Gas-phase fast-atom bombardment mass spectrometry

Abstract

Gas-phase fast-atom bombardment (FAB) mass spectra of fullerene C 60, polycyclic aromatic hydrocarbons, n-hydrocarbons, heterocyclic compounds, and others, by using high energy He, Ar, Xe, or isobutane beams, have been compared with the electron impact ionization (EI) and/or charge exchange ionization (CEI) mass spectra. The positive ion gas-phase FAB mass spectra measured show a fragmentation pattern characterized by a high intensity of lower-mass fragment ions and considerable fragmentation, the extent of which exceeded that in the He CEI mass spectra, and an intense peak corresponding to molecular ions M ·+. From the results obtained it was suggested that the internal energy distribution of M ·+ ions formed under gas-phase FAB conditions ranges widely from the non-fragmenting lower energy to considerable fragmenting higher energy. The Massey criterion for the He (8 keV) beam indicated that a single collision between a fast atom A fast and an analyte molecule M gas brings about the formation of singly and/or doubly charged molecularions as the most likely energy deposition process. The formation of multiply charged molecular and fragment ions. M ·+ and m 2+ ( z = 1–4), and the extent of fragmentation seemed to be dependent on the structure of the analyte molecules, as well as the bombarding particles used. The mechanisms of formation of positive and negative molecular ions M +, M -, and [M-H] -, under gas-phase FAB conditions, has been discussed on the basis of the positive and negative ion mass spectra obtained.