A computer simulation study of ion-molecule reactions in the mass spectrometer



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Relative ion abundances were calculated on the basis of a collision model for multiple order ion-molecule reactions. Allowance was made for the energy dependency of reaction cross sections and the existence of long-lived intermediates whose dissociation was assumed to follow unimolecular decay kinetics. Cross section and rate constant values severely affected calculated abundance curves and permitted the assignment of reaction mechanisms. The model was applied to the ethylene system under conditions where only one primary ion, C[lowered 2]H[lowered 4, raised +], was present, pressures varied from zero to ~0.03 Torr, ion paths were relatively long (0.54 cm), and drawout fields were low (<6.3 Vcm[raised -1]). Agreement between experiment and calculations based on the model was readily obtained for the above conditions when E[raised -1/2] dependent collision cross sections and energy independent dissociation rates were assumed. Calculations for more commonly used conditions of shorter ion paths (~.3 cm) and higher pressures and higher drawout fields (10-12 Vcm[raised -1]) gave very poor fits to data obtained from the literature, which were taken in the presence of several primary ions. Unsuccessful attempts were made to obtain improved fits by approximate corrections for reactions of primary ions other than Inclusion of various complications such as hard-sphere cross sections and field penetration effects did not improve agreement between calculations and experiment when energy independent unimolecular rate constants were used. Allowance for the energy dependence of unimolecular dissociation of C[lowered 4]H[lowered 4, raised +] and C[lowered 5]H[lowered 9, raised +] gave improved fits to experimental data. The calculations indicate that for tertiary complexes dissociation is competitive with collision when the dissociation rate constant is of the order of 10[raised 6] sec.[raised -1]. The effect of the energy dependence of unimolecular rates upon calculated ion abundance vs. pressure curves was evaluated in detail. It was shown that the energy dependence of rate constants for competitive dissociation processes in the CitHg+ complex formed in the ethylene system is similar to that of the C[lowered 4]H[lowered 8, raised +] ion formed by electron impact from the more stable C[lowered 4]H[lowered 8] isomers.