Determination of the fragmentation mechanisms of organophosphorus ions by H₂O and D₂O atmospheric‐pressure ionization tandem mass spectrometry II—dialkyl alkylphosphonate ions

Diethyl methylphosphonate (DEMP), diisopropyl methylpbosphonate (DIMP), diethyl isopropylphosphonate (DEIP) and diethyl ethylphosphonate (DEEP) were characterized by H 2 O and D 2 O atmospheric pressure ionization tandem mass Spectrometry (API‐MS/MS). Collision‐induced dissociation (CID)/fragmentation pathways included alkyl ions by direct cleavage, alkyl radical and water loss processes and McLafferty and McLafferty‐type rearrangements by six‐ and five‐membered ring transition states, respectively. D 2 O API proved particularly useful in that certain decomposition pathways (i.e. water and methanol neutral losses) had a statistical distribution as to the loss of an acid deuteron and proton(s). This phenomenon was manifested by two pairs of ions in the D 2 O API daughter‐ion mass spectrum for each phosphonate compound (e.g. both m/z 79/80 and 65/66 for DEMP and DIMP). The observed ion intensity ratios for these pairs of ions served as guides in the determination of their predicted ion relative abundance ratios and CID decomposition pathways. Water neutral losses as opposed to ether and alcohol neutral losses were favored for most of the protonated organophosphonate molecular ion decomposition schemes.