Showing papers by "Jacqueline F. Hamilton published in 2003"

Measurements of photo-oxidation products from the reaction of a series of alkyl-benzenes with hydroxyl radicals during EXACT using comprehensive gas chromatography

Abstract: . Photo-oxidation products from the reaction of a series of alkyl-benzenes, (benzene, toluene, p-xylene and 1,3,5-trimethyl-benzene) with hydroxyl radicals in the presence of NOx have been investigated using comprehensive gas chromatography (GCxGC). A GCxGC system has been developed which utilises valve modulation and independent separations as a function of both volatility and polarity. A number of carbonyl-type compounds were identified during a series of reactions carried out at the European Photoreactor (EUPHORE), a large volume outdoor reaction chamber in Valencia, Spain. Experiments were carried as part of the EXACT project (Effects of the oXidation of Aromatic Compounds in the Troposphere). Two litre chamber air samples were cryo-focused, with a sampling frequency of 30 minutes, allowing the evolution of species to be followed over oxidation periods of 3-6 hours. To facilitate product identification, several carbonyl compounds, which were possible products of the photo-oxidation, were synthesised and used as reference standards. For toluene reactions, observed oxygenated intermediates found included the co-eluting pair a -angelicalactone/4-oxo-2-pentenal, maleic anhydride, citraconic anhydride, benzaldehyde and p-methyl benzoquinone. In the p-xylene experiment, the products identified were E/Z-hex-3-en-2,5-dione and citraconic anhydride. For 1,3,5-TMB reactions, the products identified were 3,5-dimethylbenzaldehyde, 3,5-dimethyl-3H-furan-2-one and 3-methyl-5-methylene-5H-furan-2-one. Preliminary quantification was carried out on identified compounds using liquid standards. Comparison of FTIR and GCxGC for the measurement of the parent aromatics generally showed good agreement. Comparison of the concentrations observed by GCxGC to concentration-time profiles simulated using the Master Chemical Mechanism, MCMv3, demonstrates that this mechanism significantly over-predicts the concentrations of many product compounds and highlights the uncertainties which exist in our understanding of the atmospheric oxidation of aromatics.

Peak amplitude and resolution in comprehensive gas chromatography using valve modulation

Abstract: Comprehensive two-dimensional gas chromatography (GC x GC) allows major increases in peak capacity in comparison with separations on a single column. Interfacing the two columns in GC x GC has most often been achieved by thermal modulation. However, analyte transfer by valve modulation, in which fractions from the first column are introduced into the second column by means of a multi-port valve, is also possible. In this paper we develop a previous model of peak amplitude enhancement to compare the sensitivity and resulting resolution possible in GC x GC with thermal and valve modulation. While the peak amplitude resulting from valve modulation is inevitably much less than in thermal modulation, there is still an enhancement in comparison to single column GC. Resolution (R) in GC x GC with the two modulation methods has also been compared and is predicted to be generally improved for the valve modulation of partially resolved (R= 1) narrow peaks from the first dimension column. A comparison of peak amplitude enhancement and retained resolution using thermal and valve modulation methods is made using the example of GC x GC analysis of gasoline vapour.