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ionisation methods and their development dates back to the early 1970's. Fast particle desorption ionisation rapidly superseded the earlier field desorption method, which always suffered from complex source design and sample preparation.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 take-up 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 fig.1). 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.

Fig.1: A schematic of the mechanism of fast particle beam ionisation mass spectrometry (FAB and LSIMS)

Schematic of FAB

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. Boyth is these techniques have been largely replaced by the superior tecyhniques of electrospray ionisation and matrix-assisted laser desorption/ionisation.

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.