Analysis of a drift tube at ambient pressure: Models and precise measurements in ion mobility spectrometry

Mobility spectra for positive ions, created from a Ni-63 foil in purified a ir at ambient pressure (660 Torr) with 0.15 ppm moisture, were obtained wit h a drift tube with a discrete drift ring design at 250 degreesC as electri c fields for components were individually and independently varied. Peak ar ea, peak width, baseline intensity, drift times, and reduced mobilities (K- o) were used to measure the function and performance of each component and findings were interpreted using a model for the transport of thermalized io ns in weak electric fields at ambient pressure. Transit times and intensiti es for ions in drift tubes at ambient pressure can be understood through a detailed knowledge of the fields local to a component and derivations from theory of ion transport. Prolonged ion residence in the drift region result ed in ion transformations even for highly purified gases of low moisture at high temperature. These findings suggest that mobility spectra may be obta ined with uniformly high quality and reproducibility only under conditions when ion residence time is the primary point of reference in obtaining spec tra. Other regions of the drift tube were optimized and newly observed chem istry occurred in the aperture to detector region. The sampling of ions by such an ion shutter was found to inherently bias the ion distributions and alter actual lengths of drift regions. Consequently, drift lengths measured from physical configurations of drift tubes will be inadequate for precise measurements of drift times. These studies establish baseline measurements for evaluating drift tubes that should be generally applicable for optimiz ing performance in other drift tubes with discrete drift ring designs. Also , these results demonstrate that precise measurements in ion mobility spect rometry (IMS) will require attention to detail not heretofore carefully reg arded in modern analytical IMS. (C) 2001 American Institute of Physics.