Ion Formation from Organic Samples

According to Todd it is a common feature of FAB and LSIMS that they defy any generally acceptable mechanistic description [45]. Unlike El or Cl, where ions are generated from gaseous molecules, desorption/ionization techniques involve a state transition from liquid or solid to the gas phase and ionization of neutral molecules. Nonetheless, reviews dealing with the processes of desorption and ion formation under FAB and LSIMS conditions offer some insight [22-24]. Basi- 

The precursor model of FAB applies well to ionic analytes and samples that are easily converted to ionic species within the liquid matrix, e.g., by protonation or deprotonation or due to cationization. Those preformed ions would simply have to be desorbed into the gas phase (Fig. 10.6). TTie promoting effect of decreasing pH (added acid) on [Mh-H] ion yield of porphyrins and other analytes supports the precursor ion model [52,53], The relative intensities of [M-hH] ions in FAB spectra of aliphatic amine mixtures also do not depend to the partial pressure of the amines in the gas phase, but are sensitive on the acidity of the matrix [54]. Furthermore, incomplete desolvation of preformed ions nicely explains the observation of matrix (Ma) adducts such as [M+Ma+H] ions. The precursor model bears some similarities to ion evaporation in field desorption (Chap. 8.5.1). 

Note It has been estimated that a single impact causes the eruption of about 10 secondary neutrals, but yields only 0.02-1.5 ions [43,45,54,55]. The ions are then heading away from the surface in a supersonic expansion at speeds of about 1000 ms [22,43]. 

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