An automatic focusing structure includes a holder, a barrel and a piezoelectric actuator. The knife seat has an inwardly recessed notch and a receiving slot formed on a periphery. The differential mobility spectrometer includes a second chamber having a third end and a fourth end configured such that a fluid may flow in a second direction from the third end to the fourth end, and a second electrode system that generates an asymmetric electric field within an interior of the second chamber.