The Polarized Light Microscope in Forensic Provenance Investigations.

In recent decades the polarized light microscope (PLM) has fallen out of favor with geologists, limiting its application in forensic soil analysis.  This is unfortunate, as the PLM is an instrument that is particularly well-suited to overcoming the problems typically encountered in forensic scenarios.  Analysis by PLM is non-destructive in nature, preserving evidence for future analysis by other experts.  It is ideal for analyzing the very small sample sizes often encountered in forensic scenarios.  It is an instrument that is extremely versatile; the PLM can be used to identify naturally occurring organic and inorganic soil components along with anthropogenic substances.

The PLM can be used to determine the source rock type of a sample of unconsolidated mineral grains, either based on the mineral assemblage present in the sample or on properties observed for individual mineral or lithic grains.  Biological particles including pollen, spores, diatoms and botanical macerals can be identified and used to constrain the possible source of a sample.  In addition, individual particles observed in the sample (geologic, biological, or anthropogenic) can be isolated and re-mounted for subsequent analyses that can be useful for provenance determination.  Examples used in casework by Stoney Forensic, Inc. include qualitative chemical analysis by energy dispersive X-ray spectroscopy; surface texture analysis and high resolution imaging by scanning electron microscopy; quantitative chemical analysis by electron microprobe analysis; structural analysis by X-ray diffraction; isotopic dating (a number of techniques are available); and stable isotope analysis.  Collaboration with one or more experts is typically necessary in order to interpret the significance of the data obtained.

Several casework and literature examples will be provided to illustrate the various capabilities of the PLM and its contributions to forensic soil analysis, specifically for provenance determination.