Fast Atom Bombardment (FAB)
The FAB source was developed by Barber et al in the 1980s and is also known as Liquid Secondary Ion Mass Spectrometry (LSIM). The FAB principle consists in bombarding the sample in the solid or liquid state by a beam of primary ions or fast atoms. The primary ions are formed by the electronic impact source (Figure 1), the molecules used are often rare gases such as Argon (Ar) or Xenon (Xe). The FAB source is a soft ionization source, its use preserves the surface of the sample.
Figure 1: Fast Atoms Bombardment (FAB) ion source diagram
Figure 1 shows the diagram of the source of FAB ions, Argon molecules are introduced into the reduced pressure chamber where they are ionized by the ionization source. The ions formed are pulsed towards the collision chamber and they exchange their charges with the neutral molecules (Argon). By exchanging the charge, the ions become neutral but they still have enough kinetic energies and they pass the electrostatic gates which have the effect of eliminating ions that have not yet exchanged their charges. The fast atoms are directed at a target that contains the samples. The bombardment by atoms does not create ions, it only extracts ions from the surface. The process can be summarized as:
Ar + e– → Ar+● + 2e–
Ar+●(lent) → Ar+●(rapide)
Ar+●(rapide) Ar → Ar (rapide) Ar+●
At the output the fast atoms have kinetic energy between 3 and 10 KeV. The sample can be in liquid or solid form, in the case that the sample is liquid, it has been demonstrated that the analyte is much better sensitive by mixing with the matrix. The FAB source is therefore like MALDI but instead of using the laser, a beam of fast atoms is used. But the FAB source is much less sensitive than MALDI.
The advantage of the FAB source
It is a soft ionization source, it allows to analyze the polar compounds, ionic, it also allows to analyze the molecules of high mass (300-6000 Da), which can not be analyzed by electron ionization or ionization chemical. The compounds can be analyzed in positive or negative mode.
Noise is important for compounds with mass less than 200 Da. It does not allow to analyze composites whose mass is very high (> 10000 Da). Mass spectra are not reproducible therefore quantification is not assured.