Field dependence of mobilities for gas-phase-protonated monomers and proton-bound dimers of ketones by planar field asymmetric waveform ion mobility spectrometer (PFAIMS)

The dependence of the mobilities of gas-phase ions on electric fields from 0 to 90 Td at ambient pressure was determined for protonated monomers [(MH+(H2O)(n)] and proton bound dimers [M2H+(H2O)(n)] for a homologous series of normal ketones, from acetone to decanone (M = C3H6O to C10H20O). This dependence was measured as the normalized function of mobility alpha(E/N) using a planar field asymmetric waveform ion mobility spectrometer (PFAIMS) and the ions were mass-identified using a PFAIMS drift tube coupled to a tandem mass spectrometer. Methods are described to obtain alpha(E/N) from the measurements of compensation voltage versus amplitude of an asymmetric waveform of any shape. Slopes of alpha for MH+ versus E/N were monotonic from 0 to 90 Td for acetone, butanone, and pentanone. Plots for ketones from hexanone to octanone exhibited plateaus at high fields. Nonanone and decanone showed plots with an inversion of slope above 70 Td. Proton bound dimers for ketones with carbon numbers greater than five exhibited slopes for a versus E/N, which decreased continuously with increasing E/N. These findings are the first alpha values for ions from a homologous series under atmosphere pressure and are preliminary to explanations of alpha(E/N) with ion structure.