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

Abstract The insertion of a ‘cooling’ time period following ionization in a quadruple ion trap mass spectrometer, so as to cool ion spatial and kinetic energy distributions, also presents an opportunity for ion-molecule reactions to occur. We report here on the observation of such reactions 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 variation of experimental parameters, they can be enhanced by increasing the helium pressure (leading to kinetic energy thermalization) and by decreasing the reactant ion q z , trapping parameter. Both an increase and decrease of the rate constant was observed for the charge exchange reactions of an ion of the m / z 78 benzene molecular ion with butylbenzene and of the m / z 134 butylbenzene molecular ion with p -butylaniline as the cooling period duration was varied. The effect of reaction exothermicity on the magnitude of the rate constant for a series of charge exchange reactions of the m / z 134 butylbenzene molecular ion with substituted butylbenzenes has been explored. The results have been rationalized 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.