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A study of oleic acid and 2,4-DHB acid aerosols using an IR-VUV-ITMS: insights into the strengths and weaknesses of the technique

TitleA study of oleic acid and 2,4-DHB acid aerosols using an IR-VUV-ITMS: insights into the strengths and weaknesses of the technique
Publication TypeJournal Article
Year of Publication2009
AuthorsHanna, SJ, Campuzano-Jost, P, Simpson, EA, Burak, I, Blades, MW, Hepburn, JW, Bertram, AK
JournalPhysical Chemistry Chemical Physics
Volume11
Pagination7963-7975
Type of ArticleArticle
ISBN Number1463-9076
KeywordsATMOSPHERIC PARTICLES, CHEMICAL-ANALYSIS, ENVIRONMENTAL PARTICLES, GENERATING PARTICLE BEAMS, INVERSE FOURIER-TRANSFORM, LASER-DESORPTION/IONIZATION, ORGANIC-COMPOUNDS, QUADRUPOLE ION-TRAP, TRAP MASS-SPECTROMETER, VACUUM-ULTRAVIOLET PHOTOIONIZATION
Abstract

An investigation of oleic acid and 2,4-dihydroxybenzoic (DHB) acid aerosols was carried out using an aerosol mass spectrometer with pulsed lasers for vaporization and ionization and an ion trap for mass analysis. The extent of ion fragmentation was studied as a function of both vaporization energy and ionization wavelength. Low CO2 laser energies in the vaporization stage and near-threshold single photon ionization resulted in the least fragmented mass spectra. For DHB, only the molecular ion was observed, but for oleic acid fragmentation could not be eliminated. Tandem MS of the main fragment peak from oleic acid was carried out and provided a tool for compound identification. Photoionization efficiency curves were also collected for both DHB and oleic acid and the appearance energies of both parent and fragment ions were measured. Evidence for fragmentation occurring post-ionization is given by the similar appearance energies for both the parent and fragment ions. The results from this study were compared with those from similar experiments undertaken with time-of-flight (TOF) mass analyzers. The degree of fragmentation in the ion trap was considerably higher than that seen with TOF systems, particularly for oleic acid. This was attributed to the long storage interval in the ion trap which allows time for metastable ions to decay. Differences in the degree of fragmentation between the ion trap and TOF studies also provided further evidence for fragmentation occurring post-ionization. For 2,4-dihydroxybenzoic acid, the long delay prior to mass analysis also allowed time for reactions with background gases, in this case water, to occur.

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