ION DISTRIBUTION PROFILES IN THE DRIFT REGION OF AN ION MOBILITY SPECTROMETER

The radial distribution of ions at atmospheric pressure in the drift r egion of two ion mobility spectrometers of dissimilar dimensions was d etermined using a concentric ring detector, a detector with annulus de sign, and metallic and non-metallic blocking rings. The radial density profile of ions at the detector plate exhibited a maximum off-center beyond which ion density declined rapidly. The intensity at the center was roughly 40% less than the maximum, which to some extent reflects the ion density profile in the source. A hole in the detector plate ce nter with diameter of 3 mm or less did not measurably affect ion inten sities measured at the detector, while incremental enlargement of the hole caused a considerable decline in intensity. Finally, the radial b roadening of the ions throughout their transit in the drift region sug gested that diffusion alone could not account for the observed results .