Ion velocity distributions in an ICR spectrometer and their effect on measured rate constants

Abstract

The translational velocity distributions of ions formed by electron impact in both square and flat ion cyclotron resonance (ICR) cells are theoretically determined. These velocity distributions result from acceleration of the ions in the trapping field of the spectrometer. ICR rate constants are computed from these distributions assuming the reaction cross section is proportional to v−n. The computations are compared to thermal values for a range of n and for a range of trapping voltages. In the most severe cases the ICR rate constants are predicted to deviate from thermal values by ∼20%. The analysis considers only reactions occurring immediately following ion formation in the electron beam. Collisions with unreactive species will, of course, rapidly transform the ICR distribution to the thermal distribution. The effect of ion mass on the velocity distribution functions and rate constants was explicitly studied. The effects on the velocity distributions may be appreciable in extreme cases but the effect on the predicted ICR rate constants is small.