IMPROVED PRECISION OF RADIOCARBON MEASUREMENTS FOR CH4 AND CO2 USING GC AND CONTINUOUS-FLOW AMS ACHIEVED BY SUMMATION OF REPEATED INJECTIONS

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 cal-ibration and correction procedure to maximize accuracy