Structures and dissociation mechanisms of protonated and electron ionized methamidophos

► Under collisional activation, major fragmentations of protonated methamidophos are losses of CH3SH, CH2O and NH3. ► Mechanisms combining proton transfers and 1,2-elimination reactions, are elucidated using DFT quantum chemistry calculations. ► Formation of ions m/z 47, 94 and 95 from electron ionized methamidophos is explained via ion-neutral complexes intermediates. ► Computed proton affinity and ionization energy of methamidophos are equal to 897 kJ mol−1 and 8.77 eV, respectively, at the G4MP2 level.