δ13C and δD measurements of volatile organic compounds in a variety of emissions by thermal desorption compound specific isotope analysis

Abstract

Volatile organic compounds (VOCs) can be related to different compound classes but they all have a distinct vapour pressure allowing them to enter the atmosphere under ambient conditions. VOCs can undergo various reactions in the atmosphere and are emitted by various processes (anthropogenic and natural). Compound specific isotope analysis (CSIA) has been used in many other research studies to track the fate and source of compounds in the environment and the geological record. The majority of CSIA has been applied to extracts from soil, sediment or crude oils requiring entirely different sampling techniques compared to atmospheric samples. Applying CSIA to atmospheric compounds is a growing research field but has mainly being restricted to stable isotopes of carbon.This PhD thesis presents a novel application for adsorptive sampling on TenaxTA to analyse compound specific δ13C and δD of a range of atmospheric VOCs (C6 - C10). For the first time a 2-stage thermal desorption (TD) unit was linked to gas chromatography isotope ratio mass spectrometry (GC-irMS) and instrumental conditions were thoroughly investigated and optimised. Results obtained by using a standard mix of eleven VOCs confirmed reliability of TD-GCirMS analyses with standard deviations (SD) below instrument precision. δ values showed negligible isotopic fractionation compared to results obtained from traditional GC-irMS analysis (without TD unit) demonstrating the suitability of TD for CSIA.The technique was applied to analyse VOCs from various emission sources, e.g. car exhaust, biomass combustion and an industry stack. The results obtained have provided some insight into the formation processes of the VOCs investigated. δ13C values from an alumina refinery emission support a natural origin for the VOCs (organic material in bauxite ore). The δD values (21 to - 137 ‰) of the industry emission were consistently more enriched in D compared to δD values of VOCs previously reported making the δ values of VOCs in the industrial stack unique. Car exhaust emission from a petrol engine showed significant differences in δ values for VOCs up to 2 ‰ and 25 ‰ (δ13C and δD, respectively) at different tank fuel levels when using the same fuel batch. Car exhaust emission samples from a diesel engine showed a high content of highly complex mixture of unresolved compounds thus chromatographic baseline separation of VOCs was not achieved for stable hydrogen isotope analysis and led to unreliable δ values. The results from different biomass combustion emissions (including 5 species of C3 plants and 3 species of C4 plants) confirmed significant differences in δ13C of VOCs between C3 and C4 plants due to their specific metabolic pathways for carbon fixation in photosynthesis. The δD of VOCs derived from dicotyledons were less depleted in D compared to δD of VOCs derived from grasses (differences >27 ‰) indicating that the VOCs are derived mainly from lignin/cellulose rather than from lipids since dicotyledons contain higher amounts of lignin/cellulose.Due to the unique isotopic signatures of the VOCs from the different emission sources it was possible to distinguish their origins. Furthermore, TD-GC-irMS shows great potential to establish other emission sources in the environment and may help to gain some insight into their modes of formation.