Although it is well-recognized that criminals track dusts to and from every crime scene, dust particles on a suspects shoes are very seldom used as evidence linking the accused to the crime. The major obstacle preventing the use of this type of evidence is that the shoes have mixtures of particles arising from activity before, during and after the crime itself. Methods separating the evidentiary particle signal from background noise would enable a powerful new and widely applicable forensic capability. This capability would augment traditional footwear pattern evidence with objective quantitative associations, addressing one of the specific issues raised in the 2009 NAS report. Our prior NIJ research has shown corresponding particle sets to provide extremely strong, objective, quantitative, associative evidence. We hypothesize that by separately analyzing particles harvested from the outermost contact surfaces and the more protected recessed areas of footwear soles, we will be able to detect sequential footwear exposures and enable the widespread use of this new forensic capability. Our research goal is to test the ability of quantitative differential analyses of very small particles (VSP) on the contact surfaces and recessed areas of footwear to separate trace evidence signals attributable to different exposures. To do this we will design and conduct a series of sequential test exposures to known environments, develop and test computational methods for the quantitative differential analysis of VSP, and apply these methods to specimens collected from the contact surfaces and recessed areas of footwear. This research will provide (1) a greater understanding of how particles adhere to the soles of footwear, how they can be separated, and whether sequential exposures can be determined on this basis; (2) a set of computational methods applicable to VSP mixtures, allowing determination of proportions in mixtures, estimation of a proportion of a designated source in a mixture, and estimation of the parameters of an unknown source within a mixture; and (3) progress toward implementation of a new forensic capability that is widely applicable, with objective, quantifiable associations. Note: This project contains a research and/or development component, as defined in applicable law.