Fast Heavy Ion Induced Desorption of Molecular Ions from Small Proteins

A number of new mass spectrometric methods for biomolecules have been introduced in recent years by which it has been possible to considerably extend the mass region towards higher masses. Among many new techniques introduced,those involving neutral or charged particle bombardment of a sample seem most promising. Fast (MeV) ions, like fission fragments, are used in 252Cf-PDMS introduced by R.D. MACFARLANE and collaborators [1–3] in 1974. The use of low energy (keV) ions as in SIMS was pioneered by BENNINGHOVEN and co-workers [4]. CHAIT and STANDING [5] have used a pulsed ion source and time-of-flight technique in connection with low-energy ions. Fast (MeV) ions interact primarily with the electrons in a medium while slow (keV) ions interact with the atoms and cause atomic recoil-cascades. In spite of this difference the mass spectra of desorbed ions look very much the same. However recent measurements by our own group indicate that there are differences [6]. Fast ions are more efficient,i.e. give larger yields of molecular ions than low-energy primary ions and this seems to be even more so the larger the molecule is. Recently BARBER et al. [7] have introduced the so called FAB- (Fast Atom Bombardment) technique. This method has proven to be very efficient in most cases studied so far. Although a beam of keV neutral particles is used the basic interaction with the sample is the same as with keV ions. A maybe more significant difference from the other techniques mentioned is the use of a liquid sample “holder”,i.e. glycerol.