High-resolution ion mobility measurements

TLDR

Ion mobility spectroscopy distinguishes structural isomers of polyatomic ions and reveals their geometries. The study introduces a new apparatus for high‑resolution ion mobility measurements. The apparatus uses a pulsed laser vaporization source, an ion gate, a 63‑cm drift tube under a 14 kV electric field with ~500 Torr helium, and a quadrupole mass spectrometer for detection. The device achieves more than tenfold higher resolving power than conventional drift tubes and successfully resolves isomers of metallofullerenes, silicon clusters, and temperature‑dependent isomerization of NaCl nanocrystals.

Abstract

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 measurements is described. The apparatus consists of a pulsed laser vaporization/desorption source coupled through an ion gate to a 63-cm-long drift tube. The ion gate is a critical component that prevents the diffusion of neutral species from the source into the drift tube. Ions travel along the drift tube under the influence of a uniform electric field. At the end of the drift tube some of the ions exit through a small aperture. They are focused into a quadrupole mass spectrometer, where they are mass analyzed, and then detected by an off-axis collision dynode 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 conventional injected-ion drift tube techniques. Examples of the application of the new apparatus in resolving isomers of laser desorbed metallofullerenes, in studying silicon clusters generated by laser vaporization, and in following the isomerization of small nanocrystalline (NaCl)nCl− clusters as a function of temperature, are presented.