A flowing afterglow selected ion flow tube (FA/SIFT) comparison of SIFT injector flanges and H 3+ + N revisited

Abstract

The performances of two different but interchangeable Venturi injectors (an annulus and a hole injector) have been compared in a new flowing afterglow source-selected ion flow tube (FA/SIFT) instrument built at the University of Canterbury. The tests applied compared the relative “pumping efficiencies” of the two injectors; their ion transmission using (O 2 +); the relative ease of injecting cluster ions subject to collision induced dissociation (H 3O · +H 2O); and the extent of isomerization of ions sensitive to structural changes during the injection process (C 3H 5 +). The annulus injector was clearly superior to the hole injector in pumping efficiency. Thereafter the improvement in performance was only marginal. The greater difficulty of construction and maintenance of the annulus injector needs to be balanced against the slightly less versatile hole injector. It was necessary to direct a significant fraction of the total helium buffer gas flow through an outer, noncritical orifice to maintain satisfactory performance in the hole injector when injecting ions susceptible to collision induced dissociation. Finally, the new instrument was used to reexamine the reaction of H 3 + and N atoms, which was found to be a nonreactive system, k < 5 × 10 −11 cm 3 s −1.