IMS—A Vindication of Aristotles Physics

The medium in the definition of IMS (1.1) is central to its physics, setting it apart from MS. Ions of charge q subject to a fixed uniform E experience a constant force equal to qE. In MS analyses that proceed in vacuum, ions fly with a constant acceleration prescribed by Newton s second law of motion  

FIGURE 1.3 Schematic plots for ion velocity (sohd hne) and distance traveled (dashed hne) in MS (a) and IMS (b) regimes. Panels (c-e) exhibit detailed motion in IMS, the pressure in (d) is 2x that in (c) and the electric field intensity in (e) is Vi that in (c), with other parameters fixed. 

In the absence of field, ions fly by inertia with conserved v. These two facts are exploited in time-of-flight mass spectrometry (TOF MS), where ions are accelerated in a pusher region to different v depending on y m/z and fly through a field-free space of well-defined length to the detector. The measured flight time through that space reveals v and thus m/z of ions present. 

In contrast, objects subject to Coulomb (or any other) force in a medium reach a terminal v— the drift velocity. In IMS, different species have different v and are separated by mobility (A  

The mobility and the diffusion coefficient of an ion are connected by the Nemst-Townsend-Einstein (or simply Einstein) relationship  

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