Preliminary Characterisation of New Reference Materials (CaW-0, CaW-1 and CaW-3) for Microanalysis of Rare Earth Elements in Scheelite by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry

In this work, we prepared three CaWO₄ single crystals (CaW-0, CaW-1 and CaW-3) doped with rare earth elements (REEs) at nominal mass fractions of 50, 250 and 5000 μg g^-1 using the Czochralski method. Electron probe microanalysis (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) were employed to evaluate the within- and between-unit homogeneity of major elements and REEs, respectively. The within-unit variation (s_r) of 0.40–0.84% and between-unit s_r of 0.14–1.48% for major elements were obtained. Within- and between-unit s_r of 0.53–5.56% were also found for REEs by LA-ICP-MS spot analyses. A comparison of the s_r of repeat analyses was compared with the analytical uncertainty (u), i.e., the mean square weighted deviation (MSWD), which is close to or slightly higher than 1, indicating that no obvious chemical heterogeneity was found in these crystals. Reference values for these elements were obtained through various analytical methods across four different laboratories and applications of the reference materials to calibrating REE mass fractions in a natural scheelite were performed. No significant difference of the results was found as t values are less than 2.75 at the 99% confidence interval. The excellent linearity (R² > 0.98) between the REE mass fractions and both LA-ICP-MS signal intensity and wafer absorbance was found, but the crater height and the absolute mass transported into ICP-MS decreased when REE mass fractions as well as the colour of crystals increased. These results suggest that the reference materials have the potential to calibrate REE mass fractions in natural scheelite.