INFLUENCE OF ION ACTIVATION AND THERMALIZATION EFFECTS ON REACTION-RATE CONSTANTS IN A QUADRUPOLE ION-TRAP MASS-SPECTROMETER

The insertion of a 'cooling' time period following ionization in a qua drupole ion trap mass spectrometer, so as to cool ion spatial and kine tic energy distributions, also presents an opportunity for ion-molecul e reactions to occur. We report here on the observation of such reacti ons between the m/z 91 and m/z 92 fragment ions from butylbenzene and parent molecules. The measured rate constants for these reactions are relatively low and do not influence unduly the estimation of precursor ion internal energy from the intensity ratio of these fragment ions. The values of these reaction rate constants can be changed by variatio n of experimental parameters, they can be enhanced by increasing the h elium pressure (leading to kinetic energy thermalization) and by decre asing the reactant ion q(z) trapping parameter. Both an increase and d ecrease of the rate constant was observed for the charge exchange reac tions of an ion of the m/z 78 benzene molecular ion with butylbenzene and of the m/z 134 butylbenzene molecular ion witn p-butylaniline as t he cooling period duration was varied. The effect of reaction exotherm icity on the magnitude of the rate constant for a series of charge exc hange reactions of the m/z 134 butylbenzene molecular ion with substit uted butylbenzenes has been explored. The results have been rationaliz ed on the basis of the average dipole orientation treatment of the ion -dipole complex theory. The quasi-equilibrium theory has been used to account for the influence of ion internal energy on the reaction rate constant. (C) 1997 Elsevier Science B.V.