A Method of Rate Constant Measurement in Fourier Transform Ion Cyclotron Resonance Pulsed Valve Experiments

Abstract

Rate constant measurements in Fourier-transform ion cyclotron resonance (FT-ICR) pulsed valve experiments are complicated by the pressure changes which occur rapidly after the opening of the pulsed valve. A method is described here to overcome that complication. The time-dependent number density n(t) is sampled by determining the total ion signal I(t_n) in spectra resulting from pulsing the electron beam on at various delay times t_n after opening the valve. The I(t_n) are fit to an analytic function to give I(t) which can be integrated to give a kinetic variable comparable to pressure at fixed time or time at fixed pressure. A reactant ion isolated in the cell should disappear exponentially with ${\int }_0^{t_n}$ I(t) dt where the valve opens at time 0 admitting reactant neutral and the reactant ion concentration is sampled at time t_n. To obtain the rate constant from such an exponential decay the ionization cross section of the neutral must be known or estimated and the overall sensitivity of the instrument must be known. The latter can be determined by examining a reaction of known rate constant. The procedure is illustrated for the reaction of Re₂⁺ with cyclohexane. The implications of this treatment for other types of pulsed valve experiments are discussed.