• Robust Bayesian Analysis, an Attempt to Improve Bayesian Sequencing

      Weninger, Franz; Steier, Peter; Kutschera, Walter; Wild, Eva Maria (Department of Geosciences, The University of Arizona, 2010-01-01)
      Bayesian sequencing of radiocarbon dates deals with the problem that in most cases there does not exist an unambiguous way to define the so-called prior function, which represents information in addition to the result of the 14C measurements alone. However, a random choice of a particular prior function can lead to biased results. In this paper, "robust Bayesian analysis," which uses a whole set of prior functions, is introduced as a more reliable method. The most important aspects of the mathematical foundation and of the practical realization of the method are described. As a general result, robust Bayesian analysis leads to higher accuracy, but paid for with reduced precision. Our investigations indicate that it seems possible to establish robust analysis for practical applications.
    • Studies on the Preparation of Small 14C Samples with an RGA and 13C-Enriched Material

      Liebl, Jakob; Avalos Ortiz, Roswitha; Golser, Robin; Handle, Florian; Kutschera, Walter; Steier, Peter; Wild, Eva Maria (Department of Geosciences, The University of Arizona, 2010-01-01)
      The minimum size of radiocarbon samples for which reliable results can be obtained in an accelerator mass spectrometry (AMS) measurement is in many cases limited by carbon contamination introduced during sample preparation (i.e. all physical and chemical steps to which samples were subjected, starting from sampling). Efforts to reduce the sample size limit down to a few mu-g carbon require comprehensive systematic investigations to assess the amount of contamination and the process yields. We are introducing additional methods to speed up this process and to obtain more reliable results. A residual gas analyzer (RGA) is used to study combustion and graphitization reactions. We could optimize the reaction process at small CO2 pressures and identify detrimental side reactions. Knowing the composition of the residual gas in a graphitization process allows a reliable judgment on the completeness of the reaction. Further, we use isotopically enriched 13C (greater than or equal to 98% 13C) as a test material to determine contamination levels. This offers significant advantages: 1) The measurement of 12C/13C in CO2 is possible on-line with the RGA, which significantly reduces turnaround times compared to AMS measurements; 2) Both the reaction yield and the amount of contamination can be determined from a single test sample. The first applications of isotopically enriched 13C and the RGA have revealed that our prototype setup has room for improvements via better hardware; however, significant improvements of our sample processing procedures were achieved, eventually arriving at an overall contamination level of 0.12 to 0.15 mu-g C during sample preparation (i.e. freeze-drying, combustion, and graphitization) of mu-g-sized samples in aqueous solution, with above 50% yield.