Study of gas-phase reactivity of positive and negative even-electron ions prepared from diethylmethylphosphonate ester in an external chemical ionization source of orthogonal tandem quadrupole/ion trap instrument

摘要

Gas phase reactivity of the protonated and deprotonated diethylmethylphosphonate (DEMP), prepared under positive chemical ionization (PICI) and negative chemical ionization (NICI) conditions in an external high pressure source, was investigated using a recent homemade quadrupole filter/ion trap tandem with an orthogonal ion beam transmission. Ion chemical study with regard to unimolecular dissociation is described, especially with regard to the ion stability at different injection rf levels. The selective injection of the DEMPH~+ and [DEMP - H]~- ions seems to be a very critical step that depends upon the helium buffer gas pressure and the injection low m/z cutoff values (LMCO). Indeed, if the parent ion abundance rises up to 50% of total ion current at a low injection rf level (LMCO of 25 Th), as LMCO is increased extend fragmentations are favored. Such decompositions mainly involved 1-5 proton transfers leading to competitive and/or consecutive losses of ethylene and water. The ion abundance versus the injection rf level presents real energy resolved dependence. Ion/molecule reactions initiated by reactive fragments of externally prepared DEMPH~+ [e. g. PO~+, P(OH)_2~+, H_2C=P(OH)_2~+, O=P(OH)_2~+, H_3C-POEt~+, H_3C-PO(OCH_3)~+, H_3C(OH)_2P=OH~+, H_3C-PO(OEt)~+, CH_3(OEt)HP=OH~+, CH_3(OH)(CH_2=CHO)P=OH~+, CH_3(OH)(OEt)P=OH~+, CH_3(OCH_3)(CH_2=CHO)P=OH~+] have been investigated. These fragment ions, selected using a quadrupole filter, induce selective ion/molecule reactions with the DEMP molecules directly introduced into the ion trap cell. The exothermic reactions observed are (1) a proton transfer process and (2) proton-bound dimer formation, as well as stepwise addition/elimination reactions in which the first electrophilic attack at the phosphorous site of the DEMP is followed by ethylene loss. On the other hand, by selecting and injecting negative ions like alkoxide species (externally formed from aliphatic alcohols in NICI), nucleophilic attacks such as a S_N2-P pathway on DEMP can occur. However, most of the ions are generated by elimination and/or deprotonation mechanisms. Alkoxide size and alcohol nucleophilic character significantly influence the orientation of the various ion/molecule reactions.