Molecular velocity distribution effects in kinetic studies by time-resolved mass spectrometry

Abstract

Mathematical models were developed to describe the sampling of a rapidly reacting gas by time-resolved mass spectrometry. The flow of reaction mixture from a sampling orifice in the reactor wall to the mass spectrometer ion source was treated by application of the Boltzmann equation. The calculations demonstrate that for sufficiently rapid reactions the molecular velocity distribution of the detected species causes a distribution of arrival times at the ion source which complicates the kinetic analysis. The time-dependent signal is composed of two parts; one due to the rate of chemical reaction and the other due to the ion source arrival time distribution. Calculations done on the basis of both first and second order kinetics indicate the upper limit to the reaction rate which can be measured without causing substantial errors in rate constant determinations when conventional types of concentration time plots are used. These limits hold for effusive flow through the orifice, and at least to a first approximation also hold for convective and transition flow.