δD and δ13C analyses of atmospheric volatile organic compounds by thermal desorption gas chromatography isotope ratio mass spectrometry

This paper describes the establishment of a robust method to determine compound specific delta D and delta C-13 values of volatile organic compounds (VOCs) in a standard mixture ranging between C-6, and C-10 and was applied to various complex emission samples, e.g. from biomass combustion and car exhaust. A thermal desorption (TD) unit was linked to a gas chromatography isotope ratio mass spectrometer (GC-irMS) to enable compound specific isotope analysis (CSIA) of gaseous samples. TenaxTA was used as an adsorbent material in stainless steel TD tubes. We determined instrument settings to achieve a minimal water background level for reliable delta D analysis and investigated the impact of storage time on delta D and delta C-13 values of collected VOCs (176 days and 40 days of storage, respectively). Most of the standard compounds investigated showed standard deviations (SD) < 6 parts per thousand (delta D) when stored for 148 days at 4 degrees C. However, benzene revealed occasionally D depleted values (21 parts per thousand SD) for unknown reasons. delta C-13 analysis demonstrated that storage of 40 days had no effect on VOCs investigated. We also showed that breakthrough (benzene and toluene, 37% and 7%, respectively) had only a negligible effect (0.7 parts per thousand and 0.4 parts per thousand, respectively) on delta C-13 values of VOCs on the sample tube. We established that the sample portion collected at the split flow effluent of the TD unit can be used as a replicate sample for isotope analysis saving valuable sampling time and resources. We also applied TD-GC-irMS to different emission samples (biomass combustion, petrol and diesel car engines exhaust) and for the first time delta D values of atmospheric VOCs in the above range are reported. Significant differences in delta D of up to 130 parts per thousand were observed between VOCs in emissions from petrol car engine exhaust and biomass combustion (Karri tree). However, diesel car emissions showed a high content of highly complex unresolved mixtures thus a baseline separation of VOCs was not achieved for stable hydrogen isotope analysis. The ability to analyse delta D by TD-GC-irMS complements the characterisation of atmospheric VOCs and is maybe used for establishing further source(s). (C) 2011 Elsevier B.V. All rights reserved.