Origin of Ions in FI-MS

FI mass spectra can show signals due to reactions of the analyte with the emitter surface or between molecules adsorbed to that surface. In case of acetone, it was demonstrated that [M+H] ions are produced mainly by a field-induced proton-transfer reaction in the physically adsorbed layer [65]. The mechanism of this field-induced reaction depends on the existence of tautomeric structures of the neutral molecule. Besides the [M+H] ions, [M-H] radicals are formed in an overall reaction analogous to CHs formation in Cl  

Furthermore, the radicals formed upon field-induced hydrogen abstraction can lead to polymerization products on the emitter surface. The mechanism of this field polymerization helped to elucidate the phenomenon of activation of field emitters, i.e., the growth of microneedles on the emitter surface under the conditions of field ionization of certain polar organic compounds. [59] 

Note Analytes possessing exchangeable hydrogens also tend to form [Mh-H] ions in FI-MS. Occasionally, the quasimolecular ion occurs in favor of the molecular ion that can be of lower intensity or almost be absent (Fig. 8.13). Criteria to distinguish from [Mh-H] ions have been published. [60] 

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