Quantitative determination of rare earth elements in scheelite via LA-ICP-MS using REE-doped tungstate single crystals as calibration standards

Abstract

The lack of homogeneous and matrix-matched scheelite reference materials makes it difficult for quantitative determination of rare earth element (REE) concentrations in scheelite via LA-ICP-MS. In this work, three REE-doped tungstate single crystals (i.e., Dy:NaGd(WO4)2, Yb:NaGd(WO4)2, and Nd:NaGd(WO4)2) were selected as calibration standards. REEs in these crystals were found to distribute homogeneously with desirable homogeneity (signal intensity RSD < 4%). To determine the effective concentration of REEs in these crystals, three dissolution methods were comparatively studied for dissolving tungstate single crystals as well as scheelite, the results were precise (RSD: 0.15–3.21%) among these dissolution methods and consistent with reference values (obtained by EPMA or reported values) with ratios of pneumatic nebulization ICP-MS (pretreated by HNO3 + HF/HNO3 + HF + H3PO4/HNO3 + HF + tartaric acid) to EPMA 0.98–1.07%. Therefore, the simplest dissolution method (i.e., HNO3 + HF) was proposed for dissolving tungstate single crystals and scheelite. After that, REE concentrations in these crystals were treated as reference values to calibrate those in scheelite. REE concentrations in scheelite detected by LA-ICP-MS agreed well with those detected by pneumatic nebulization ICP-MS with relative percentage differences < 5% except for Yb (7.07%). It can be concluded that tungstate single crystals can be applied to calibrating REE concentrations in scheelite, and therefore an analytical method for quantitative determination of REE concentrations in scheelite via LA-ICP-MS using tungstate single crystals as calibration standards was successfully established.