Pre‐ionization effects in the mass spectrometry of some azoxybenzenes

The mass spectrometry of azoxybenzene, and of several of its substituted variants, has been investigated under both electron ionization (EI) and chemical ionization (CI) conditions. The EI spectra largely confirm earlier mechanistic conclusions of Bowie et al. (J. Chem. Soc. B, 621 (1967)), but extend this work to consideration of the mechanism of formation of the [M − O]+˙ ion. This unexpected ion has been shown, using techniques of tandem mass spectrometry, to arise from pre-ionization reduction to the corresponding azobenzene probably at the ion source surfaces. These pre-ionization effects cause the observed spectra to vary markedly with the conditions, including partial pressure of the sample, within the EI source. Under CI conditions (both methane and isobutane reagent gases) the pre-ionization reduction process is even more marked, and can involve reductive cleavage to the appropriate substituted anilines. Addition of water vapour favours the latter process over formation of [M − O + H]+. Analogous processes have been reported previously for nitroaromatics, suggesting that mass spectra of aromatic compounds substituted with oxygenated nitrogen atoms may all prove to be susceptible to similar effects.