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Differential Analysis of Very Small Particles (VSP) From the Contact Surfaces and Recessed Areas of Footwear

NCJ Number
Forensic Science International Volume: 298 Dated: May 2019 Pages: 106-114
Date Published
February 2019
9 pages
This study conducted differential analysis of very small particles (VSP) recovered separately from contact and recessed surfaces of footwear, following a controlled series of environmental exposures.

Prior work has shown that the contact surfaces of footwear rapidly lose very small particles (VSP) when walking on dry soil. Other research, with more general sampling from shoe soles, has shown that particles can persist much longer. Given rapid losses from contact surfaces, the current study hypothesized that non-contact, recessed areas of footwear retain particles from prior exposures. In this study, work boots and tennis shoes were exposed by walking distances of 250 m, sequentially, in three environmental sites. VSP were harvested separately from contact and recessed surfaces using a moist swabbing procedure. VSP were analyzed by microscopy and the proportions attributable to each site were determined using a newly developed statistical model. The principal findings are that (1) contact surfaces of footwear are dominated by VSP attributable to the most recent site of exposure; (2) recessed surfaces of footwear retain VSP from prior exposures; (3) sole type appears to be a source of major differences in the amount of VSP from prior exposures remaining in recessed areas; and (4) when VSP attributable to prior exposures are found, there is no clear trend for dominance between earlier exposures. Based in these findings, the study concludes that in cases where the last site visited is of interest, VSP from contact surfaces of footwear will give a nearly pure sample of that site. In cases where prior sites visited are of interest, the VSP from the contact surfaces can be used as a background signal, providing a means for differential analysis of VSP mixtures found in the recessed areas of the sole. The strong influence of shoe sole type on the retention of VSP from prior exposures indicates that efforts to explore the rates of VSP loss from recessed areas will need to take characteristics of the sole type into account. The absence of a clear trend for dominance between earlier exposures suggests that once traces are lodged within the recessed areas of footwear soles they can be retained for long periods of time. These findings support prior research on the retention of particles on footwear that shows periods of long retention following rapid initial losses of trace materials. The study advises that differential analysis of VSP is a significant contribution to the methods available for the deconvolution of particle mixtures into fractions that are characteristic of their contributing sources. (publisher abstract modified)

Date Published: February 1, 2019