Technical progress in AMS microscale radiocarbon analysis

A graphitization technique for making precise and accurate radiocarbon measurements on small samples containing less than 100 μg of carbon was developed. The development of a microscale radiocarbon analysis technique is very important for advanced research in various fields of study, especially organic geochemistry. A detailed examination of the sample preparation process for microscale radiocarbon analysis and data analysis using IAEA-C1, IAEA-C6 and NIST HOXII standards show encouraging results. Small amounts of CO2 (10-50 μg C) from each standard were reduced to graphite over an iron catalyst (∼1 mg) at 670 deg. C. In this study, a newly developed ultralow-background graphite reactor was also used. Measured 14C/12C ratios of small samples were significantly decreased compared with those of samples with ∼1 mg C probably because small amount of carbon induced isotopic fractionation effect. This fractionation appears to be directly related to a decrease in the ion current generated by smaller sample sizes. We confirmed that this fractionation could be compensated for by measuring the samples relative to standards of identical size. We also achieved low-background measurement during the entire graphitization process with this newly established system. Detailed results of sample preparation and data analysis are discussed