A new method to graphitize CO₂ from atmospheric air for radiocarbon analysis

Full text: The concentrations of carbon dioxide (CO₂), that is an important greenhouse gas, are increasing rapidly in the atmosphere since the industrialization. The anthropogenic emissions of 14C free CO₂ from fossil fuels, such as coal, oil or gas, reduces the 14C to 12C ratio in the atmosphere [2]. When the background 14CO₂ level is well known, even the local surplus of anthropogenic CO₂ can directly be calculated from locally measured CO₂ concentration and 14CO₂ abundance [3]. A new simple and fast method to prepare samples for radiocarbon measurements of atmospheric CO₂ has been developed and tested. Atmospheric air is collected in air sampling bags (5 L per sample) at site. The bags are transported to the laboratory, where CO₂ is isolated from the remaining air with a modified molecular sieve trap (220 mg Zeolite 13X) which is implemented in the automated graphitization line (AGE3) [4]. Using the AGE3, the CO₂ is then graphitized in a fully automated way and subsequently measured by accelerator mass spectrometry (AMS). Compared with other techniques [5] this new method avoids the use of caustic sodium hydroxide (NaOH) for sampling in the field. Neither does it require liquid nitrogen for CO₂ separation. The presented method was intensively tested and characterized. Cross contamination between consecutive samples has been calculated to be as low as 1.1 ±0.3‰ and a blank level of 0.0016±0.004 F14C were found. A programmable air loading box to collect and graphitize up to seven air samples at a time without the need of manually changing bags ensure reliable and repeatable results. A four-month set of radiocarbon measurements on local atmospheric CO₂ proves the applicability of the new method.