A Rule to Account for Mass Shifts of Even-Electron Fragment Ions in Fast-Atom Bombardment Mass Spectrometry

A rule is proposed for prediction of correct mass numbers of fragment ions in fast-atom bombardment (FAB) ionization mass spectrometry. The term “mass shift” in the rule is defined as the increment or diminution for the mass of an observed peak against the mass calculated from the corresponding part of the structural formula of a sample molecule. For positively-charged fragment ions, the mass shift is either 0, +1, +2 or even +3, according to the nature of the cleaved bond, and 0, −1 or −2 for negatively-charged fragment ions. The rule is based on the recognition that, with a few exceptions, fragment ions formed under FAB ionization are even-electron species and should have a stable structure which includes either (a) charged carbon atoms stabilized by conjugation with double bonds or with any heteroatom-containing functional groups, or (b) positively- or negatively-charged heteroatoms. The rule is also applicable to product ions observed in collision-induced dissociation.