  We present a mechanism to sort out particles of different masses in an
asymmetric channel, where the entropic barriers arise naturally and control the
diffusion of these particles. When particles are subjected to an oscillatory
force, with the scaled amplitude $a$ and frequency $\omega$, the mean particle
velocity exhibits a bell-shaped behavior as a function of the particle mass,
indicating that particles with an optimal mass $m_{op}$ drift faster than other
particles. By tuning $a$ and $\omega$, we get an empirical relation to estimate
$m_{op} \sim (a\,\omega^2)^{-0.4}$. An additional static bias, applied in the
opposite direction of the rectified velocity, would push the particles of
lighter mass to move in its direction while the others drift opposite to it.
This study is useful to design lab-on-a-chip devices for separating particles
of different masses.
