NH4F assisted high pressure digestion of geological samples for multi-element analysis by ICP-MS

Sample decomposition is a fundamental and critical stage in the process of geochemical sample analysis using ICP-MS. Digestion in mixtures of HF and HNO3 acids is the conventional method of dissolution of geological samples. However, the use of HF is very corrosive and toxic. In this study, decomposition techniques using mixtures of less toxic and safer NH4F and HNO3 in high pressure digestion bombs have been investigated for different types of rock reference materials. Various sample digestion parameters were optimized through the analysis of granite GSR-1 or G-2, which is difficult to dissolve because of presence of resistant minerals like zircon. It was found that the ratio of NH4F(w)/HNO3(v) within the range of 0.33–0.83 has optimum digestion capabilities. Fifty milligrams of granite samples can be completely dissolved by using 0.20–0.50 gram NH4F under given digestion conditions. Our results also indicate that the increase of NH4F/HNO3 ratio or the weighed sample amount or the complete dryness of the treated powders during sample digestion will induce fluoride precipitation. In the case of granites GSR-1 and G-2, the recovery yields of Sc, Y, Sr, REE (excluding Ce) and Th are significantly lower in the presence of fluoride precipitation. The addition of 1–2 ml HNO3 to the treated powder after NH4F/HNO3 attack, before evaporation, can apparently suppress the formation of insoluble fluorides. A digestion time of 10 h in screw top PTFE-lined stainless steel bombs at 190 °C is sufficient to dissolve granite GSR-1. The developed method was applied to the digestion of a series of international geological reference materials. Results obtained using NH4F/HNO3 were similar to published values for mafic to felsic igneous rocks and shale. These results thus show that the NH4F/HNO3 digestion can be used with confidence for a wide variety of geological samples.