SUSTAINED OFF-RESONANCE IRRADIATION FOR COLLISION-ACTIVATED DISSOCIATION INVOLVING FOURIER-TRANSFORM MASS-SPECTROMETRY - COLLISION-ACTIVATED DISSOCIATION TECHNIQUE THAT EMULATES INFRARED MULTIPHOTON DISSOCIATION

A method for collision-activated dissociation (CAD) involving Fourier transform mass spectrometry (FTMS) is described. In conventional FTMS-CAD an ion is translationally excited by applying a short duration (< 500-mu-s) resonant electric field pulse. Subsequent inelastic collisions with a neutral target result in fragmentation. It is now shown that CAD can also be effected by application of a sustained (greater-than-or-equal-to 500 ms) off-resonance (SORI) electric field pulse. The differences for CAD between "on-resonance" and "off-resonance" irradiation involve the maximum translational energy obtained by the ions, the time scale for excitation and the number of collisions effecting dissociation. SORI-CAD results in sequential activation of ions by multiple collisions of low (< 10 eV) kinetic energy ions with the target. As a consequence, only small increments of internal energy are transferred to the ion throughout the duration of the electric field pulse. This mode of activation is analogous to infrared multiphoton dissociation (IRMPD) by using low-power continuous wave (CW) CO2 lasers which allow the lowest energy decomposition pathway to be sampled. SORI-CAD is, likewise, a selective probe of the lowest energy decomposition pathway of ions. SORI-CAD of a number of model systems is described and potential applications of this new CAD method to chemical problems are discussed.