Fast Atom Bombardment Mass Spectrometry:Ionization Mechanism.

Abstract

Some ideas have been presented to understand ion formation under matrix fast atom bombardment (FAB) conditions. A state of hydrogen-bonding hydrophilic solvation, named “quasi-preformed state”, has been introduced to explain the ion formation reflecting condensed-phase, and the both “quasi-preformed state” and a state of “preformed ion” have been defined on the basis of the Bjerrum's proposition for ion-pairs in solution. Some examples of preformed ion have been described. The extents of electronic excitation leading to the formation of molecular ions M+·, under matrix FAB conditions, have been examined by detecting FAB-induced fluorescence from a thin layer of alkali-halides (LiCl, NaCl, KCl, and CsCl), and by using some compounds of which ionization energies are known. The results obtained indicate that matrix FAB conditions are sufficient in energy for M+· formation (by about 10 eV electronic excitation) and are insufficient for M2+ formation (by about 20 eV electronic excitation). The mechanisms of the formation of various ions, [M+H]+, [M+Na]+, [M-H]+, M+·, M-·, and [M-H]-, under matrix FAB conditions have been described. A model for FAB ionization, named “cavity” model, has been introduced to deal quantitatively with the rate of ion formation or ion yields from matrix surface. The “cavity” model requires that the ion formation occurs in both processes, i) the collision cascade reflecting condensed-phase and ii) the ion/molecule reactions reflecting gas-phase. The influence of fast atom species (He, Ar, and Xe) on the ion yields which may directly relate to the size of cavity or crater has been examined by comparing with the spectral patterns in the gas-phase FAB mass spectra of m-nitrobenzyl alcohol. It has been roughly estimated that a “cavity” formed with Ar or Xe beam contains several 100 molecules.