Fast atom bombardment ion source

R. S. Annan, H. J. Kochling, J. A. Hill, and K. Biemann. Matrix-Assisted Laser Desorption Using a Fast-Atom Bombardment Ion Source and a Magnetic Mass Spectrometer. Rapid Commun. Mass Spectrom., 6(1992) 298-302. 

Fig. 5. Overall diagram of a fast atom bombardment ion source.

The overall diagram of a fast-atom bombardment ion source is depicted in Fig. 5. This is fairly simple, consisting of three main elements (1) an atom gun, (2) a sample inlet, and (3) an ion-extraction system. 

The overall diagram of a fast atom bombardment ion source is fairly simple. It consists of three main elements i) an atom gun, ii) a sample inlet, and iii) an ion extraction system. The atom gun is made up of an evacuated chamber that encloses a plate to which a high voltage potential (nominally 8 kV) is apphed. The gas to be used for the "bombardment" is allowed into the chamber, through an appropriate inlet, where it is ionised by the high potential plate (see equation (22)). The ion beam so created is repelled by the same plate (at 8 kV) and, to a certain extent, regains its neutral character by electron-capture or by charge-resonance processes such as those exemplified by equations (23) and (24). 

A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

Fast-atom bombardment (FAB) is one of a number of ionization techniques which utilize a matrix material, in which the analyte is dissolved, to transfer sufficient energy to the analyte to facilitate ionization. In FAB, the matrix material is a liquid, such as glycerol, and the energy for ionization is provided by a high-energy atom (usually xenon) or, more recently, an ion (Cs+) beam. In conventional FAB, the solution of analyte in the matrix material is applied to the end of a probe which is placed in the source of the mass spectrometer where it is bombarded with the atom/ion beam. 

Recent attention has focused on MS for the direct analysis of polymer extracts, using soft ionisation sources to provide enhanced molecular ion signals and less fragment ions, thereby facilitating spectral interpretation. The direct MS analysis of polymer extracts has been accomplished using fast atom bombardment (FAB) [97,98], laser desorption (LD) [97,99], field desorption (FD) [100] and chemical ionisation (Cl) [100]. 

The mass spectra of mixtures are often too complex to be interpreted unambiguously, thus favouring the separation of the components of mixtures before examination by mass spectrometry. Nevertheless, direct polymer/additive mixture analysis has been reported [22,23], which is greatly aided by tandem MS. Coupling of mass spectrometry and a flowing liquid stream involves vaporisation and solvent stripping before introduction of the solute into an ion source for gas-phase ionisation (Section 1.33.2). Widespread LC-MS interfaces are thermospray (TSP), continuous-flow fast atom bombardment (CF-FAB), electrospray (ESP), etc. Also, supercritical fluids have been linked to mass spectrometry (SFE-MS, SFC-MS). A mass spectrometer may have more than one inlet (total inlet systems). 

More recently, attention has turned to the aftertreatment of commercially available mordant dyes on wool with iron(II) and iron(III) salts as a potential source reduction approach to eliminating chromium ions from dyebath effluent [34]- The anticipated improvements in fastness performance were achieved. The structures of the conventional 1 2 iron-dye complexes formed on the wool fibres were characterised by negative-ion fast-atom bombardment spectroscopy and HPLC analysis [35]. 

Mass spectrometry is traditionally a gas phase technique for the analysis of relatively volatile samples. Effluents from gas chromatographs are already in a suitable form and other readily vaporized samples could be fairly easily accommodated. However the coupling of mass spectrometry to liquid streams, e.g. HPLC and capillary electrophoresis, posed a new problem and several different methods are now in use. These include the spray methods mentioned below and bombarding with atoms (fast atom bombardment, FAB) or ions (secondary-ion mass spectrometry, SIMS). The part of the instrument in which ionization of the neutral molecules occurs is called the ion source. The commonest method of... 

In standard FAB, the surface of the matrix solution is depleted of analyte and suffers from radiational damage during elongated measurements. Refreshment of the surface proceeds by diffusion (limited by the viscosity of the matrix) or evaporation. Continuous-flow fast atom bombardment (CF-FAB) continuously refreshes the surface exposed to the atom beam. [107,108] The same effect is obtained in slightly different way by the frit-fast atom bombardment (frit-FAB) technique. [109,110] In addition, both CF-FAB and frit-FAB can be used for online-coupling of liquid chromatography (LC, Chap. 12) [111] or capillary electrophoresis (CE) to a FAB ion source. [112]... 

More recently, studies concerning cluster ion formation from solid or deeply cooled liquid alcohols [120-122] have gained new interest. [123,124] Low-temperature fast atom bombardment (LT-FAB) of frozen aqueous solutions of metal salts provides a source of abundant hydrated metal ions. [125-127] Organic molecules can also be detected from their frozen solutions. [128] Such LT-FAB applications are particularly interesting when enabling the detection of species that would otherwise not be accessible by mass spectrometry, because they are either extremely air- and/or water-sensitive [129,130] as the phosphaoxetane intermediate of the Wittig reaction [131] or insoluble in standard FAB matrices. [106,132]... 

Most of the direct and indirect (transport) interfaces described here use chemical ionization (c.i.) ion-sources, which are not well suited to such polar, non-volatile compounds as tri- and higher oligosaccharides. The thermospray interface, which can operate on an ion-evaporative mode, is capable of producing intact molecular ions from such nonvolatile, polar molecules and should be useful in oligosaccharide analysis. Molecules of this type, however, can also be easily analyzed by fast-atom-bombardment ionization, and use of this technique, coupled to direct liquid introduction and moving-belt interfaces, has been reported. The latter system has been applied to complex oligosaccharide analysis. ... 

Electron ionization (El) was the primary ionization source for mass analysis until the 1980s, limiting the chemist to the analysis of small molecules well below the mass range of common bioorganic compounds. This limitation motivated the development of the techniques commonly known as ESI, 1 MALDI, 2 and fast atom bombardment (FAB) 3,4 (Table 1). These ion sources allow for rapid and easy peptide analyses that previously required laborious sample preparation or were not possible with electron ionization. The mechanism of ionization these ion sources employ, which is somewhat responsible for their ability to generate stable molecular ions, is protonation and/or deprotonation. 

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