A METHOD OF RATE-CONSTANT MEASUREMENT IN FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE PULSED VALVE EXPERIMENTS

Rate constant measurements in Fourier-transform ion cyclotron resonanc e (PT-ICR) pulsed valve experiments are complicated by the pressure ch anges which occur rapidly after the opening of the pulsed valve. A met hod is described here to overcome that complication. The time-dependen t 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 vario us delay times t, after opening the valve. The I(t(n)) are fit to an a nalytic function to give I(t) which can be integrated to give a kineti c variable comparable to pressure at fixed time or time at fixed press ure. A reactant ion isolated in the cell should disappear exponentiall y with integral(O)(tn) I(t) dt where the valve opens at time 0 admitti ng reactant neutral and the reactant ion concentration is sampled at t ime t(n). To obtain the rate constant from such an exponential decay t he ionization cross section of the neutral must be known or estimated and the overall sensitivity of the instrument must be known. The latte r can be determined by examining a reaction of known rate constant. Th e procedure is illustrated for the reaction of Re-2(+) with cyclohexan e. The implications of this treatment for other types of pulsed valve experiments are discussed.