The University of Arizona

Improved Precision of 14C Measurements for CH4 and CO2 Using GC and Continuous-Flow AMS Achieved by Summation of Repeated Injections

Cameron P McIntyre, Ann P McNichol, Mark L Roberts, Jeffrey S Seewald, Karl F von Reden, William J Jenkins

Abstract


Compound specific radiocarbon measurements can be made instantaneously using a gas chromatograph (GC) combustion system coupled to a 14C AMS system fitted with a gas ion source. Samples below 10 µg C can be analyzed but the precision is reduced to 5–10% because of lower source efficiency. We modified our GC for CH4 and CO2 analysis and injected samples multiple times to sum data and increase precision. We attained a maximum precision of 0.6% for modern CO2 from 25 injections of 27 µg C and a background of ~0.5% (40 kyr) for ancient methane. The 14C content of dissolved CO2 and CH4 in water samples collected at a deep-sea hydrothermal vent and a serpentine mud volcano was measured and the results for the vent sample are consistent with previously published data. Further experiments are required to determine a calibration and correction procedure to maximize accuracy.

DOI: 10.2458/azu_js_rc.55.16348


Keywords


gas chromatography; AMS; radiocarbon; methane; carbon dioxide; gas analysis; precise

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