Total Rock Dissolution Using Ammonium Bifluoride (NH₄HF₂) in Screw-Top Teflon Vials: A New Development in Open-Vessel Digestion

Complete sample digestion is a prerequisite for achieving reproducible and accurate analytical results for geological samples. Open-vessel acid digestions successfully dissolve mafic samples, but this method cannot achieve complete dissolution of felsic samples, because of the presence of refractory minerals such as zircon. In this study, an efficient and simplified digestion technique using the solid compound NH₄HF₂ in a screw-top vial has been developed for multielement analysis of different types of rock samples. NH₄HF₂ has a higher boiling point (239.5 °C) than conventional acids such as HF, HNO₃ and HCl, which allows for an elevated digestion temperature in open vessels, enabling the decomposition of refractory phases. Similar to HF, HNO₃ and HCl, ultrapure NH₄HF₂ can be produced using a conventional PFA sub-boiling (heating and cooling) purification system. A digestion time of 2–3 h for 200 mg NH₄HF₂ in a Savillex Teflon vial at 230 °C is sufficient to digest 50 mg of the felsic rock GSP-2, which is ∼6 times faster than using conventional closed-vessel acid digestion at 190 °C (high-pressure PTFE digestion bomb). The price of a Savillex Teflon vial is far less than the price of a high-pressure PTFE digestion bomb (consisting of a PTFE inner vessel and an outer stainless steel pressure jacket). Moreover, the NH₄HF₂–open-vessel acid digestion is not hampered by the formation of insoluble fluorides. We have successfully applied the NH₄HF₂–open-vessel acid digestion to the digestion of a series of international geological reference materials, including mafic to felsic igneous rocks and shales. This method provides an effective, simple, economical, and comparatively safe dissolution method that combines the advantages of both the open- and closed-vessel digestion methods