地質学的試料の希土類元素測定における酸分解法の改良 : 密閉式PFA容器による分解ならびに130℃による蒸発乾固

抄録

It is shown that accurate rare-earth elements (REE) data in 22 geological rock reference samples (JB-1a, JB-2, JB-3, JA-1, JA-2, JA-3, JR-2, JR-2, JF-1, JG-1a, JG-2, JG-3, JGb-1. W-2, DNC-1, BIR-1, AGV-1, G-2, SDC-1, QLO-1, RGM-1 and STM-1) can be obtained by slight modification of the commonly used acid digestion procedure combined with ICP-AES analysis. The modified procedure consists of the decomposition of the samples with a sealed PFA vessel using acid mixture (HF+HClO₄+HNO₃+HCl) for 4 days on a 130℃ hot plate and the drying of the sample at 130℃ which is lower than commonly used temperatures, followed by dissolution with HCl and column separation of REEs. Our method minimizes the amount of undecomposed minerals and prevents the precipitation of fluorides and perchlorates during the digestion procedure. Twenty two geological standard samples of diverse matrices issued by Geological Survey of Japan (GSJ) and United States Geological Survey (USGS) were digested to assess the efficiency of the digestion procedure. Very small amount of undecomposed spinels and zircons were found in the digestion residues of JB-1a and JG-1a, respectively. The analytical results of REE for JB-1a and JG-1a agree well with the reported working values, indicating that the amount of undissolved minerals has practically small effect on REE data. Fluorides and perchlorates were found a fter the drying of the decomposed sample solutions at 180℃ for three samples (JG-1a, JG-2 and JR-2), on the other hand no precipitation is formed for all the samples digested when the drying was done at 130℃. The fluorides and perchlorates were shown to be responsible for the significant under-estimation of light REE concentrations in the samples. The REE data obtained by the drying at 130℃, however, showed good agreement (within 5% in most cases) with the working values. Thus, the formation of insoluble fluorides during the digestion procedure can be avoided by keeping the evaporation temperature at 130℃, without any additional agents. Our acid digestion method combined with ICP-AES yields accurate REE results, in good agreement with the working values for various standard rocks including granitic rocks.