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During the past 50 years, man has become acutely aware of the air pollution pervading our environment. Thus he has focused a considerable amount of scientific study on the chemical and physical properties of gaseous and particulate contaminants. A key element in the classification of particulate pollution is the sampling procedure used to collect particles, with the emphasis on separating the respirable from the non-respirable fraction. Generally, impaction techniques have been utilized for size collection of particulates, but these procedures can be affected by the nature of the impaction collection technique which can cause the particles to bounce from one stage to the next, particularly when larger sample quantities are desired for chemical analysis and gravimetrics. Recently EPA, working with several other investigators, perfected a dichotomous collection device, based on virtual impaction principles, which separates and collects particles into two size ranges, 0 to 3.5 microns and 3.5 to 20 microns. Particle bounce at the collection surface is not a problem with this sampler because particles are captured by filtration. The operation and design of manual and automated virtual impactors are discussed. In addition, the application of x-ray fluorescence to the analysis of particulates collected with the dichotomous sampler is described.