The presence of trace molecules in air may be determined using an ion mobility
spectrometer. Such devices may be used in the fields of explosives detection, identification
of narcotics, and in applications characterized by the presence of very low airborne
concentrations of organic molecules of special interest. The sensitivity of such
instruments may depend upon on the method of gas sampling utilized. A virtual wall
gas sampling system can greatly improve the sampling efficiency, particularly when
the sampling needs to be performed at a distance from the air intake and large
volumes need to be sampled. The virtual wall gas sampling system consists of an
intake gas flow and a separate group of one or more sheet-like gas flows, which
may be either mutually deflected to move with a circular motion or may be formed
into a cylindrical bounding surface.
