3. Anomalous metastable peaks
In the 1950s, improvements in vacuum technology revived interest in atomic ion–surface bombardment as a means of surface analysis using SIMS [33]. However, studies of polyatomic ion collisions with surfaces did not initiate until the late 1970s. Interest in this research area was triggered by the serendipitous discovery of anomalous metastable peaks in double-focusing sector instruments first reported by Beynon et al. in 1968 [34]. The double-focusing sector instrument is composed of both a magnetic and an electric sector connected by a field-free region. Metastable ions undergoing unimolecular or collision-induced dissociation (CID) in the field-free region have been used extensively for structural characterization of organic molecules using MS [35]. In double-focusing instruments, metastable ions are observed as fairly broad features; the position of the metastable peak can be calculated based on the m/z values of the precursor and the fragment ion. Anomalous metastable peaks were observed at m/z values that did not correspond to the calculated peak positions [34], [36] and [37]. In fact, the positions of the anomalous peaks indicated that corresponding fragment ions had a higher velocity than predicted by the conservation of momentum. A systematic study by Beynon and co-workers demonstrated that anomalous metastable peaks originate from dissociation of precursor ions induced by glancing-incidence collisions with the magnetic sector’s upper and lower walls [37]. Subsequently, SID was examined by incorporating a 5 cm long, stainless-steel tube between the ionization source and magnetic sector [38]. Fragment ions of comparable intensity were observed using SID and high-energy CID of small ions. In addition, charge inversion was observed for collisions of Br+ with surfaces, resulting in the formation of Br− through a two-electron process [38].