A method for carbon stable isotope analysis of methyl halides and chlorofluorocarbons at pptv concentrations

A pre-concentration system has been validated for use with a gas chromatography/mass spectrometry/isotope ratio mass spectrometer (GC/MS/IRMS) to determine ambient air C-13/C-12 ratios for methyl halides (MeCl and MeBr) and chlorofluorocarbons (CFCs). The isotopic composition of specific compounds can provide useful information on their atmospheric budgets and biogeochemistry that cannot be ascertained from abundance measurements alone. Although pre-concentration systems have been previously used with a GC/MS/IRMS for atmospheric trace gas analysis, this is the first study also to report system validation tests. Validation results indicate that the preconcentration system and subsequent separation technologies do not significantly alter the stable isotopic ratios of the target methyl halides, CFC-12 (CCl2F2) and CFC-113 (C2Cl3F3). Significant, but consistent, isotopic shifts of -27.5parts per thousand to -25.6parts per thousand do occur within the system for CFC-11 (CCl3F), although the shift is correctible. The method presented has the capacity to separate these target halocarbons from more than 50 other compounds in ambient air samples. Separation allows for the determination of stable carbon isotope ratios of five of these six target trace atmospheric constituents within ambient air for large volume samples (! 10 L). Representative urban air analyses from Belfast City are also presented which give carbon isotope results similar to published values for C-13/C-12 analysis of MeCl (-39.1parts per thousand) and CFC-113 (-28.1parts per thousand). However, this is the first paper reporting stable carbon isotope signatures for CFC-11 (-29.4parts per thousand) and CFC-12 (-37.0parts per thousand). Copyright (C) 2005 John Wiley Sons, Ltd.