Particles above a certain size possess so much momentum that they can not follow the air stream and strike a collection surface, which is available for later analysis of mass and composition.
Removal of particles from an air-stream by impaction followed by mass and composition analysis has always been a different approach as to filter sampling, yet has been little utilized for routine analysis because of lack of suitable analytical methods.
The most clear and important advantage of impaction, as opposed to filtration, is that two key aerosol parameters, size and composition, can be simultaneously established.
[citation needed] For two of the most common configurations, an orifice and an infinite slot, theoretical predictions can be made and empirically verified that give the cuts point and shape of the collection efficiency of an impaction stage.
By varying the speed of the air stream and the sharpness of the bend, one can separate particles into numerous size classifications while retaining a sample for analysis.