INVESTIGATION OF DIFFERENCES IN ELEMENTAL COMPOSITION BETWEEN SOILS FROM DIFFERENT LOCATIONS WITH SIMILAR LITHOLOGIES: FORENSIC APPLICATIONS

This work was performed in the context of forensic analysis of soil evidence for the purpose of associating (or excluding) soil found on a suspect’s person or possessions with (or from) the soil at a crime scene or alibi location. The elemental analysis methods presented here are also applicable to geology, archaeology, agriculture and environmental science.

When a soil comparison is performed and two soil samples are associated, the significance of this association is important as well. Could similar soils from different locations also be associated (false positive association)? Studies have shown that spatial heterogeneity in elemental composition (and a number of other characteristics commonly employed in forensic soil analysis) increases with distance and varies from region to region. Another approach to this question is to examine the compositional differences from different regions within a particular lithological class.

Archival surface (0-5 cm) samples from the USGS Soil Geochemical Landscapes of the Conterminous United States study were used. Regions with similar primary and secondary lithologies (parent rock types) were identified using data from the USGS Mineral Resource Program. Sub-samples of approximately 0.5 g were spiked with internal standard, homogenized, and pressed into pellets. Pellets were analyzed using both laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) and laser-induced breakdown spectroscopy (LIBS). Both techniques eliminate the need for acid extraction or digestion protocols. LA-ICP-MS offers high sensitivity and precision. LIBS offers low cost screening, field-portable capabilities, and higher sensitivity than XRF in the low mass range.

The differences in elemental composition between different regions with similar lithology are reported with both LA-ICP-MS and LIBS data using uni- and multivariate statistical and data reduction methods such as analysis of variance (ANOVA) and principal components analysis (PCA). Similarities within lithologies are contrasted with stark differences between samples of different lithologies, even for those samples originating at close geographic proximity.