COOLING TIME AND PRESSURE EFFECTS ON COMPETITIVE THERMALIZATION ACTIVATION PROCESSES BY RESONANCE EXCITATION ON ITMS

The classical n-butylbenzene model is used to estimate the qualitative variation of the precursor ion internal energy by the resonant excita tion method in an ion-trap mass spectrometer. The kinetic energy therm alization process is confirmed by insertion of a cooling time (less th an or equal to 10 ms under our conditions). An initial internal energy relaxation process due to collisions between the molecular ion and ne utral species of n-butylbenzene is proposed, The larger amount of inte rnal energy deposited in ions as the helium pressure is increased can be explained in terms of the number of collisions between the molecula r ion and helium atoms, the competition between ion dissociation and i on ejection, the rate constant of dissociation and the secular axial f requency resolution of the isolated ions. Increased analyte pressure d oes not accelerate the ion relaxation process, probably due to the spa ce charge effects. In summary, variation of helium pressure and coolin g time permit optimization of ion internal energy deposition leading t o enhanced sensitivity in MS/MS experiments.