the invention relates to low flow rate cascade impactors for sampling aerosols , notably but not limited to pharmaceutical aerosols . the impactor stages serve as both orifice plate and collecting cup , simplifying collection and analysis . the impactor is designed to operate at flow rates approximating the inspiratory flow rates of young children and infants . also presented is a method of and apparatus for applying a coating material to the collection surface of the stages after an impactor is assembled for use . the method entails generation of a polydisperse aerosol and sampling into the impactor . the coating substance improves the trapping of particles on the stages . the apparatus and method of application limit the amount of coating material applied and confines it to the regions of particle impact opposite the stage orifices .