Robert R. Hudgins

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 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 ...

High-resolution ion mobility measurements for silicon cluster anions and cations

High-resolution ion mobility measurements have been performed for silicon cluster anions and cations, Sin− and Sin+, n=6–55. New isomers have been resolved for every cluster size larger than Si20. The results for the anions and the cations have the same global features. However, changing the charge often causes a shift in the isomer distribution, or causes new isomers to emerge. For example, the transition from prolate geometries to more-spherical ones is shifted to larger cluster sizes for the anions than for the cations. The mobilities of the anions are systematically smaller than those of the cations, presumably because of differences in the exterior electron densities.

High resolution ion mobility measurements for gas phase proteins: correlation between solution phase and gas phase conformations

Abstract Our high resolution ion mobility apparatus has been modified by attaching an electrospray source to perform measurements for biological molecules. While the greater resolving power permits the resolution of more conformations for BPTI and cytochrome c , the resolved features are generally much broader than expected for a single rigid conformation. A major advantage of the new experimental configuration is the much gentler introduction of ions into the drift tube, so that the observed gas phase conformations appear to more closely reflect those present in solution. For example, it is possible to distinguish between the native state of cytochrome c and the methanol-denatured form on the basis of the ion mobility measurements; the mass spectra alone are not sensitive enough to detect this change. Thus this approach may provide a quick and sensitive tool for probing the solution phase conformations of biological molecules.

Structural information from ion mobility measurements: applications to semiconductor clusters

Ion mobility measurements are one of the few methods presently available that can directly probe the structures of relatively large molecules in the gas phase. Here we review the application of ion mobility methods to the elucidation of the structures of semiconductor clusters (Sin, Gen, and Snn). We describe the new high-resolution implementation of the technique and the advanced methods of mobility calculations that are crucial for the correct analysis of the experimental data.

Conformations of GlynH+ and AlanH+ Peptides in the Gas Phase

High-resolution ion mobility measurements and molecular dynamics simulations have been used to probe the conformations of protonated polyglycine and polyalanine (Gly(n)H and Ala(n)H+, n = 3-20) in the gas phase. The measured collision integrals for both the polyglycine and the polyalanine peptides are consistent with a self-solvated globule conformation, where the peptide chain wraps around and solvates the charge located on the terminal amine. The conformations of the small peptides are governed entirely by self-solvation, whereas the larger ones have additional backbone hydrogen bonds. Helical conformations, which are stable for neutral Alan peptides, were not observed in the experiments. Molecular dynamics simulations for Ala(n)H+ peptides suggest that the charge destabilizes the helix, although several of the low energy conformations found in the simulations for the larger Ala(n)H+ peptides have small helical regions.

HIGH-RESOLUTION ION MOBILITY STUDIES OF SODIUM CHLORIDE NANOCRYSTALS

Abstract High-resolution ion mobility measurements have been used to examine the geometries of (NaCl) n Cl − clusters with up to 49 NaCl units. Steps in the relative inverse mobilities as a function of the number of NaCl units are due to completion of the 4 × 3 × 3, 4 × 4 × 3, 4 × 4 × 4, and 5 × 4 × 4 cuboids. For clusters with more than 30 NaCl units several families of isomers, with different cuboid or incomplete cuboid geometries, have been resolved at room temperature. The resolved isomers are assigned to specific geometries by comparing the measured mobilities to mobilities calculated for geometries optimized using an ionic potential.

High-resolution ion mobility measurements of indium clusters: electron spill-out in metal cluster anions and cations

Abstract High-resolution ion mobility measurements have been used to examine In n − and In n + clusters with n =2–30. The cluster–He scattering cross-sections of the anions are systematically larger than those of the cations. The difference (∼25% for In 2 to ∼2% for In 30 ) is attributed to the electron density extending further from the surface of the cluster in the anions than in the cations. Geometric cross-sections estimated from electron densities determined from a charged jellium model are in good agreement with the experimental data: the change in cross-section with charge state is reproduced over the entire range.

Experimental Studies of the Structures and Isomerization of Atomic Clusters

Ion mobility measurements can be used to separate structural isomers of atomic clusters and to provide information about their geometries and isomerization processes. The principles behind ion mobility measurements and the methods used to calculate mobilities for comparison with the experimental data are briefly reviewed. With the development of high resolution ion mobility measurements, it is now possible to separate many more structural isomers than could be resolved using conventional techniques. Some recent results for carbon and silicon clusters are described. For sodium chloride nanocrystals several families of structural isomers have been resolved and the results show that dramatic shape transformations can occur at room temperature for these species.