Effects of stepwise excitation on fragmentation of gas phase ions: An especially striking case

Abstract

AbstractDramatically different fragmentation patterns are obtained for 4‐ethyl‐2,6,7‐trioxa‐1‐phosphabicyclo‐[2.2.2]octane‐1‐oxide upon electron ionization (EI) and for the corresponding molecular ion on collision‐induced dissociation (CID). Two reasons for this behaviour have been discovered. (i) Irreversible multistep isomerization of the molecular ions occurs prior to collisional activation in mass spectrometry/mass spectrometry (MS/MS). Isomerization reactions have been characterized by isotopic labelling and by examining structures of relevant unlabelled and labelled fragment ions by MS/MS. The extent of isomerization can be controlled by varying the amount of internal energy of the molecular ions. This has been done by changing the number of thermalizing collisions which the ions undergo with neutral molecules in the ion source. (ii) When multiple collisions are used to dissociate the molecular ions, the initially stable fragmentation products undergo extensive further decomposition. As a result, abundant phosphorus‐containing fragment ions are obtained for the bicyclic phosphate in high‐pressure CID, whereas electron ionization leads to predominant hydrocarbon ions. A minor change in the structure of this molecule has major effects on the fragmentation behaviour: high‐ and low‐energy collisional activation spectra of the molecular ion of the corresponding phosphite are identical with the 12 e V EI mass spectrum of the neutral.