Forensic Comparison of Soil Samples Using Nondestructive Elemental Analysis

The smallest traces of crime: Trace elements in forensic science

BACKGROUND

Securing the evidence in various investigative situations is often associated with trace analysis, including fingerprints or blood groups. However, when classic and conventional methods fail, trace elements, such as copper, zinc, fluorine, and many others found in exceedingly insignificant amounts in organisms, may prove useful and effective.

METHODS

The presented work reviews articles published between 2003 and 2023, describing the use of trace elements and the analytical methods employed for their analysis in forensic medicine and related sciences.

RESULTS & CONCLUSION

Trace elements can be valuable as traces collected at crime scenes and during corpse examination, aiding in determining characteristics like the sex or age of the deceased. Additionally, trace elements levels in the body can serve as alcohol or drug poisoning markers. In traumatology, trace elements enable the identification of various instruments and the injuries caused by their use.

Mineralogical and elemental data for soil discriminating and geolocation tracing

One of the key tasks of soil analysis in forensic sciences is to provide information about its diversities and geolocation. In fact, soil analysis is relevant for forensic geologists. In this study, a total of 80 soil samples were collected from eight Chinese cities (10 samples per city). Different minerals and their relative percentages were analyzed by the X-ray diffraction (XRD) method. In addition, the relative amounts of montmorillonite, kaolinite, amphibole, feldspar, calcite, and dolomite provided information about the origin of a soil, either if it came from a northern or southern city of China. The oxide weight percentages of 10 elements of Al₂O₃, SiO_2, Fe₂O₃, K₂O, Na₂O, MgO, CaO, P₂O₅, MnO, and TiO₂ were also obtained by using X-ray fluorescence (XRF) from the 80 soil samples. Moreover, principal component analysis (PCA) and hierarchical clustering analysis (HCA) methods were performed for dimensionality reduction, elemental marker identification and soils classification to the city they came from purposes. The eighty soils analyzed in this study could be tracked correctly to their city of origin. The K-Nearest Neighbors (KNN) model was done to evaluate the prediction ability based on the soil elemental composition, and it was confirmed by cross validation methods. The results demonstrated that mineralogical and elemental composition can provide powerful information for soil discrimination and source tracing.

A case study in forensic soil comparison

Soil examination can provide useful forensic information about the spatial location and suspect's activities. Many techniques have been applied for soil comparison and provenance determination in criminal investigations. Pollen and diatom identification, which has the potential to provide an independent ecological assessment of soil evidence, is currently underused in forensic soil analysis. This work presents a case study of application of these methods to help criminal investigation in a murder case, which happened in an irrigation ditch in Hunan Province, southern China. Soils from the suspect's clothes, the exact crime scene spot in the irrigation ditch, along the ditch and the reference ditches were collected and analyzed. In addition to the element and mineral analysis, pollen and diatom assemblages were analyzed for further comparison. The statistical methods of hierarchical cluster and cosine similarity analysis were carried out to assist in soil comparison and provenance determination. The results showed that soil on the suspect's clothes had a high probability to share the same source with the soil from the crime scene in the irrigation ditch. The suspect confessed to murder based largely on the soil examination result even without other evidences.

Human-associated microbial populations as evidence in forensic casework

In forensic investigations involving human biological traces, cell type identification is often required. Identifying the cell type from which a human STR profile has originated can assist in verifying scenarios. Several techniques have been developed for this purpose, most of which focus on molecular characteristics of human cells. Here we present a microarray method focusing on the microbial populations that are associated with human cell material. A microarray with 863 probes targeting (sets of) species, specific genera, groups of genera or families was designed for this study and evaluated with samples from different body sites: hand, foot, groin, penis, vagina, mouth and faeces. In total 175 samples from healthy individuals were analysed. Next to human faeces, 15 feline and 15 canine faeces samples were also included. Both clustering and classification analysis were used for data analysis. Faecal and oral samples could clearly be distinguished from vaginal and skin samples, and also canine and feline faeces could be differentiated from human faeces. Some penis samples showed high similarity to vaginal samples, others to skin samples. Discriminating between skin samples from different skin sites proved to be challenging. As a proof of principle, twenty-one mock case samples were analysed with the microarray method. All mock case samples were clustered or classified within the correct main cluster/group. Only two of the mock case samples were assigned to the wrong sub-cluster/class; with classification one additional sample was classified within the wrong sub-class. Overall, the microarray method is a valuable addition to already existing cell typing techniques. Combining the results of microbial population analysis with for instance mRNA typing can increase the evidential value of a trace, since both techniques focus on independent targets within a sample.