Gas-phase Fast Atom Bombardment Research Articles

Gas-phase fast-atom bombardment mass spectrometry

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.

Fast Atom Bombardment Mass Spectrometry:Ionization Mechanism.

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.

Formation of M +· ions and electronic excitation under fast-atom bombardment conditions by using a liquid matrix

Japan The mechanism for the formation of molecular ions M(+·) under fast-atom bombardment (FAB) conditions with a liquid matrix is discussed on the basis of the mass spectra of pyrene, coronene, and fullerene C60 obtained by using electron impact, gas-phase fast-atom bombardment, and gas-phase fast-molecule bombardment techniques. The obtained results suggest that formation of the M(+·) ions under FAB conditions is not due to direct collisions between analytes M and fast atoms A, but is due to collision interactions between M and recoiling matrix molecules B or matrix ions. It has been confirmed, furthermore, that the FAB conditions with a liquid matrix are sufficient in energy for formation of singly charged ions M(+·) and insufficient for the formation of multiply charged ions M (z+) (z=2, 3) of pyrene, coronene, and C60.

Some Characteristics of Gas-Phase Fast Atom Bombardment Ionization Mass Spectrometry: Chromium Hexacarbonyl and Trolox.

Gas-phase fast atom bombardment (GFAB) mass spectra of the chromium hexacarbonyl molecule were compared with its charge-exchange ionization (CEI) mass spectra. It was found that the fragmentation pattern of chromium hexacarbonyl ions obtained by He-CEI mass spectrometry most resembles those measured in GFAB, and so it seems that energy deposition during GFAB implies involvement of a resonance-like energy transfer from fast atoms and may be similar or higher than that in He-CEI reaction. No great difference was observed in GFAB mass spectrum for a variety of fast bombarding particles including He, Ar, Xe, Cs, and isobutane. A high pressure GFAB mass spectrum of trolox measured using a Cs beam demonstrated its reactive nature that exchange reaction between hydroxy protons and Cs+ ions seems to take place in gas phase for the molecule and its neutral fragments while a Cs-FAB mass spectrum measured under a relatively low sample gas pressure showed only a fragmentation pattern characterized by a high abundance of fragment ions. Since Cs is only an alkali metal among fast bombarding particles, its chemical nature may be responsible for the observed reactivity with the target molecule.

Ion Formation and Degradation of C70by Gas-Phase Fast-Atom Bombardment with Helium Beam

Abstract The formation of multiply-charged molecular ions C70 z+ (z=1-4) and fragments due to successive C2 losses from C70 z+ ions were observed by using gas-phase fast-atom bombardment technique with high-energy (8keV) helium beam.

Ion Formation and Fragmentation in Gas-Phase Fast-Atom Bombardment Mass Spectrometry of C60

Abstract The observation of multiply-charged ions C60 Z+ (z=1-4) and fragment ions (C60 - C2n)z+ (z=1-4, n=1-12) has been reported using a new ionization/ collision technique, called gas-phase fast-atom bombardment with a high-energy (8 keV) helium beam.

Ionization and fragmentation of C 60 by gas-phase fast-atom bombardment using He, Ar and Xe beams

Mass spectra of fullerene C 60 were obtained by gas-phase fast-atom bombardment (FAB) technique using a high-energy (8 keV) He, Ar or Xe beam. The gas-phase FAB spectra of C 60 using Ar and Xe beam showed a characteristic fragment pattern consisting of the peaks of both even and odd number carbon ion species C + n, having relative maxima peaks at m/z 84 (C 7), 132 (C 11), 180 (C 15) 228 (C 19) and 276 (C 23), and relative minima peaks at m/z 96 (C 8) or 108 (C 9), 156 (C 13), 204 (C 17) and 252 (C 21). The spectra with Ar and Xe beam did not show triply-charged ion peaks, whereas the spectrum with He beam showed abundant triply-charged fragment and C 3+ 60 ion peaks. The mechanism for gas-phase FAB ionization was discussed.

Gas-phase fast atom bombardment mass spectra of fullerenes and pyrene

Mass spectra of fullerenes C 60and C 70 and pyrene were obtained using the gas-phase fast atom bombardment (FAB) ionization technique using high-energy (7 keV) helium beam. Positive ion gas-phase FAB spectra of their compounds show the peaks corresponding to multiply-charged molecular ions M z+ ( z = 1–3) and abundant fragment ions, whereas the electron ionization (EI) spectra only slightly show fragment peaks. The gas-phase FAB spectral patterns of the fullerenes, consisting of the fragment peaks (C 60 − C 2 n ) z+ and (C 70 − C 2 n ) z+ ( z = 1–3, n = 1–12), are similar to the corresponding collision-induced dissociation patterns. The negative ion gas-phase FAB spectrum shows the molecular ion peaks C − 60 and C − 70, and only slightly shows fragment peaks. The results obtained indicate that gas-phase FAB mass spectrometry can be used as a new technique for the collision study of extremely stable compounds.