A device used to remove from a flowing gas stream a predictable fraction of particles within a narrow size range based upon electrical mobility. The electrical mobility is defined as the electrical velocity of the particle divided by the field strength. This device consists of two cylindrical electrodes, an outer ground electrode and an inner rod to which a precise negative voltage is applied. The sample flow is introduced into an annular gap between the two electrodes and flows through the device along with an inner core of particle- free sheath air without mixing. Particles with positive charges are attracted through the sheath air toward the negatively charged center electrode. Only those particles within a narrow mobility range pass through a slit near the bottom of the center electrode.