An Examination of X-Ray Fluorescence (XRF) and X-Ray Diffraction (XRD) Analysis of Soils and the Transfer Issue.

Soil is a common form of trace evidence found adhering to almost everything, including people, vehicles, and physical evidence items. The ability to compare soil samples from a crime scene to a suspect’s property, or effects, in order to determine if there is a relationship between them can prove extremely valuable. However, there are several unanswered questions about how the properties of a soil change when material is transferred from the ground to a carrier such as automobile tires, shoe treads, or onto clothing as mud spatter. The goal of this ongoing project is to systematically employ x-ray diffraction (XRD) and x-ray fluorescence (XRF) analysis in order to assess their suitability for linking transferred soil material to their possible sources. Powder XRD is commonly utilized as a powerful forensic tool for identification of crystalline materials, including differentiating components of mixtures. XRD is used to determine the bulk mineralogy of soil and to see if, and how, the mineral profile changes when soil transferred. XRF is used to determine elemental composition. A major limitation of XRF is that the most common elemental constituents of minerals (ex. silicon, oxygen, aluminum and magnesium) are not detectable. However, heavier elements (>K) can be readily detected, some with great sensitivity. This suggests that it may be possible to differentiate geologic materials based on their trace element content. Both of these techniques are non-destructive and can be employed with little or no sample preparation. It is often possible to analyze transferred material in situ on clothing or other carriers. The question under investigation is whether using these techniques together will prove robust enough to be successful in linking transferred soil to its potential source. On a side note, preliminary research has demonstrated that this technique can be helpful in relating traces of mineral cosmetics (like eye shadow) on fabric to a bulk product.