Mechanistic studies of multipole storage assisted dissociation

The degree and onset of fragmentation in multipole storage assisted dissoci ation (MSAD) have been investigated as functions of several hexapole parame ters. Strict studies of hexapole charge density (number of ions injected) a nd hexapole storage time were made possible by placing a pulsed shutter in front of the entrance to the mass spectrometer. The results obtained show t hat the charge density is the most critical parameter, but also dependencie s on storage time, radio-frequency (rf) -amplitude, and pressure are seen. From these data, and from simulations of the ion trajectories inside the he xapole, a mechanism for MSAD, similar to the ones for sustained off-resonan ce irradiation (SORI), and for low energy collisionally induced dissociatio n in the collision multipole of a triple quadrupole mass spectrometer, is p roposed. It is believed that, at higher charge densities, ions are pushed t o larger hexapole radii where the electric potential created by the rf fiel d is higher, forcing the ions to oscillate radially to higher amplitudes an d thereby reach higher (but still relatively low) kinetic energies. Multipl e collisions with residual gas molecules at these elevated energies then he at up the molecules to their dissociation threshold. Further support for th is mechanism is obtained from a comparison of MSAD and SORI spectra which a re almost identical in appearance. (C) 2000 American Society for Mass Spect rometry.