This is the peer reviewed version of the following article: Archbold, M., Redeker, K.R., Davis, S., Elliot, T, Kalin, R.M. (2005) A method for compound specific stable isotope measurements of methyl halides and chlorofluorocarbons at pptv concentrations. Rapid Communications in Mass Spectrometry, 19(3): 337-342., which has been published in final form at http://dx.doi.org/10.1002/rcm.1791 . This article may be used for non- commercial purposes in accordance With Wiley Terms and Conditions for self-archiving [http://olabout.wiley.com/WileyCDA/Section/id-815640.html] A method for compound specific stable isotope measurements of methyl halides and chlorofluorocarbons at pptv concentration. Marie E. Archbold*, Kelly R. Redeker, Simon Davis, Trevor Elliot and Robert M. Kalin Environmental Engineering Research Centre, School of Civil Engineering, Queen’s University of Belfast, David Keir Building, Stranmillis Rd., Belfast BT9 5AG, UK Abstract 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 13 C/ 12 C 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 (CCl 2 F 2 ) and CFC-113 (C 2 Cl 3 F 3 ). Significant, but consistent, isotopic shifts of -27.5‰ to -25.6‰ do occur within the system for CFC-11 (CCl 3 F), 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 13 C/ 12 C analysis of MeCl (-39.1‰) and CFC-113 (-28.1‰). However, this is the first paper reporting stable carbon isotope signatures for CFC-11 (-29.4‰) and CFC-12 (-37.0‰). Introduction Compound-specific isotope ratio mass spectrometry has been previously used to define levels of contribution from specific reservoirs to atmospheric budgets of CO 2 , CO, N 2 O and CH 4. 1,2,3 . Recently, there has been increased interest in rare atmospheric components, such as halogenated volatile organic compounds (HVOCs) 4,5,6,7,8,9,10,11,12,13 .