Evaluation of a two-step thermal method for separating organic and elemental carbon for radiocarbon analysis

摘要

We thoroughly characterized a system for thermal separation of organic carbon(OC) and elemental carbon (EC) for subsequent radiocarbon analysis. Differentorganic compounds as well as ambient aerosol filter samples were introducedinto an oven system and combusted to CO in pure O. The mainobjective was to test which combustion times and temperatures are best suitedto separate OC and EC. The final separation step for OC was combustion at360 °C for 15 min. Combustion at this temperature proved enough toremove several organic test substances from the filter (including highmolecular weight humic acid) but did not remove substantial amounts of EC.For isolation of EC, OC first needs to be completely removed from the filter.This was achieved by water extraction of the filter, followed by combustionof the water insoluble OC at 360 °C and combustion at an intermediatetemperature step of 2 min at 450 °C. This last step removed the mostrefractory OC together with some EC. Finally, the remaining EC was combustedto CO at 650 °C. The recovery of black carbon after theintermediate 450 °C step was approximately 80%. Several testsprovided strong evidence that OC was removed efficiently during theintermediate temperature step: (i) brown carbon, indicative of refractory OC,was removed; (ii) the fraction modern of EC did not decrease significantly ifthe temperature of the intermediate step was further increased. Based ontests with various organic compounds, we estimated that charred organiccarbon could contribute 4–8% to an elemental carbon sample that wasisolated according to our method.