Forensic Comparison of Soil Samples

Abstract

As a preliminary experiment to test the discriminating ability of forensic soil analysis techniques and obtain area-specific information, soil samples were collected from eight areas near the eastern branch of the National Forensic Service (NFS) located in Gangwondo, an eastern province of South Korea. The soil samples were collected from five spots within each sample area using a small-scale (1 m²) soil sampling technique; for each of these five spots, two samples were collected from two places in each spot, (i) one from the surface and (ii) another from 30 cm below the surface. For each sample, the color of the sample with particle size in the range 53–500 μm and the major constituents were determined using a spectrophotometer and X-ray fluorescence spectrometer (XRF), respectively. The carbon content and carbon isotope ratio of the part of the sample of particle size below 53 μm were measured using an element analyzer-isotope ratio mass spectrometer (EA-IRMS). The canonical discriminant and XRF analyses showed an excellent color discriminating ability of 87.5 % and 88.8 %, respectively, with respect to the major constituents. The EA-IRMS results showed that the soils obtained from a 30-cm depth below the surface were generally more enriched in δ¹³C (0/00) than the surface soils, and that the surface soils contained a higher carbon amount (%). The canonical discriminant analysis confirmed 100 % discriminating ability when all three soil characteristics (i.e., color, composition, and content) were used in the analysis. Out of the two functions obtained from the analysis, Function 1 exhibited greater potential for explaining the SiO₂, Fe₂O₃, and TiO₂; thus, Area 6 and 7 could be more easily differentiated than the other areas using this function. Function 2 exhibited greater potential for explaining color factor b* (δ¹³C and C content), and could more efficiently differentiate Area 2 and 5. However, different results were obtained within the same area based on the soil depth. Therefore, when performing a comparative sampling analysis in forensic science, due care should be taken to prevent the mixing of adjacent soils from various depths. Better results can be achieved by collecting soil samples from different spots within the same area.