Relating IMS Data to Molecular Structure

Beyond a much higher speed, the major distinction of IMS from condensed-phase separations is that it is also a stmctural characterization tool of broad utility. This capability, central to the analytical profile and potential of IMS, arises from the possibility to compute the mobility (under some conditions) for any hypothetical geometry reasonably accurately. In this section, we review the approaches to calculation of ion mobilities in gases and point out the challenges of extending those methods to differential IMS. 

Even those calculations are exact only within the framework of 

FIGURE 1.20 Mobilities of ions in noble gases measurements (circles) and calculations using ab initio interaction potentials (lines). (From Viehland, L.A., Lozeille, J., Soldm, P., Lee, E.P.F., Wright, T.G., J. Chem. Phys., 121, 341, 2004.) 

In summary, the two problem areas of state-of-the-art mobility calculations are the neglect of inelasticity of molecular collisions, especially with respect to rotation, and poor quality or absence of force fields for ion-molecule interactions. However, the impossibility of rigorously solving the Schrodinger equation for polyatomic molecules has stimulated rather than precluded continuous improvement and application of approximate quantum chemistry methods. 

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