Methods to detect volatile organic compounds using selected-ion chemical ionization mass spectrometry

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  • March 21, 2019
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  • Wendling, Karen Saretta
    • Affiliation: College of Arts and Sciences, Department of Chemistry
Abstract
  • The results described in this dissertation detail several methods for detecting volatile organic compounds (VOCs) using selected-ion chemical ionization (SICI) in a quadrupole ion trap mass spectrometer (QITMS). In this work, various reagent ions were generated using constant or pulsed glow discharge ionization in a custom glow discharge source. The reagent ions were then brought into the quadrupole ion trap mass spectrometer where VOCs analytes could be ionized by a selected reagent ion species. Many of the current methods to detect VOCs utilize chemical ionization with hydronium reagent ions. The hydronium reagent ionizes most VOCs via proton transfer ionization. Analyte fragmentation is minimal so most VOCs are detected as [M + H]+ ions. The limitation of this method, however, is that isomeric or isobaric VOCs cannot be distinguished in the mass spectrum. Six VOCs were the primary focus of this work: isomers isoprene and cyclopentene, isomers methyl vinyl ketone and methacrolein, and isobars 2-methylfuran and cyclohexene. The results described in this dissertation focus on two methods to detect and distinguish VOCs. First, several different reagent ion species were used to ionize the VOCs. The hydronium reagent ionized all six VOCs via proton transfer. Nitric oxide (NO+) reagent ions were used to ionize isoprene, cyclopentene, 2-methylfuran, and cyclohexene via charge-transfer ionization. When methacrolein was ionized, [M – H]+ ions were formed. Protonated acetone reagent ions were used to selectively ionize isoprene, methyl vinyl ketone, and 2-methylfuran. Second, the VOC ions were dissociated using collision-induced dissociation (CID) or infrared multiphoton photodissociation (IRMPD). In general, dissociating isomers or isobars produced product ions having indistinguishable mass-tocharge ratios. Dissociation of the molecular ions of 2-methylfuran and cyclohexene, however, produced diagnostic product ions. Using IRMPD for a 50 ms irradiation time to dissociate protonated methyl vinyl ketone, methacrolein, and a third isomer, crotonaldehyde, produced an unexpected result: methacrolein remained essentially undissociated while the other two isomers dissociated to products having different mass-to-charge ratios. The ability of SICI-QITMS to detect VOCs using a variety of reagents and to study VOC ions using dissociation methods shows that SICI is a promising alternative to current VOC detection methods.
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  • In Copyright
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  • Glish, Gary
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  • Open access
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  • October 11, 2010

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