Dissociation of H3O+, NO+ and O2+• reagent ions injected into nitrogen carrier gas in SIFT-MS and reactivity of the ion fragments

Abstract A comparative study has been made of the utility of helium and nitrogen as the carrier gas in selected ion flow tube mass spectrometry, SIFT-MS. A most important parameter is shown to be the injection energy, Elab, into the nitrogen carrier gas of the reagent cations H3O+, NO+ and O2+• produced in the microwave discharge ion source. If Elab is too high, partial fragmentation of the reagent ions occurs in the hard collisions with nitrogen molecules, which does not occur to a significant extent in their collisions with lighter helium atoms. Then, the H3O+ ions fragment producing highly reactive ions, including OH+ and H2O+•, that can ionise and dissociate the N2 molecules, and the fraction of the non-dissociated H3O+ can associate with N2 molecules producing H3O+N2 adduct ions. These events promote the production of hydrated hydronium ions, H3O+(H2O)1,2,3, which become dominant even when the carrier gas and sample gas are only a modest humidity. The NO+ reagent ions become electronically excited and partially fragment to either O+• or N+ ions. These events result in their reactions with the N2 molecules producing excessive O2+• ions in the carrier gas. Similarly, the O2+• reagent ions become electronically excited and fragment to O+• ions that react with the N2 molecules producing large fractions of NO+ ions. These processes result in larger fractions of “impurity ions” than are desirable for gas phase analysis using SIFT-MS, which is especially serious for H3O+ reagent ions, but less so for NO+ and O2+•, which can still be used, with caution, for trace gas analysis. It is shown by reducing Elab that fragmentation can be minimised, and it is suggested that by increasing the carrier gas temperature or exploiting drift tube methods, adduct ion formation can be inhibited and thus partially “purify” the reagent ions to approach the situation that pertains to helium carrier gas.