New synthesis method of silica-gel for lead isotope analysis

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A new synthesis method for the silica-gel activator involving the hydrolysis of silicic acid ester was examined. Tetraethoxysilane (TEOS) was used as silicic acid ester. The silica-gel was obtained in mixed solvents of water (6.415 ml) and ethanol (0.230 ml) by hydrolysis of 0.230 ml of TEOS, using 0.125 ml of ammonia solution (20 wt.%) as a catalyst.An ion beam current (208Pb) of 4.0×10-11 A or more can be obtained with 200 ng of lead samples (NIST SRM 981) using our new synthesized silica-gel activator. Moreover, the ion beam current decayed less than 40% over the course of about 30 min. No significant change of isotopic composition was observed within a single run. The uncertainties for the 208Pb/204Pb, 207Pb/204Pb and 206Pb/204Pb ratios of NIST SRM 981 (100-200 ng) over 16 separate experiments were 0.027%, 0.020% and 0.015% (1σ%), respectively.These observations suggest that the silica-gel synthesized from silicic acid ester can be applied to the lead isotope analyses. It is well known that the size, size distribution and specific surface of silica can be controlled during the synthesizing silica-gel from silicic acid ester. Therefore, the present research makes it possible to investigate the relation between the capability of silica-gel as an activator for the lead isotope analyses, and the physicochemical properties of the silica-gel.(Communicated by Ikuo KUSHIRO, M. J. A., Feb. 12, 2003)
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