Investigation into Background Levels of Small Organic Samples at the Nerc Radiocarbon Laboratory

Recent progress in preparation/combustion of submilligram organic samples at our laboratories is presented. Routine methods had to be modified/refined to achieve acceptable and consistent procedural blanks for organic samples smaller than 1000 μg C. A description of the process leading to a modified combustion method for smaller organic samples is given in detail. In addition to analyzing different background materials, the influence of different chemical reagents on the overall radiocarbon background level was investigated, such as carbon contamination arising from copper oxide of different purities and from different suppliers. Using the modified combustion method, small amounts of background materials and known-age standard IAEA-C5 were individually combusted to CO2. Below 1000 μg C, organic background levels follow an inverse mass dependency when combusted with the modified method, increasing from 0.13 ± 0.05 pMC up to 1.20 ± 0.04 pMC for 80 μg C. Results for a given carbon mass were lower for combustion of etched Iceland spar calcite mineral, indicating that part of the observed background of bituminous coal was probably introduced by handling the material in atmosphere prior to combustion. Using the modified combustion method, the background-corrected activity of IAEA-C5 agreed to within 2 σ of the consensus value of 23.05 pMC down to a sample mass of 55 μg C.