Simultaneous determination of insoluble fluoride-forming and high field-strength element abundances in rock samples by ICP-QMS through isotope dilution-internal standardisation

同位体希釈-内標準ICP-QMSによる岩石中の化学的挙動の異なる微量元素群(IFFEs, HFSEs)の濃度同時測定手法

鏡味 沙耶   ; 横山 哲也*

Kagami, Saya; Yokoyama, Tetsuya*

岩石に含まれる異なる2つの元素群(Insoluble fluoride-forming elements: IFFEsとHigh field strength elements: HFSEs)の濃度測定では、試料の前処理過程でそれぞれの化学的挙動が異なるため、それぞれのフラクションを必要としていた。本研究では、試料の酸分解前に主要元素組成を任意に調整することで、IFFEsとHFSEsの濃度を同一のフラクションから測定する手法を開発した。本研究の手法を用いて標準岩石試料のIFFEs/HFSEs濃度を測定した結果、多くの元素に対して、得られた分析値のばらつきは4%未満であり、また、従来法による分析値と5%以内で一致した。この手法は、試料量が少ない貴重なサンプルやIFFEs/HFSEsに富む微小鉱物を含む不均質な試料の濃度測定に対して特に有効である。

We developed a new method for the determination of the mass fractions of insoluble fluoride-forming elements (IFFEs) and high field strength elements (HFSEs) in rock samples by ICP-QMS. Unlike conventional methods in which the two elemental groups were measured separately using two solutions prepared from distinct sample aliquots, the new technique prepares a solution by acid digestion from a single sample aliquot, then divide the solution into two fractions that are dedicated to the measurement of IFFEs and HFSEs, respectively. The problem regarding the incorporation of IFFEs/HFSEs into insoluble fluorides during HF digestion was overcome by adjusting the Ca-Al-Mg composition of the sample before acid digestion. The acceptable compositional range was Ca/(Ca+Al) 0.43 and Mg/(Mg+Al) 0.40 when the sample was decomposed on a hot plate, while more restricted condition of Ca/(Ca+Al) 0.40, Mg/(Mg+Al) 0.40, and Al/(Mg+Ca) 1.7 was required for the digestion under high pressure and temperature using Teflon bomb. This method achieved the repeatabilities of 4% for most IFFEs and HFSEs, and yielded the mass fractions of most IFFEs and HFSEs that were consistent with the data obtained by conventional methods within 5%. The method is applicable not only to precious samples but also to heterogeneous samples containing accessory minerals enriched in IFFEs/HFSEs.