Simultaneous Determination of Insoluble Fluoride-Forming and High Field-Strength Element Abundances in Rock Samples by ICP-QMS through Isotope Dilution-Internal Standardisation

A new method for precise measurement of the mass fractions of insoluble fluoride-forming elements (IFFEs) and high field-strength elements (HFSEs) in rock samples by ICP-QMS was developed. Unlike previous methods, in which the two elemental groups are measured separately using two solutions prepared from distinct sample aliquots (i.e., separate methods), this technique prepares a solution by acid digestion from a single sample aliquot, then divides the solution into two fractions that are dedicated to the measurement of IFFEs and HFSEs. The incorporation of IFFEs/HFSEs into insoluble fluorides during HF digestion was overcome by adjusting the Ca-Al-Mg composition of the sample by Al addition before acid digestion. The acceptable composition was Ca/(Ca+Al) < 0.43 and Mg/(Mg+Al) < 0.40 with hot plate digestion, whereas the limited condition of Ca/(Ca+Al) < 0.40, Mg/(Mg+Al) < 0.40 and Al/(Mg+Ca) < 1.86 was required for digestion under high pressure and temperature. For samples with an unacceptable composition, the change in Ca-Al-Mg composition by Mg/Ca addition should be prohibited because the reaction of HF with the added solution before sample decomposition forms fluorides that coprecipitate HFSEs. The developed method yielded mass fractions consistent with data from the separate methods (+/- 5%) and achieved a repeatability of < 4% for most elements.