A comparison of the ion chemistry for mono-substituted toluenes and anilines by three methods of atmospheric pressure ionization with ion mobility spectrometry

Ion mobility spectra for a series of mono-substituted toluenes and a series of mono-substituted anilines were obtained using three different methods of atmospheric pressure ionization including photoionization, chemical ionization from a 63Ni source, and chemical ionization from a corona discharge source. The product ion peak intensities were measured as functions of analyte concentration at 323 K in a purified air atmosphere. Two, and sometimes three, product ion peaks were observed in spectra from chemical ionization with the 63Ni source and it is suggested that the major peak, due to the protonated molecule, arose in both series by proton transfer from H3O+(H2O)n. The second peak with diminished intensity and longer drift time than the protonated molecule can be seen with the toluenes and was understood to be the NO+ adduct, formed from the reactant ion NO+(H2O)n. Electron transfer from the anilines to the latter ion yields the molecular ions, identified by having the same reduced mobility coefficients as the molecular ions produced by photoionization. The structure of these product ions was determined by investigations using the coupling of ion mobility spectrometry with atmospheric pressure photoionization and mass spectrometry (APPI–IMS–MS). The relative abundances of both the NO+ adducts with the toluenes and the molecular ions with the anilines are enhanced with a corona discharge source where relatively more NO+(H2O)n is produced than in a 63Ni source. Ab initio calculations show that only the protonated anilines of all the product ions are significantly hydrated with 1 ppmv of moisture in the supporting atmosphere of the ion mobility spectrometer.