HIGH-RESOLUTION ION MOBILITY MEASUREMENTS

Gas phase ion mobility measurements can resolve structural isomers for polyatomic ions and provide information about their geometries. A new experimental apparatus for performing high-resolution ion mobility me asurements is described. The apparatus consists of a pulsed laser vapo rization/desorption source coupled through an ion gate to a 63-cm-long drift tube. The ion gate is a critical component that prevents the di ffusion of neutral species from the source into the drift tube. Ions t ravel along the drift tube under the influence of a uniform electric h eld. At the end of the drift tube some of the ions exit through a smal l aperture. They are focused into a quadrupole mass spectrometer, wher e they are mass analyzed, and then detected by an off-axis collision d ynode and by dual microchannel plates. The apparatus is operated with a drift voltage of up to 14 000 V and a helium buffer gas pressure of around 500 Torr. The resolving power for ion mobility measurements is over an order of magnitude higher than has been achieved using convent ional injected-ion drift tube techniques. Examples of the application of the new apparatus in resolving isomers of laser desorbed metalloful lerenes, in studying silicon clusters generated by laser vaporization, and in following the isomerization of small nanocrystalline (NaClCl- clusters as a function of temperature, are presented. (C) 1997 America n Institute of Physics.