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Microscale AMS (super 14) C measurement at NOSAMS.

Ann Pearson, Ann P McNichol, Robert J Schneider, Reden F von, Yan Zheng


Techniques for making precise and accurate radiocarbon accelerator mass spectrometry (AMS) measurements on samples containing less than a few hundred micrograms of carbon are being developed at the NOSAMS facility. A detailed examination of all aspects of the sample preparation and data analysis process shows encouraging results. Small quantities of CO (sub 2) are reduced to graphite over cobalt catalyst at an optimal temperature of 605 degrees C. Measured (super 14) C/ (super 12) C ratios of the resulting targets are affected by machine-induced isotopic fractionation, which appears directly related to the decrease in ion current generated by the smaller sample sizes. It is possible to compensate effectively for this fractionation by measuring samples relative to small standards of identical size. Examination of the various potential sources of background (super 14) C contamination indicates that the sample combustion process is the largest contributor, adding ca. 1 mu g of carbon with a less-than-modern (super 14) C concentration.


combustion;isotope fractionation;mathematical methods;size;C 14 C 12;accuracy;graphite;native elements;data processing;accelerator mass spectroscopy;mass spectroscopy;spectroscopy;sample preparation;methods;C 14;carbon;isotopes;radioactive isotopes;carbon dioxide;stable isotopes;absolute age

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