Compound-Specific Radiocarbon Analysis by Elemental Analyzer-Accelerator Mass Spectrometry: Precision and Limitations

We examine instrumental and methodological capabilities for microscale (10-50 mu g of C) radiocarbon analysis of individual compounds in the context of paleoclimate and paleoceanography applications, for which relatively high-precision measurements are required. An extensive suite of data for C-14-free and modern reference materials processed using different methods and acquired using an elemental-analyzer-accelerator-mass-spectrometry (EA-AMS) instrumental setup at ETH Zurich was compiled to assess the reproducibility of specific isolation procedures. In order to determine the precision, accuracy, and reproducibility of measurements on processed compounds, we explore the results of both reference materials and three classes of compounds (fatty acids, alkenones, and amino acids) extracted from sediment samples. We utilize a MATLAB code developed to systematically evaluate constant-contamination-model parameters, which in turn can be applied to measurements of unknown process samples. This approach is computationally reliable and can be used for any blank assessment of small-size radiocarbon samples. Our results show that a conservative lower estimate of the sample sizes required to produce relatively high-precision C-14 data (i.e., with acceptable errors of <5% on final C-14 ages) and high reproducibility in old samples (i.e., (FC)-C-14 approximate to 0.1) using current isolation methods are 50 and 30 mu g of C for alkenones and fatty acids, respectively. Moreover, when the (FC)-C-14 is >0.5, a precision of 2% can be achieved for alkenone and fatty acid samples containing >= 15 and 10 mu g of C, respectively.