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Fast Atom Bombardment

Fastatom bombardment is an ionization method of mass spectrometry. In thismethod ions are produced in a mass spectrometer from non volatile organic molecule by bombarding the compound in the condensed phase with energy rich neutral particle. It is used to analyze non-volatile and high-polarity compounds.  The material to be analyzed is mixed with a non-volatile chemical protection environment known as a ‘matrix’.  It isbombarded under vacuum with a high energy (4000 to 10000 electron volts) beam of atoms.  The atoms are usually from an inert gas such as argon or xenon.  Common matrices contain glycerol.   Matrix is a very important factor in Fast atom bombardment.  It is a liquid with a high vapor pressure, so that it doesn't dry out easily under the vacuum.  The analyte is soluble in the matrices.  The use of matrices in Fast atom bombardment also brings the peaks from matrices into a typical spectrum.


Method of Ionization in Fast Atom Bombardment:

Thebombarding ions as usually rare gases either Xe or Ar these atoms are first ionized and then passed through an electric field to increase their kinetic energy.  After acceleration, the fast moving ions pass into a chamber containing further gas atoms in which bombardment of the ions and atom lead to charge exchange;

Ar +. + Ar(thermal)   Ar(fast)   +   Ar +. (thermal)


The fast atom that is formed retains most of the original kinetic energy of the fast ions and continues in the same direction.

Thenegatively charged deflection plate can be used to direct the ions away.  Positive or negative ions may be formed in the bombardment process with extraction of ions from the sample plate being dependent onwhether the extraction slits are held at a positive or negative voltage.  The rapid sample heating from the noble- gases reduce sample fragmentation.


Working Of Fast Atom Bombardment


 

SchematicRepresentationOfFAB



Thesample is usually applied to the plate in the form of an inert in volatile liquid (matrix material), like glycerol the liquid matrix helpsto reduce the lattice energy, which must be overcome to desorb an ion from a condenced phase, and also damage done by bombardment.  In order to achieve efficient ionization, it is important to form a monolayer of sample at the surface of the matrix material and to make sure that the sample is well dissolved.  The sample is applied to a probe and then introduced to the ion source via vacuum interlock.                                                      


Advantages Of Fast Atom Bombardment:


  1. Ions can be generated from sample at room temperature i.e. volatilization is not required.
  2. Molecular, quasimolecular, and fragment ions are formed.  
  3. The sample sensitivity is high.
  4. The life time of sample is long and beam currents are quite stable.