The authors briefly review the development of the virtual impactor which, as an inertial particle separator according to aerodynamic sizes, has played a unique role in particle sampling, concentration, classification, and generation. Its performance characteristics in size separation are predictable by theoretical model calculations. However, its behavior in terms of internal wall losses has thus far defied quantitative analysis, and its ultimate control has eluded most practitioners in virtual impactor design. Through experimentation, the authors identify the relevant parameters in a virtual impactor and indicate their relative sensitivity and acceptable ranges of variability. With the detailed illustration of specific high-efficiency virtual impactor design, which has a cutpoint of 2.5 μm and wall losses of under 1%, it is demonstrated the underlying principles cited are crucial to minimizing losses and may be generally applicable to future developments.
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