Dissociation mechanisms in positive chemical ionization

The ion formation mechanisms in positive chemical ionization are presented in Chapter 3. The M+ ions issued from charge exchange almost do not fragment in the conditions under which they are formed in GC-MS (unlike the M+ ions formed by electron ionization). Methane chemical ionization often directly supplies fragment ions by removing a hydride or alkyl anion. 

The adduct ions such as MNH4+ do not fragment significantly in the mass spectrometer or dissociate themselves into M + NH/ at m/z 18 in case of an increase in internal energy (in MS/MS in particular)— which is of absolutely no interest from an analytical view. 

We are interested exclusively in the fragmentation of protonated molecules. Compared to El, Cl is a soft ionization mode. This means that the MH+ ions issued from Cl have much less internal energy than the M+ ions produced in El and therefore fragment themselves much less than the latter. 

FIGURE 9.54 Elimination of ammonia from protonated primary amine (top) and water from protonated alcohol (bottom) in positive chemical ionization. 

FIGURE 9.55 Protonation of bntylbenzene followed by hydrogen transfer on the charged carbon atom leading to bntene elimination in positive chemical ionization. 

---