Collision induced dissociations of ions produced in the field free regions of a triple sector mass spectrometer

Molecular or fragment ions can be prepared by a large variety of reactions in the field free regions of a mass spectrometer; these reactions involve spontaneous dissociation as well as collision induced excitation of singly or doubly charged precursors. Some typical examples show the different means which can lead to a given ion and how they can be studied by collisional activation. It also appears that in most cases the use of a triple sector (E/B/E) mass spectrometer facilitates these experiments as interfering artefacts are suppressed.     

Electron impact induced fragmentation of β-allenic and γ-acetylenic alcohols

The main fragmentation pathway of ionized hydroxyallenes (1) consists of a methyl loss. Extensive deuterium-labelling experiments indicate that the terminal allenic carbon is implied in this fragmentation. Collisional activation spectra indicate a propenyl-acylium structure (a) for these [M – CH₃]⁺ ions which can originate from a 1,4-hydroxyl migration followed by hydrogen rearrangements. Isomeric hydroxyacetylenes (2) behave similarly, also giving rise, by methyl loss, to acylium ions a. It is proposed that 2^{+ ˙} is irreversibly isomerized into 1^{+ ˙} by a 1,3-hydrogen transfer ‘catalysed’ by the hydroxy group. The proposed internal proton-bound complex justifies also the easier loss of water from 2⁺˙. Ethyl loss is also a prominent fragmentation for the hydroxyallene and hydroxy-acetylene homologues.     

Mass and kinetic energy spectrometries of alkylketenes

Real-time analysis of alkylketenes synthesized by flash vacuum thermolysis (FVT) of acid chlorides has been performed by mass spectrometric techniques using a triple sector (E/B/E) spectrometer. The major high energy reactions of the molecular ions consist of β-cleavages relative to the C?C bond of the ketene function, followed by decarbonylation. The unimolecular reactions of the metastable molecular ions are often different as shown by collisional activation of the fragment ions produced in a field-free region of the spectrometer and also from estimated heats of formation.     

Structure of gas phase [C5H6]+˙ ions

Charge-stripping spectra have been used to differentiate ionized cyclopentadiene from its acyclic isomers. The minimum amounts of translational energy lost during the charge-stripping processes and the relative charge-stripping efficiencies, which are also structurally important parameters, have been measured for these ionic species. [C₅H₆]⁺˙ ions, formed by dissociative ionization of various precursors in the ion source are found, usually, to be a mixture of cyclic and acyclic ions. In contrast, [C₅H₆]⁺˙ ions, derived from the dissociation of metastable molecular ions from a series of organic compounds, have the cyclopentadienyl structure. This structure was confirmed by collision-induced dissociation of ions formed in the first field-free region of a triple sector mass spectrometer.     

Metastable fragment ions from carbinolamine ethers

The secondary decomposition of fragment ions produced by electron impact of carbinolamine ethers was studied by mass analysed ion kinetic energy spectrometry. A new case was observed of ions of the same structure leading to the same fragments through different mechanisms competing in the metastable time frame and accompanied by a different partitioning of the excess energy (internal vs translational). The role of a fractional positive charge localized at saturated centres in the reactivity of these gaseous organic ions is discussed.