N 2O: A case study for acceleration region kinetics in a double-focusing mass spectrometer with a conventional nier-type ion source

Abstract

A method is presented for the investigation of unimolecular reactions of short-lived ionic species and ion/molecule reactions occurring in the acceleration region of a double-focusing mass spectrometer equipped with a standard Nier-type ion source. This method, which is described as acceleration region kinetics (ARK), has been applied initially to an investigation of ionic species formed from nitrous oxide. N 2O, 15NNO, N 15NO, N 2 18O and 15NH 4NO 3 have been employed in this study. The temporal range of dissociative lifetimes accessible within the acceleration region is estimated as 10 −9–10 −7 s. The determination of half-lives of N 2O +·* in non-competing reactions to yield NO + and N +· 2 has been carried out using ARK and has been compared with the results of linked-scans at constant B 2/ E, where B and E represent magnetic and electric fields, respectively. Releases of kinetic energy have been measured in the second field-free region of the mass spectrometer for reactions of N 2O +·, N 2O 2+ and isotopically substituted analogues. The kinetic energy release associated with extrusion of the central N atom in isotopically labelled N 2O +· is virtually indistinguishable from that accompanying loss of the terminal N atom. Collision induced dissociation has been employed to demonstrate the contribution of N 2O +· in excited states to the dissociative channel leading to N +· 2 and O, a reaction having a greater energy requirement than that leading to NO + and N ·. The release of kinetic energy associated with charge separation of the dication, N 2O 2+, to form NO + and N + was observed to be greater than published values, and the existence of the competing channel to form N +· 2 and O +· is disputed. Isotopic labelling has been used to support the claim that ion signals ascribed previously to N +· 2 and O +· are due to CO +· and O +· from charge separation of CO 2+ 2 dications isobaric with N 2O 2+ dications.