Rapid bulk rock decomposition by ammonium fluoride (NH₄F) in open vessels at an elevated digestion temperature

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• 作者：Zhaochu Hu ; Wen Zhang ; Yongsheng Liu ; Haihong Chen ; Richard M. Gaschnig ; Keqing Zong ; Ming Li ; Shan Gao ; Shenghong Hu
• 关键词：Sample digestion ; Rock ; Ammonium fluoride ; Open vessels ; ICP-MS
• 刊名：Chemical Geology
• 出版年：2013
• 出版时间：26 September, 2013
• 年：2013
• 卷：355
• 期：Complete
• 页码：144-152
• 全文大小：1058 K

文摘

Complete dissolution is essential for achieving accurate analytical results for geological samples. Hydrofluoric acid is the most effective mineral acid for breaking up strong Si-O bonds to form SiF₆^? 2 ions in acidic solution. However, HF is rather inefficient in the open-vessel acid digestion of refractory minerals such as zircon due to its low boiling point (boiling point of 38.3 % HF = 112 ¡ãC). In this study, a decomposition technique using the solid compound NH₄F in an open vessel (Savillex Teflon vial) has been investigated for the multi-element analysis of various rock reference materials. The higher boiling point (260 ¡ãC) of NH₄F allows for an elevated digestion temperature in open vessels, which enables the decomposition of refractory phases. Using this method, Zr was completely recovered from the granodiorite GSP-2 reference material in 1-1.5 h at 250 ¡ãC, which is ~ 12 times faster than using conventional closed-vessel acid digestion at 190 ¡ãC (high-pressure PTFE digestion bomb). There is a positive correlation between the test sample mass and the amount of NH₄F required for complete recovery of Zr, Hf, Nb, Ta, and heavy rare earth elements in GSP-2, which is due to the resistance of zircon (which is a major reservoir for heavy rare earth elements) to decomposition at lower reagent to sample ratios. The recommended proportion of NH₄F, relative to the amount of sample powder required for complete element recovery, is 6:1. Unlike in NH₄F-assisted high-pressure acid digestion, our results clearly indicate that adding HNO₃ severely inhibited the digestion capabilities of NH₄F for refractory minerals such as zircon in open vessels. The most outstanding advantage of the new method is that the digestion can be performed in a conventional Savillex Teflon vial instead of a high-pressure PTFE digestion bomb. Moreover, NH₄F?open-vessel acid digestion is not hampered by the formation of insoluble fluorides, which represents another important advantage of this new sample decomposition method. Similar to HF and HNO₃, ultra-pure NH₄F can be produced using a conventional PFA sub-boiling purification system, and it does not induce new interference species in ICP-MS analysis. Additionally, this reagent is removed by taking the sample to dryness, which is important to keep the total dissolved solid of the final solution presented to the instrument low. NH₄F has many of the same safety handling requirements of HF, especially when being heated to dryness to remove the Si as SiF₄, and excess reagent as HF, so safety issues are still a concern. The developed NH₄F?open-vessel acid digestion method has been successfully applied to the digestion of a series of international geological reference materials. This simple, effective, and comparatively safe dissolution method shows great potential for the digestion of geological samples.