Methods for High-Precision 14C AMS Measurement of Atmospheric CO2 at LLNL

Abstract

Development of radiocarbon analysis with precision better than 2 has the potential to expand the utility of 14CO2 measurements for carbon cycle investigations as atmospheric gradients currently approach the typical measurement precision of 25. The accelerator mass spectrometer at Lawrence Livermore National Laboratory (LLNL) produces high and stable beam currents that enable efficient acquisition times for large numbers of 14C counts. One million 14C atoms can be detected in approximately 25 min, suggesting that near 1 counting precision is economically feasible at LLNL. The overall uncertainty in measured values is ultimately determined by the variation between measured ratios in several sputtering periods of the same sample and by the reproducibility of replicate samples. Experiments on the collection of 1 million counts on replicate samples of CO2 extracted from a whole air cylinder show a standard deviation of 1.7 in 36 samples measured over several wheels. This precision may be limited by the reproducibility of oxalic acid I standard samples, which is considerably poorer. We outline the procedures for high-precision sample handling and analysis that have enabled reproducibility in the cylinder extraction samples at the <2 level and describe future directions to continue increasing measurement precision at LLNL.