The transfer of diatoms from freshwater to footwear materials: An experimental study assessing transfer, persistence, and extraction methods for forensic reconstruction

Abstract

In recent years there has been growing interest in environmental forms of trace evidence, and ecological trace evidence collected from footwear has proved valuable within casework. Simultaneously, there has been growing awareness of the need for empirical experimentation to underpin forensic inferences. Diatoms are unicellular algae, and each cell (or 'frustule') consists of two valves which are made of silica, a robust material that favours their preservation both in sediments and within forensic scenarios. A series of experiments were carried out to investigate the transfer and persistence of diatoms upon common footwear materials, a recipient surface that has historically been overlooked by studies of persistence. The effectiveness of two novel extraction techniques (jet rinsing, and heating and agitation with distilled water) was compared to the established extraction technique of hydrogen peroxide digestion, for a suite of five common footwear materials: canvas, leather, and 'suede' (representing upper materials), and rubber and polyurethane (representing sole materials). It was observed that the novel extraction technique of heating and agitation with distilled water did not extract fewer diatom valves, or cause increased fragmentation of valves, when compared to peroxide digestion, suggesting that the method may be viable where potentially hazardous chemical reactions may be encountered with the peroxide digestion method. Valves could be extracted from all five footwear materials after 3min of immersion, and more valves were extracted from the rougher, woven upper materials than the smoother sole materials. Canvas yielded the most valves (a mean of 2511/cm) and polyurethane the fewest (a mean of 15/cm). The persistence of diatoms on the three upper materials was addressed with a preliminary pilot investigation, with ten intervals sampled between 0 and 168h. Valves were seen to persist in detectable quantities after 168h on all three upper materials. However, some samples produced slides with no valves, and the earliest time after which no diatom valves were found was 4h after the transfer. Analysis of the particle size distributions over time, by image analysis, suggests that the retention of diatoms may be size-selective; after 168h, no particles larger than 200μm could be found on the samples of canvas, and >95% of the particles on the samples of suede were less than or equal to 200μm. A pilot investigation into the effects of immersion interval was carried out upon samples of canvas. Greater numbers of valves were extracted from the samples with longer immersion intervals, but even after 30s, >500 valves could be recovered per cm, suggesting that footwear may be sampled for diatoms even if the contact with a water body may have been brief. These findings indicate that, if the variability within and between experimental runs can be addressed, there is significant potential for diatoms to be incorporated into the trace analysis of footwear and assist forensic reconstructions.

Freshwater diatom transfer to clothing: Spatial and temporal influences on trace evidence in forensic reconstructions

“…Diatom analysis used in this manner is relatively recent, with the potential to offer valuable environmental and circumstantial intelligence during crime reconstructions. A limited number of empirical research studies have previously considered diatom transfer from freshwater and terrestrial environments onto clothing and footwear materials, and the methods appropriate for their collection and analysis [15,[20][21][22]. Despite this initial focus, additional research is required to investigate the transfer and persistence dynamics of diatoms in experimental scenarios pertinent to forensic casework.…”

“…An initial study by Scott et al [15], found that diatoms transferred to new/used 100% cotton clothing surfaces following different periods of contact (3 minutes to 24 hours) with freshwater and soil environments. Furthermore, Levin et al [21] examined freshwater diatom transfer to five different footwear materials (soles and uppers) following three immersion intervals (30 seconds to 30 minutes). Though both studies identified the diatoms present by morphology or by genus, neither considered the potential influence of particulate variability on diatom transfer to clothing.…”

“…technique when approaching different types of clothing garments. Future research may wish to test a more diverse range of trace evidence collection procedures for the recovery of diatoms from clothing [11,21], particularly in relation to the assessment of larger transfer surface areas.…”

“…The exclusionary assessment of trace materials transferred between crime scenes and persons or items of forensic interest, often includes environmental indicators as diverse as soil mineral grains [12,13], algae [14,15], pollen [16,17], and microbial communities [18,19]. Although most published studies in forensic ecology focus upon terrestrial crime scene environments, the value of aquatic trace evidence, particularly diatoms, is increasingly being articulated [15,[20][21][22].…”

Crime reconstruction and the role of trace materials from crime scene to court

“…Empirical studies addressing the transfer and persistence of trace materials have developed since the early work of Pounds and Smalldon () (fibers), Brewster, Thorpe, Gettinby, and Caddy () and Hicks, Vanina, and Margot () (glass), and Morgan et al () and Bull, Morgan, Sagovsky, and Hughes () (general particulates). There is now a growing body of published work that addresses a wide range of different physical traces (Bitter, (smoke residues), Maitre et al, (GSRs), Palmer, Sheridan, Puckett, Richardson, & Lo, and Slot et al, (fibers), Levin, Morgan, Scott, & Jones, (diatoms), Morgan, Scott, Ainley, & Bull, (soils) Morgan, Flynn, et al, , and Morgan, Allen, King, & Bull, (pollen)).…”