Ion Dissociation in the Drift Tube of a Time-of-Flight Mass Spectrometer: Spurious Fragments Arising from Charge-Transfer and Dissociation Reactions of Retarded Ions

Abstract

Spurious fragment peaks that simulate fragment‐ion dissociation products formed in the drift‐tube reaction region have been observed in a time‐of‐flight mass spectrometer adapted for kinetic studies of ion dissociation processes. These peaks arise from processes occurring after the reactant ion has passed through the reaction region and entered the potential barrier that is normally used to separate reactants and products by selectively shifting their flight times. The flight‐time shifts characteristic of such spurious peaks depend on the distance from the ion source to the potential barrier, whereas the shifts characteristic of normal fragment peaks are independent of this distance. Comparisons of experimentally measured flight‐time shifts with analytically calculated values identify a fragment peak observed in association with the N₂⁺ mass‐28 peak in N₂O as a spurious fragment peak, which represents not dissociation products of N₂⁺ formed in the reaction zone, but neutrals formed inside the potential barrier. The absence of other fragment peaks then establishes that the neutrals are formed by the charge‐transfer process $${\mathrm{N}}_2^{+}{\displaystyle {lim}_{\mathrm{fast}}}+{\mathrm{N}}_2\mathrm{O}{\displaystyle {lim}_{\mathrm{thermal}}}\to {\mathrm{N}}_2^0{\displaystyle {lim}_{\mathrm{fast}}}\mathrm{+(}{\mathrm{N}}_2\mathrm{O}{)}^{+}·{\displaystyle {lim}_{\mathrm{thermal}}}$$ Since such spurious fragment peaks can, as in this case, be much larger than any real fragment peaks present, a cautious approach to peak identification is essential when using the TOF—potential barrier technique for ion dissociation studies. The recognition of such spurious fragment peaks is greatly facilitated by making flight‐time shift measurements at more than one value of the distance from the ion source to the potential barrier.