Negative ions generated by reactions with oxygen in the chemical ionization source. I. Characterization of gas‐phase and wall‐catalyzed reactions of fluorene, anthracene and fluoranthene

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Abstract
The methane negative ion chemical ionization (NICI) mass spectra of polycyclic aromatic hydrocarbons are usually dominated by molecular, M− ˙ or M  H− ions; however, ions resulting from additions to M have also been reported. Some of these ions have been observed at [M + 14]− ˙, [M + 15]−, [M + 30]− and [M + 32]−˙ and have been attributed to reactions with either oxygen‐containing impurities in the buffer gas or alkyl radical species generated by ionization of a hydrocarbon buffer gas. In this study, the NICI spectra of fluorene, anthracene and fluoranthene were studied in detail using quadrupole and Fourier transform mass spectrometers. Spectra were acquired when reactive species such as oxygen, water, nitrous oxide and carbon dioxide were added to the nitrogen buffer gas. Experiments with deuterated methane were also carried out. These studies indicated that buffer gas impurities affect the NICI spectra; however, gas‐phase ion‐molecule reactions were not responsible for all of the observed products. In addition to electron‐ and ion‐molecule reactions, ions were observed that resulted from wall‐catalyzed oxidation reactions followed by electron capture. These reactions were enhanced by the addition of oxygen and elevated ion source temperatures. Depending upon the parent PAH structure, oxidation products such as ketones, quinones and anhydrides were formed.