Differential Sampling of Footwear to Separate Relevant Evidentiary Particles from Background Noise

David A. Stoney, Ph.D.
Paul L. Stoney, M.B.A


Everywhere people walk, very small particles (VSP) transfer to and from their footwear. The mere presence at a crime scene requires this contact and transfer. VSP on footwear are routinely ignored by forensic science unless there are obvious accumulations of soil or conspicuous visible particles. There is an extraordinary, untapped potential to exploit VSP found on footwear.

One predominant challenge remains to unlock this potential: VSP are invariably present as mixtures of materials that can originate before, during, or after any event of forensic interest. Their usefulness depends on our ability to separate a reliable, relevant evidentiary "signal" from the background noise.

This project tested the ability to separate particle signals on the contact surfaces of footwear soles using differential analysis of loosely held, moderately held, and strongly held particle fractions. Project objectives were to: (1) design and conduct a series of realistic environmental exposures suitable for testing of VSP transfer, retention and fractionation, (2) design, develop and test a differential sampling protocol suitable for testing the physical fractionation of VSP signals, and (3) test the ability to use differential sampling to separate specific VSP signals of interest.

Three environmental exposure sites were chosen to have different, characteristic particle types (soil minerals). Shoes of two types (work boots and tennis shoes) were tested, accumulating particles by walking 250 m in each environment. Some shoes were exposed to only one environment; others were exposed to all three, in one of six different sequences.

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Sampling methods were developed to separate particles from the contact surface of the shoe based on how tightly they were held to the sole. Loosely held particles were removed by walking on paper, moderately held particles were removed by electrostatic lifting, and the most tightly held particles were removed by moist swabbing.

The resulting numbers and types of particles were determined using forensic microscopy. Particle profiles from the different fractions were compared to test the ability to objectively distinguish the order of exposure to the three environments.

Without exception, the samples resulting from differential sampling are dominated by the third site in the sequential footwear exposures. No noticeable differences are seen among the differential samplings of the loosely, moderately and strongly held particles: the same overwhelming presence of the third site is seen. It is clear from these results that the third (final) exposure results in the nearly complete removal of any particles that were transferred to the contact surfaces of the shoe from the first and second exposures. This occurs regardless of the exposure sequence and regardless of which specific site was used for the third exposure. It is also clear that under the experimental conditions loosely, moderately and strongly held particles are affected similarly, without any detectable enrichment of the earlier exposures among the more tightly held particles.

This finding is significant in that it fails to follow prior research focused on the persistence of trace evidence generally, and on footwear specifically, that strongly supports the hypothesis that, after transfer to an item, some particles are tightly held (and retained longer), while others are loosely held (and more rapidly lost). Contact surfaces of footwear, under the experimental conditions, are clearly an exception. Given that prior research has shown, in comparable studies, that a generalized sampling of footwear soles (from both contact and recessed areas) shows the retention of particles from earlier contacts, the clear implication of the present research is that, although particles on the contact surfaces of footwear are removed and replaced, those that are present on the more recessed areas of the sole are not.

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The results of this project have important implications for guiding follow-on research, notably: (1) research on differential sampling of footwear should continue, focusing on the difference between particle populations found on contact surfaces and those found on recessed areas, and (2) research on related computational and statistical methods to objectively interpret mixtures of particles should continue, focusing on multivariate methods that take advantage of both qualitative and quantitative distinctions among traces and possible sources. This resource was prepared by the author(s) using Federal funds provided by the U.S.

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Article posted March 27, 2019