MICROSCALE AMS C-14 MEASUREMENT AT NOSAMS

Techniques for making precise and accurate radiocarbon accelerator mas s spectrometry (AMS) measurements on samples containing less than a fe w hundred micrograms of carbon are being developed at the NOSAMS facil ity. A detailed examination of all aspects of the sample preparation a nd data analysis process shows encouraging results. Small quantities o f CO2 are reduced to graphite over cobalt catalyst at an optimal tempe rature of 605 degrees C. Measured C-14/C-12 ratios of the resulting ta rgets are affected by machine-induced isotopic fractionation, which ap pears directly related to the decrease in ion current generated by the smaller sample sizes. It is possible to compensate effectively for th is fractionation by measuring samples relative to small standards of i dentical size. Examination of the various potential sources of backgro und C-14 contamination indicates that the sample combustion process is the largest contributor, adding ca. 1 mu g of carbon with a less-than -modern C-14 concentration.