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

Abstract

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 electrop 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.

Oxidation reactions of dibenzothiophene subjected to negative chemical ionization with oxygen

“…[5]], as well as ions stemming from a complete oxidative degradation of 1 (e.g.. The ion [M + O 2 ] -, could either be a cluster ion between M and O 2 ' [9,10] (in which case it would belong to the ion-molecule-reaction products; the al- [30] can be excluded for reasons given below), or an oxidation product of 1 such as the corresponding sulfone 3 [3] or its rearrangement product, the sulfinate 4. In this case a surface-catalyzed reaction has to be taken into account.…”

“…These spectra differ grossly from those where 02 only plays a partial role as a contaminant: v. Ardenne et al [6], using a low pressure discharge in as side-products of aromatic compounds when nonreactive reagent gases are used [9,10]. Their genesis is explained by ion-molecule reactions of sample molecules with 0-' or OH-derived from trace amounts of O 2 and H 20 present in the ion source [9,10]. [M -H]-ions may either be formed by dissociative electron capture or by deprotonation by basic species such as 0-' or OH- [9,10].…”

“…Their genesis is explained by ion-molecule reactions of sample molecules with 0-' or OH-derived from trace amounts of O 2 and H 20 present in the ion source [9,10]. [M -H]-ions may either be formed by dissociative electron capture or by deprotonation by basic species such as 0-' or OH- [9,10]. Besides 0-' (which in the experiments of Hunt et al [2) was removed by the addition of H 2), the basic reagent ions°2 ' and OH- [11] are present in the 02 plasma and it is, therefore, rather surprising that M-' (m/z 184) should be observed [2] in the NO(02) mass spectrum of 1, whereas [M -H]-and not M-' is formed [8] even when electron capture promoting hydrocarbon reagent gases such as i-C 4 H lO are used.…”

Separation of isomeric polycyclic aromatic hydrocarbons by GC-MS: Differentiation between isomers by positive chemical ionization with ammonia and dimethyl ether as reagent gases

“…Chargeexchange positive- [3][4][5] and negative-ion [6][7][8] chemical-ionization mass spectrometry, (PICI and NICI), with methane or a mixture of methane in argon as reagent gases have been used to differentiate between isomeric PAHs. NICI using carbon dioxide alone [9] or in combination with a mixture of nitrogen and oxygen [10,11] has been reported recently.…”

The role of ion‐bound cluster formation in negative ion mass spectrometry

“…The first is direct electron transfer to analytes which exhibit higher electron affinities than oxygen (EA 0.43 eV) leading to radical anions (M – ); the second is proton transfer from the analyte to O 2 – if the gas‐phase basicity of the superoxide is higher than that of the deprotonated analyte ([M–H] – ). It is speculated that both pathways proceed via [M+O 2 ] – cluster formation, which may result from the reaction of M with O 2 – or of M – with neutral O 2 , but this cluster type has rarely been observed in mass spectra (EA = electron affinity, ∆G acid = gas‐phase acidity)…”