-unit variation (S_r) ranged from 3.82 % to 9.86 %, the between-unit S_r varied from 3.94 % to 7.85 %, and the mean square weighted deviation (MSWD) was close to 1, indicating minimal chemical heterogeneity. These results are comparable to those of certified reference materials and other home-made calibration standards. Femtosecond LA-ICP-MS mapping further confirmed the homogeneous distribution of REEs within the CaF₂ crystal. Stability tests conducted from April to November 2024 demonstrated the crystal's good long-term stability. The method's accuracy was validated by calibrating REE concentrations in another CaF₂ single crystal (CaF₂-005), with measured REE concentrations closely matching reference values. The relative deviations (D_r) primarily ranged from -10.10 % to 12.79 %, which is significantly more accurate than the non-matrix-matched calibration using NIST SRM 610. These findings demonstrate that this method provides reliable accuracy and holds promise for application in the accurate determination of REE concentrations in CaF₂-matrix samples, such as CaF₂ luminescent materials and natural fluorite ores.