AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Comparison of Heterogeneous Oxidation Products of Branched and Normal Alkanes, as Characterized by Two-dimensional Gas Chromatography with Vacuum Ultraviolet High-Resolution Time-of-Flight Mass Spectrometry
CHRIS RUEHL, Theodora Nah, Gabriel Isaacman, David Worton, Arthur Chan, Katheryn Kolesar, Christopher Cappa, Allen H. Goldstein, Kevin Wilson, Univeristy of California, Berkeley
Abstract Number: 517 Working Group: Aerosol Chemistry
Abstract Previous research has shown that the molecular structure of alkanes (i.e., degree of branching and cyclization) influences the rate at which they are oxidized in the gas phase. The importance of molecular structure in heterogeneous oxidation is more complicated, however, because it may involve the distribution of phases within the particle (e.g., bulk-surface), particle viscosity, and/or distinct secondary (radical-propagating) reactions. We generated submicron particles composed of either squalane, n-octacosane, or n-triacontane, three saturated alkanes of similar volatility but different degrees of branching, and oxidized them in a NOx-free flow-tube photo-reactor. We characterized the molecular composition of the products using two-dimensional Gas Chromatography, with High-Resolution Time-Of Flight Mass Spectrometric detection utilizing Vacuum UltraViolet photoionization (GCxGC/VUV-HRTOFMS). This soft ionization technique permits detection of the parent ion; along with high resolution, which distinguishes between different compounds with the same nominal molecular mass, GCxGC/VUV-HRTOFMS allows for a more complete set of molecular product identifications than previously reported, including precursors with up to four oxidized carbons (up to two each of alcohols and carbonyls). In addition, we identify many of the smaller molecules that are products of fragmentation reactions. We report the relative yields of these compounds over a range of 0.25 to 6.6 hydroxyl radical lifetimes, to determine the influence of alkane branching on heterogeneous oxidation. We found that overall, the branched alkane (squalane) oxidized more rapidly than the normal alkanes. Oxidation of branched and normal alkanes led to comparable carbonyl functionalization. However, while hydroxyl functionalization was also important for the branched alkane, it was strongly suppressed for the normal alkanes. Furthermore, fragmentation reactions were much more prevalent for the branched alkane precursor. These results suggest that alkane branching allows for a wider range of heterogeneous photooxidation pathways, while alkanes containing only primary and secondary carbons (e.g., n octacosane) predominantly form ketones and aldehydes without fragmentation during heterogeneous photooxidation.