Small samples or those with high specific radiocarbon activity are commonly diluted with low 14C carrier to produce adequate target material with isotope ratios amenable for AMS analysis. For projects where absolute specific activity is important, carbon content of the diluted sample must be established via: (1) independent parallel processing for 14C and sample carbon, (2) measurement of sample carbon content prior to carrier addition, or (3) translation of activity concentration to specific activity where sample carbon content is known or assumed. Each of these methods has drawbacks, and it can be difficult to calculate high precision specific activity for small unique samples with unknown or variable carbon content.
Carbon isotope dilution can be used for specific 14C activity determination through measurement of 13C/12C after addition of a carrier with 13C/12C different from natural levels. We have tested the method by diluting samples with a measured quantity of 13C-enriched carrier as 13CO2 or 13C-urea. With adequate combustion and graphitization yields, 13C/12C of the final AMS sample falls between that of the unknown (1% 13C) and the carrier (99% 13C), facilitating calculation of sample carbon content. Specific radiocarbon activity of the diluted sample is subsequently calculated using measured 14C content and calculated sample size, after a small correction for processing and analysis backgrounds.
The utility of carbon isotope dilution is significant because it allows for simultaneous measurement of sample size and 14C content, allowing accurate determination of specific activity in very small unique samples. Proof of principle work was completed using the FN spectrometer at Purdue University; subsequent work has focused on the application of the methodology to the FN and 1MV spectrometers at the Center for Accelerator Mass Spectrometry. We will present these results and discuss the strengths and limitations of the method.
This work was performed in part under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.
See more of Poster Session II
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)