Dissociative excitation of gas-phase ions. A comparison of techniques utilizing ion cyclotron resonance spectroscopy and angle-resolved mass spectrometry

Abstract

A comparison is made between electron-impact dissociation of ions in an ion cyclotron resonance (ICR) spectrometer and collision-induced dissociation performed in a mass spectrometer capable of selecting scattering-angles. These experiments both probe ion structure by monitoring unimolecular dissociation following excitation. The techniques have been applied to the molecular ions of trifluorotoluene, phenetole, aniline, n-propyl-benzene, and perfluoropropylene. Plots of relative fragment-ion abundance versus electron energy (in the range 2–9 eV) are compared to similar plots in which the abscissa is the scattering angle (known to be related in some systems to internal energy). The plots are found to be qualitatively similar for the two techniques for several of the molecular ions, but significantly different for two. For one of these, perfluoropropylene, a lack of dependence of fragment-ion abundance upon scattering angle was observed, implying that the ion internal energy does not vary with scattering angle. The differences between the electron- and particle-impact behavior may stem from the fact that fluorescence or rotational excitation are implicated. Although both methods suffer from some uncertainty in defining precisely the internal energy deposition they are simple to effect yet yield important fundamental information about ion dissociation chemistry.