  We identify a single-particle drift resulting from collisional interactions
with a background species, in the presence of a collisionality gradient and
background net flow. We analyze this drift in different limits, showing how it
reduces to the well known impurity pinch for high-Z i impurities. We find that
in the low-temperature, singly-ionized limit, the magnitude of the drift
becomes mass-dependent and energy-dependent. By solving for the resulting
diffusion-advection motion, we propose a mass-separation scheme that takes
advantage of this drift, and analyze the separative capability as a function of
collisionally dissipated energy.
