Evaluation of the digestion capability of ammonium bifluoride for the determination of major and trace elements in Ti-rich minerals by ICP-MS

Abstract

Complete sample digestion is a prerequisite to achieve accurate and precise results of elemental concentrations and isotopic compositions in Ti-rich minerals of interest, such as refractory rutile. In this study, a rapid digestion protocol using ammonium bifluoride (NH₄HF₂) was developed to completely digest large amounts of rutile, ilmenite, titanite and perovskite. These Ti-rich samples (50 mg) can be completely digested in 4 hours with the use of 300 mg of NH₄HF₂ at 230 °C, and each 200 mg of rutile and ilmenite ore can be completely dissolved with NH₄HF₂ digestion and without insoluble residues present. Furthermore, NH₄HF₂ digestion was found to be effective in dissolving the different sizes (31–150 μm) of Ti-rich minerals. Compared to common HNO₃–HF closed-vessel acid digestion, the proposed NH₄HF₂ digestion provides consistent results for major Ti and thirty-six other trace elements in a series of ilmenite ore reference materials and in-house Ti-rich minerals. It was found that elemental concentrations (Ti, Sc, V, Cr, Nb, Ta, Hf and U) in rutile measured by inductively coupled plasma-mass spectrometry (ICP-MS) using NH₄HF₂ digestion were consistent with the values measured in the pressed powder pellet of rutile by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). However, lower recoveries of Ti, V, Nb and Ta in rutile were obtained from closed-vessel acid digestion, possibly due to the formation of white precipitates during the evaporation/redissolution process. The proposed NH₄HF₂ digestion is safe and more effective in quickly dissolving Ti-rich materials and suitable for the determination of quantities of major and trace elements.