THE KINETIC ION MOBILITY MASS-SPECTROMETER - MEASUREMENTS OF ION MOLECULE REACTION-RATE CONSTANTS AT ATMOSPHERIC-PRESSURE

An instrumental method for measuring the rate constants of ion-molecule reactions in an atmospheric pressure buffer gas is described here. The instrument consists of an ion mobility spectrometer, a mass spectrometer, and an associated gas-handling plant. The rate constants are measured by two different operational modes of the instrument. One of these does not require mass spectrometric sampling of the reaction mixture and, therefore, completely avoids measurement errors that am commonly associated with aperture sampling of a high-pressure ionized gas. The application of this instrument to the S(N)2 nucleophilic displacement reactions of chloride anions with a series of alkyl bromides in nitrogen buffer gas at 640 Torr and 125-degrees-C is described, and the results are compared with those obtained by others at lower pressure.