Stoney David A, Bowen Andrew M, Ausdemore Madeline, Stoney Paul L, Neumann Cedric, Stoney Frederick P
Stoney Forensic, Inc., 14101-G Willard Road, Chantilly, VA, 20151-2934 USA.
Department of Mathematics and Statistics, South Dakota State University, Brookings, SD 57007 USA.
Forensic Sci Int. 2019 Mar;296:39-47. doi: 10.1016/j.forsciint.2018.12.020. Epub 2019 Jan 14.
It has previously been shown that a walk of 250 m (approximately 175 steps/shoe) in a new environment is sufficient to remove and replace particles present on the contact surfaces of footwear. However, it is unknown how quickly this replacement occurs. This paper describes experiments measuring how quickly (in terms of steps) this loss and replacement of VSP occurs. Three environmental exposure sites were chosen to have different, characteristic particle types (soil minerals). Footwear of two types (work boots and tennis shoes) were tested, accumulating particles by walking 250 m in one site, followed by walking a designated number of steps (per shoe) in a second site. Very small particles (VSP) were harvested from contact surfaces of the footwear (those surfaces in direct contact with the ground when walking) using a moist swabbing procedure. The resulting numbers and types of VSP were determined using forensic microscopy and the proportions of VSP attributable to the first and second site were determined by particle combination analysis using a latent Dirichlet allocation model. The principal findings from this study are (1) that the contact surfaces of footwear are dominated by VSP attributable to the most recent site of exposure, (2) that walking in a new location rapidly removes and replaces VSP from the contact surfaces of footwear, (3) major replacement occurs in 5 to 10 steps and nearly complete replacement occurs by 25 steps, (4) the character of the loading site may influence the initial rate of loss and replacement of VSP (during the first five steps), and (5) overall, the loss and replacement of VSP on footwear in these experiments can be reasonably described by the relationship 1/(Steps + 2.68). The significance of these findings is considerable. 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 necessary beginning for efforts to resolve mixtures of VSP arising from different exposures. The usefulness of VSP on footwear evidence depends on developing this ability so that a reliable, relevant evidentiary "signal" can be separated from background noise (or signals from other exposures).
此前已有研究表明，在新环境中行走250米（约每只鞋175步）足以清除并替换鞋类接触面上的颗粒。然而，这种替换发生的速度尚不清楚。本文描述了测量VSP这种损失和替换发生速度（以步数计）的实验。选择了三个环境暴露地点，其具有不同的特征性颗粒类型（土壤矿物质）。测试了两种类型的鞋（工作靴和网球鞋），先在一个地点行走250米以积累颗粒，然后在第二个地点行走指定数量的步数（每只鞋）。使用湿擦拭程序从鞋类的接触表面（行走时与地面直接接触的那些表面）采集非常小的颗粒（VSP）。使用法医显微镜确定所得VSP的数量和类型，并使用潜在狄利克雷分配模型通过颗粒组合分析确定归因于第一个和第二个地点的VSP比例。本研究的主要发现是：（1）鞋类的接触表面主要是归因于最近暴露地点的VSP；（2）在新地点行走会迅速清除并替换鞋类接触表面上的VSP；（3）主要替换发生在5至10步，到25步时几乎完全替换；（4）加载地点的特征可能会影响VSP损失和替换的初始速率（在前五步中）；（5）总体而言，这些实验中鞋类上VSP的损失和替换可以用关系1/（步数 + 2.68）合理描述。这些发现的意义重大。在最后访问的地点令人关注的情况下，鞋类接触表面的VSP将给出该地点几乎纯净的样本。在先前访问的地点令人关注的情况下，接触表面的VSP可作为背景信号，为解析来自不同暴露的VSP混合物的努力提供必要的起点。鞋类证据上VSP的有用性取决于开发这种能力，以便能够从背景噪声（或来自其他暴露的信号）中分离出可靠、相关的证据“信号”。
