Compound specific radiocarbon analysis at the AWI-MICADAS facility (2018)

Compound specific radiocarbon analysis (CSRA) of n-alkanes, n-alkanoic acids or other biomarkers is a powerful, yet challenging tool to determine the cycling and fate of organic carbon in environmental settings. For example, the radiocarbon ages of long chain n-alkanoic acids of terrestrial origin extracted from high latitude marine sediments aid to decipher the impact of climate change on the carbon cycle, and vice versa.&#13;\nThe quality of Interpretations made from radiocarbon analysis highly depend on the precise determination of and the correction for external carbon contamination incorporated during sample preparation and measurement. Since recent advances in accelerator mass spectrometry (AMS) and direct gas measurements made it possible to analyze samples as small as 5 gC, even smallest non-confined and corrected contaminations could impair 14C results and associated interpretations significantly. CSRA requires laborious and demanding wet chemical extraction and purification steps followed by preparative gas chromatography for the isolation of individual compounds and is thus prone to incorporation of external carbon. Traditionally, isolated compounds were combusted in evacuated quartz tubes and produced CO2 was purified on vacuum lines for subsequent 14C analysis. A recent study calculated the amount of carbon contamination for n-alkanoic acids prepared for 14C analysis to be 5.0 ± 0.15 gC (Sun et al. in prep) highlighting the necessity for blank carbon corrections and a modified preparation protocol to reduce the contamination for very small CSRA samples. &#13;\nHere we report on a modified analytical procedure replacing the time-consuming vacuum line preparation by direct combustion of analytes using an elemental analyzer (EA). The EA is directly connected to the AMS (Mini Carbon Dating System, MICADAS, Ionplus) via a gas interface system (GIS) enabling the direct and fast (~4 samples per hour) 14C analysis of purified CO2 generated from the analytes. Immediately prior to the EA combustion isolated compounds were transferred in solution (1 gC/l DCM) into 50 l liquid tin capsules, the solvent was carefully evaporated at 35 °C on a hot plate and tin capsules were tightly packed and added to the EA sequence. Initial results of n-alkanoic acids extracted and processed as fatty acid methyl esters (FAMEs) from a fossil sediment (Messel Shale) and modern apple peel (collected in 2013) show a significant blank reduction to 1.7 ± 0.2 gC. The data show that the blank reduction is directly associated to the change from vacuum line preparation to EA combustion. The blank contributions for other compound classes (e.g., n-alkanes) are currently being evaluated alongside the validation of this method by processing mixtures of model compounds of different classes with known and varying 14C concentration. &#13;\n&#13;\nReferences&#13;\nSun, S., Meyer, V., Doman, A., Winterfeld, M., Hefter, J., Dummann, W., McIntyre, C., Montlucon, D. B., Haghipour, N., Wacker, L., Gentz, T., van der Voort, T., Eglinton, T. I., Mollenhauer, G., (in prep.) 14C blank assessment in small-scale compound-specific radiocarbon analysis of lipid biomarkers and lignin phenols