A New Automated Extraction System for 14C Measurement for Atmospheric CO2

Abstract

The radiocarbon content of atmospheric CO2 (∆14CO2) has long been of interest to atmospheric and Earth system researchers. Recent improvements in 14C measurement precision and reduction in sample size requirements have now made it possible to measure ∆14CO2 within existing trace gas sampling networks, most notably as a method to quantify recently added fossil-fuel-derived CO2 in the atmosphere. At INSTAAR, in collaboration with NOAA/ESRL, ~600 atmospheric samples from around the globe are prepared each year, and that number is anticipated to grow in connection with various monitoring and data assimilation efforts. To accommodate the growing demand and reduce per sample costs, we developed an automated extraction system to quantitatively isolate CO2 from whole air for AMS 14C analysis. Twenty samples can be extracted in 1 fully automated run, taking 10-12 hr to complete and requiring only about 1 hr of operator time, a substantial improvement over the manual extraction system. CO2 is extracted cryogenically by flowing the whole air over a liquid nitrogen trap, after first removing water in a trap at -85 C. Large volume vacuum lines are used to extract ~30 mol of CO2 in less than 10 min, keeping contamination from leaks to a minimum and allowing rapid processing and greater throughput. 13C measurements on the resultant CO2 demonstrate that extraction is quantitative, and extractions of 14C-free air show that no significant modern contamination occurs. Replicate analyses of standard materials indicate that both mean values and precision are comparable to those for the manual extraction system.