| Stable carbon isotopic composition studies can be extremely useful to better understand distributions, sources and removal mechanisms of NMHCs. The use of isotopic composition measurements of NMHCs in the atmosphere is presently limited by the lack of suitable methods. The key problems are generally low mixing ratios in the range of a few ppbv or less and the complexity of the mixtures, consisting of a substantial number of different compounds. The purpose of this study was to develop a suitable method to analyze the carbon isotope abundance ratios for atmospheric NMHCs at ppbv and sub-ppbv levels. Tests and experiments were performed to validate this methodology.;The developed method is based on the combination of a cryofoccusing step, 2D-gas chromatographic separation, quantitative combustion of all the separated hydrocarbons to CO2 and finally the determination of δ 13C in the CO2 using isotope ratio mass spectrometry. It was demonstrated that for a well resolved compound and sufficient amount of analyte (5ng or more of carbon) the method is accurate to about 0.5 permill.;Samples from different locations characterized by different types of emission sources (e.g. vehicle exhaust, fuel evaporation, biomass burning, refinery), as well as ambient samples were analyzed to test the method's suitability for the analyses of real air samples. The method was also applied to kinetic isotope effect studies in the reaction of hydrocarbon with hydroxyl radical. There is no record on any previous measurements regarding the kinetic isotope effects in the reactions of NMHCs with hydroxyl radical, thus these measurements are first in their kind. |
