A Novel Use of Negative Ion Mobility Spectrometry for Measuring Electron Attachment Rates

Abstract

A new experimental approach based on swarm techniques was proposed for the measurement of rate constant of an electron attachment process at atmospheric pressure by means of negative ion mobility spectrometry. In this technique, sample is continuously delivered to the drift gas, which then enters into the drift region where it reacts with a counterflowing swarm of electrons injected by the shutter grid. As a result, negative ions are formed in the drift tube and a tail appears in the ion mobility spectrum. It is shown that the tail fits to an exponential function and the magnitude of the electron attachment rate constant value can be extracted from the plot of ln(ion intensity) versus the drift time. Unlike conventional electron swarm technique, it is not necessary, in this method, to perform the experiment at various concentrations of the sample. The approach proposed was typically examined for CCl4, CHCl3, and CH2Cl2 and the rate constants were estimated for a range of mean electron energies (〈〉 ) 0.28-0.88 eV) in nitrogen buffer gas at 300 K. Good agreement between the rate constant measured in the present work with those reported in earlier works is observed.