Formation and suppression of AlF₃ during HF digestion of rock samples in Teflon bomb for precise trace element analyses by ICP-MS and ID-TIMS

An aluminum fluoride, AlF₃, forms during HF digestion of felsic rock samples, for trace element and isotope geochemistry, which use a Teflon bomb at high temperature and pressure. The AlF₃ incorporates trace elements (Rb, Sr, Y, Cs, Ba, REE, Pb, Th, and U), and can not be decomposed by conventional methods such as evaporation with HClO₄. The production of this AlF₃ results in lower yields and poor accuracy in analyses of these trace elements by ICP-MS. The formation of AlF₃ is controlled by the chemical composition of the rock samples, in particular, AlF₃ does not form during decomposition of mafic samples with relatively high (Mg+Ca)/Al ratios. We have developed a new method to suppress the AlF₃ formation, in which excess Mg is added to the sample prior to acid digestion in the bomb. This new method makes it possible to accurately determine the trace element compositions of higher-Al rock samples with lower concentrations of Mg and Ca (e.g. rhyolite and granite). In the trace element analyses by TIMS with isotope dilution techniques (ID-TIMS), AlF₃ formation hinders the achievement of isotope equilibrium, resulting in erroneous results. However, the Mg-addition method removed this problem by suppression of AlF₃ formation.