DGI is a 5-stage cascade low pressure impactor. An impactor has two co-linear plates; the other one acts as a collection surface for the particles, the other one has small nozzle or nozzles in it to control the flow velocity. After the sample passes through the nozzles it is turned sharply before the collection plate; particles larger than stage cut diameter cannot follow the flow stream lines but are impacted on the collection plate. Particles smaller that the stage cut diameter continue to the following impactor stages where they are further size classified or collected.