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
superseded the earlier field desorption method, which always suffered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, though, saw 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 culminate 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)
