Date of Degree

9-2021

Document Type

Dissertation

Degree Name

Ph.D.

Program

Criminal Justice

Advisor

Thomas A. Kubic

Committee Members

Nicholas D. K. Petraco

John R. Lombardi

Gary McPherson

Subject Categories

Analytical Chemistry | Forensic Science and Technology | Soil Science

Keywords

forensic science, forensic soil analysis, scanning electron microscopy, SEM-EDS, minerals, clays

Abstract

Soil, that complex mixture of minerals, organic particles, chemicals, and anthropogenic materials, is ubiquitous and easily transferred, which gives it forensic relevance. Forensic soil analysis has traditionally included the identification and comparison of both organic and inorganic components, the inorganic portion being most commonly examined by light microscopy and other instrumental techniques. Predominantly, the inorganic materials in soil are naturally-occurring minerals, contributed as grains from the surrounding bedrock, which gives a soil its regional character. That parent material then undergoes physical, chemical, and anthropogenic changes producing wide geographic variation within a single region. This variation forms the basis for potential forensic comparison and inclusion/exclusion decisions.

Although the forensic value of soil analysis has been repeatedly demonstrated for over 100 years, the comparison of evidentiary soil samples languishes in the modern forensic laboratory. This underutilization is due, in part, to the dangerous and time-consuming nature of preparing a collected sample for analysis by careful separation and chemical digestion of the organic components, but also to the difficulty in maintaining qualified forensic microscopists. In addition to these pragmatic concerns, the field faces substantial difficulty, due to the diversity of soil types and evidentiary types, when attempting to meet modern legal and scientific demands for standardized methods and numerical results. A simplified examination scheme, incorporating easily duplicated preparation and mounting steps, automated analysis of individual mineral grains, and statistical approaches for reporting results, could provide an accessible method for publicly-funded laboratories to screen samples before seeking out a forensic soil analyst.

This research is a preliminary step towards realizing the potential of automated elemental analysis of individual soil particles for criminal justice applications. A protocol was developed that would allow for low-cost, low-risk, practical sample preparation and mounting of mineral grains for SEM-EDS analysis without the use of resin embedding techniques or polishing. In the process of that development, the nature of commercially-available “known” minerals was investigated, the destructiveness of plasma cleaning assessed, and a variety of mounting methods evaluated. Parameters were developed for fast, acceptably accurate automatic SEM identification of particles and the EDS spectral acquisition from those particles. The limits of a software package for classifying the results of that automated search and analysis were determined; upon determining the shortcomings of the instrument’s current software, the research was expanded to include the creation of an R program for the classification of mineral grains, based on their EDS spectra. The results were analyzed comparatively and through the use of multivariate chemometrics. Avenues for further research are also presented.

The successful classification results presented in this research suggest that automated SEM-EDS analysis of inorganic soil particles down to the clay size range (50m) may, in the future, be a useful and widely accessible tool for the screening analysis of otherwise neglected soil evidence. The presented technique provides some quantitative, statistical reference for the individuality of soil samples, which serves as a foundation for both a defense against a Daubert challenge and possible future databasing of soils.

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