Sample Dilution for AMS 14C Analysis of Small Samples (30-150 mu g C)

We investigated sample dilution as a technique for accelerator mass spectrometry (AMS) radiocarbon analysis of very small samples (down to 30 mu g). By diluting such samples up to a total weight of 200 mu g, we can still perform reliable AM S measurements and improve the success rate significantly for targets that are difficult to measure. A disadvantage of this dilution technique is a loss of measurement precision. In addition, calculations of the C-14/C-12 isotope ratios and the uncertainties therein are not straight forward because of peculiarities in isotope fractionation processes in the AMS system. Therefore, to make sample dilution a routine method in our laboratory, we did extensive theoretical and experimental research to find the optimum conditions for all relevant parameters. Here, we report on the first detailed study dealing with all aspects of sample dilution. Our results can be applied in general. As an illustrative test case, we analyze C-14 data for CO2 extracted from an ice core, from which samples of 35 mu g C or less are available.