A method and apparatus is provided for detecting an impurity in a sample where an index can be calculated to assess purity in the presence of n major components with signal averaging or noise-filtering automatically built-in. Samples may range in size from a few nanometers to at least tens of micrometers, may be dispersed in any fluid medium, and may be sorted on the basis of size, shape, optical characteristics, charge, and other physical properties. Side inlets are defined between the cover plate and the air guiding member.