A numerical and experimental investigation comparing two configurations for a slit nozzle virtual impactor, one using a three-partitioned horizontal inlet and the other using a typical vertical inlet, was conducted to improve collection efficiency and reduce wall loss. All parameters, such as the length and width of the acceleration nozzle, the width of the collection nozzle, the distance between the two nozzles, and the inlet flow rate, were kept constant, and only the inlet configuration was changed. The ratio of the minor to the total flow rate was fixed at 0.1. Parametric analysis was performed to study the flow-rate influence of incoming clean air and aerosol on the performance of the three-partitioned horizontal inlet virtual impactor. The square root of the Stokes number, (Stk)1/2, was used to characterize the collection efficiency and wall loss of both configurations. It was observed that by using the three-partitioned horizontal inlet, the cut-off diameter was reduced from (Stk)1/2 = 0.80 to (Stk)1/2 = 0.41, and the wall loss near the cut-off diameter decreased from 25%–45% to below 4.5%. Moreover, using the three-partitioned horizontal inlet reduced the contamination by particles smaller than the cut-off size in the minor outflow section of the virtual impactor to almost 0%.