A new method to graphitize CO2 from atmospheric air for radiocarbon analysis
Mr Philip Gautschi1, Dr Lukas Wacker1, Prof Hans-Arno Synal1
1Lip, Eth Zurich, , Switzerland
The concentrations of carbon dioxide (CO2), that is an important greenhouse gas, are increasing radpidly in the athmosphere 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 . 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 .
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) . 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  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.
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Philip Gautschi studied Physics at ETH Zurich, where he currently works as a PHD student.