Impact type gas-particle separation is applied to ultra-fine particles by judiciously altering the aerodynamic drag on the particles with respect to their inertia by operation at sub-atmospheric pressures. An impact separator has a housing having a channel in which particles flow in a fluid such as air, a purified gas or mixtures thereof. A plurality of blades are placed in the channel for affecting the motion of the particles and the fluid with respect to each other. A second channel branches from the first channel to receive particles diverted and separated from the flow of the fluid in the first channel. A flow mechanism creates a sub-atmospheric pressure in the channel.