Fast Atom Bombardment (FAB) & Liquid Secondary
Ion Mass Spectrometry (LSIMS)
There are a number of fast particle beam desorption ionisation
methods and their development dates back to the early 1970's. Fast
particle desorption ionisation rapidly superseded the earlier field
desorption method, which always suffered from complex source design
and sample preparation. In the mid. 1970's, plasma desorption (PD),
which made use of
252Cf fission to initiate analyte ion
desorption [1], saw limited successes. The technique had a slow uptake
mainly due to the difficulties and dangers surrounding the handling of
252Cf
and it went into decline with the introduction of LSIMS in the late
1970's [2]. The development of fast particle desorption culminated
with the development of FAB by Michael Barber at UMIST in the early
1980's [3].
The techniques of FAB and LSIMS are very similar in concept and design
as they both involve the bombardment of a solid spot of the
analyte/matrix mixture on the end of a sample probe by a fast particle
beam (see figure). The matrix (a small organic species like glycerol
or 3-nitro benzylalcohol) is used to keep a homogenous sample surface.
The particle beam is incident onto the surface of the analyte/matrix
spot, where it transfers its energy bringing about localised
collisions and disruptions. Some species are ejected (sputtered) from
the surface as secondary ions by this process. These ions are then
extracted and focussed before passing to the mass analyser. The
polarity of ions produced depends on the source potentials - the
figure shows a positive ion beam being formed.
In FAB, the particle beam is a neutral inert gas (Ar or Xe) at 4-10
keV and in LSIMS, the particle beam is ions (usually Cs
+)
at 2-30 keV. Both methods are comparatively 'soft' ionisation methods
- very little residual energy is possessed by the ions after
desorption - making them particularly suited to the analysis of low
volatility analytes. The resulting spectra consist largely of intact
molecular species (e.g. [M+H]
+ and [M+Na]
+) with
some minor structural fragmentation. The low mass region of the
spectra are, however, dominated by matrix and matrix/salt cluster
ions. Both of these techniques have been largely superceeded by the
superior techniques of electrospray ionisation (ESI) and
matrix-assisted laser desorption/ionisation (MALDI).
References:
[1] K.D. MacFarlane and D.F. Torgerson,
InternationaI
Journal of Mass Spectrometry and Ion Physics, 21,
1976,
p81.
[2] A. Benninghoven and W.K. Sichtermann,
Analytical
Chemistry, 50,
1978,
p1180.
[3] M. Barber
et al.,
Journal of the Chemical Society -
Chemical Communications,
1981,
p325 and
Nature, 293,
1981, p270 and
Analytical
Chemistry, 54,
1982,
p645A.