Soil Forensic Examinations Using Advanced Laboratory Source and Synchrotron XRD Techniques.

Forensic soil science, as a newly developed discipline of soil science, has matured to the extent that well-defined questions and crime scene investigations are being addressed in increasingly refined ways to assist both law enforcement and environmental agencies.  Soils, regolith, minerals and man-made mineral particles such as bricks (soil-regolith materials) are being used in specialized forensic investigations to associate a soil-regolith sample taken from an item, such as shoes, clothing, shovel or vehicle, with a specific location. The majority of forensic cases involving soil-regolith materials are usually complex and the challenges of associating relevant information from one source with another often requires the application of new, sophisticated field and laboratory methods.

Forensic evidence is often available only as very small amounts of soil-regolith material, which may have trace amounts of mineral particles.  For example, the soil material may have: (i) small paint or plastic flecks containing traces of minerals such as rutile, (ii) small brick or burnt soil particles (<0.5mm) containing polycrystalline minerals such as cristobalite and mullite and (iii) unburnt and burnt bone fragments. Traditional X-ray diffraction (XRD) techniques using low-background Si wafer holders are useful for measuring XRD patterns from samples with weights as low as several milligrams. These techniques are generally too insensitive to measure XRD patterns from samples weighing less than a milligram. However, the mineralogy of small fragments (<0.5mm) can be determined with micro-XRD techniques using fine (submillimetre) monocapillary attachments on laboratory XRD instruments. While these instruments are adequate for determining dominant components, synchrotron XRD with high X-ray intensity provides far greater sensitivity and resolution than laboratory source XRD systems. This enables the identification of minute amounts of mineral components. Through case studies, this presentation will demonstrate how advanced field, laboratory and synchrotron approaches have been critical in developing coherent, predictive, soil-regolith models, from microscopic to landscape scales, to solve soil-regolith-based criminal and environmental investigations.