ABSTRACT
Objects suspended in a gas or liquid medium always experience spontaneous diffusion due to Brownian motion at finite temperature. With no other forces, the diffusion in an isotropic medium has no preferred direction. When an external force (of electric, magnetic, or gravitational origin) is applied, an object will also move along its vector with the speed controlled by the characteristics known as mobility. In particular, a Coulomb force exerted by electric field upon charged particles seeks to transpose them along the field lines, toward decreasing potential for positive ions and in reverse for negative ions. The above motions in a medium are superposed on any flow of the medium itself. The diffusion and mobility properties of objects are closely related: both depend on the object nature, which carries information about it and allows separating different species.1 The use of that fact with respect to ions in electromagnetic fields is called ion mobility spectrometry (IMS).
