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In thephysics ofaerosols,aerosol impaction is the process in whichparticles are removed from anair stream by forcing thegases to make a sharp bend. Particles above a certain size possess so muchmomentum that they can not follow the air stream and strike a collection surface, which is available for later analysis ofmass 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 keyaerosol parameters,size andcomposition, can be simultaneously established.[1]
There are many advantages of impaction as asampling method.[citation needed] For two of the most common configurations, anorifice and aninfinite slot,theoretical predictions can be made and empirically verified that give the cuts point and shape of the collection efficiency of an impaction stage. The air stream moves over the sample, not through it as in filtration, reducingdesiccation andchemical transformations of the collected sample. Almost complete control of the type of surface on which the particles are impacted, as opposed to the limited choice of filter types. 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.
There are also several disadvantages to impaction as a sampling method.[citation needed] Only a limited amount of material is available for mass and compositional analysis, as one can not collect more than a fewmonolayers of particles before particle bounce and mis-sizing are a potential problem.
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