静岡大学における^<14>C年代測定用ベンゼン合成法

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In this study, we improved the benzene synthetic system for radiocarbon dating constructed by Inomata (1992 MS) at Shizuoka University and describe the method in detail. A decarbonation system for carbonate samples such as shells and corals was additionally installed into the benzene synthetic system in order to measure carbonate ^<14>C-ages. We used 10 benzene samples, such as buried woods, charcoal, commercial cokes and fossil shells to measure ^<14>C using a liquid scintillation counter. The yields of these 10 samples during sample combustion were more than 99% after correction for the charcoal carbon content, and during decarbonation were 69〜94% with a mean value of 84%. The yields from carbon dioxide into acetylene were 80〜96% with a mean value of 90%, and from acetylene into benzene were 76〜95% with a mean value of 86%. Carbon isotopic fractionation during combution into CO_2 from charcoal samples was less than 1%o and that through the whole procedure of benzene synthesis was about ±3‰. The purity of benzene synthesized was 98.7〜99.9% by NMR and there were no impurities such as acetone, or ethyl alcohol, but 0.06〜1.34% toluene was detected. Using this system, a buried wood sample collected from the Kawagodaira pyroclastic flow was synthesized into benzene. The measured ^<14>C-age was 3,400±80 y B.P., which was slightly older than before reported.
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