Soil traces in forensic practise
Authors: Kotrlý Marek 
Published: December 2016

Introduction:

Requirements for analysis of relicts of soils, abrasions, or contamination of clothing, footwear, human body, vehicle, tools, etc., are relatively frequently encountered in forensic practice. There are two basic types of tasks ‐ We can compare these traces with reference samples of soils from places where the traces may have occurred, or a more complicated task ‐forecasting an unknown location of the origin of a pedological trace. In both cases, it is a relatively complex examination involving a wide range of individual investigations, whereas to a forensic lab can be submitted practically any material produced by activities of a human being and nature. Therefore, an organic part, pollen, relicts of soil microorganism shells, etc. are assessed separately within the analysis of pedological traces

Methodology:

Basic techniques are optical light microscopy in transmitted and reflected light, polarization, fluorescence, next are utilised SEM/EDS (WDS), XRF, FTIR, Raman spektroscopy, XRD, micro XRD, etc.

Technique of cathode luminescence (CL) capable of distinguishing material based on its genesis, thus usually also according to a different place of occurrence, were introduced to differentiate mineralogical phases of practically identical chemical composition, optical characteristics, structure and similar inclusions.

Biological material frequently present in the traces is analysed separately, plant and animal relicts are analysed, including microscopic shells and relics.

Anthropogenic material (glass fragments, slag, construction materials, etc.) is analysed separately by other techniques and may increase the probability measure of a match (hit) between traces and reference samples.

For forecasting of unknown locations, where the contamination by soil may have occurred, systems of geographic information (GIS) are used, in which data are connected with detailed geological maps, both uncovered and covered, and with pedological maps.

The complex of methodologies is based on mineralogical and physical‐chemical methods better embracing particularities of soil composition than some applied procedures arising from purely chemical base. Nevertheless, the introduced system at the same time is not omitting even biological and anthropogenic materials that usually represent a very important comparative material. Genetic methods that could compare both information from plant fragments and soil microorganisms are a big potential into the future.

Most of the analytical methods described yield quantitative data for data comparison. One of the possibilities bringing to soil analysis quantitative data that can be used e.g. for statistical comparison of traces and reference samples among each other are systems of automatic analysis of mineral grains (based on SEM/EDS). These systems have been available at the market for several years, nevertheless, to date samples of mineral grains recovered from soil traces have been prepared into casting compounds and then have been cut into polished section forms. Only then were samples analysed in systems of automatic mineralogical analysis. Our laboratory for the first time carried out in cooperation with the producer a number of detailed experiments with analysis of samples prepared directly from mineral grains of a soil trace, fixed at an adhesive stub with carbon target for SEM in diameter of 30mm. The experiments were at first conducted with model samples prepared from known mineral phases and then with samples from common soils. Several series of random samples were taken in a final stage from different places and the ability to determine differences among individual sampling points was tested. It was found that the differences are traceable also among sampling sites that were at a distance less than 20m. This technology, of course, is not a panacea, but appropriately compliments a complex analysis of soil phases.

Summary:

The use of new methods considerably extends possibilities of typical microscopic procedures in a forensic field and allows to obtain the needed quantitative data in a forensic analysis of pedological phases, differentiation of analogous mineral phases, or the option of analysis of the organic phase directly in SEM chamber.

The technology described suitably supplements the complex analysis of soil phases.