Using GC×GC-ToF-MS to characterise SVOC from diesel exhaust emissions

Abstract:

Despite intensive research over the last 20 years, a number of major research questions remain concerning the sources and properties of road traffic-generated particulate matter. There are major knowledge gaps concerning the composition of primary vehicle exhaust aerosol, and its contribution to secondary organic aerosol (SOA) formation. These uncertainties relate especially to the semi-volatile component of the particles. Semi-Volatile Organic Compounds (SVOC) are compounds which partition directly between the gas and aerosol phases under ambient conditions, and include compounds with saturation concentrations roughly between 0.1 and 10⁴ µg m^-3. The SVOC in engine exhaust are typically hydrocarbons in the C₁₅-C₃₅ range. They are largely uncharacterised, other than the n-alkanes, because they are unresolved by traditional gas chromatography and form a large hump in the chromatogram referred to as Unresolved Complex Mixture (UCM).

In this study, samples were collected from the exhaust of a diesel engine with and without abatement devices fitted. Engine exhaust was diluted with air and collected using both filter and impaction (MOUDI), to resolve total mass and size resolved mass respectively. Particle size distribution was evaluated by sampling simultaneously with a Scanning Mobility Particle Sizer (SMPS). 2D Gas-Chromatography Time-of-Flight Mass-Spectrometry (GC×GC-ToF-MS) was exploited to characterise and quantify the composition of SVOC from the exhaust emission. The SVOC was observed to contain predominantly n-alkanes, alkyl-cyclohexanes and aromatics; similar to both fresh lubricating oil and fuel. Preliminary results indicate that the contribution of diesel fuel to the exhaust SVOC composition is dominant at high speeds, and a more pronounced contribution from lubricating oil is observed at low speeds. Differences were also observed in the SVOC composition when using different fuel types, engine lubricants, starting temperatures and collecting samples with and without abatement devices fitted. The wealth of compounds identified and quantified in the C₁₅-C₃₅ range included PAH, esters, carboxylic acids, alkanes, alkenes, alcohols and hopanes.