Assessment of the persistence and transference of inorganic gunshot residues

Author: Nicholas Lucas

Lucas, Nicholas, 2019 Assessment of the persistence and transference of inorganic gunshot residues, Flinders University, College of Science and Engineering

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The interpretation of gunshot residue (GSR) evidence presents a number of challenges in evaluating the significance of a particular finding. Therefore, to ensure that evidence is placed in the appropriate context, a systematic approach to reviewing the factors that influence the significance of a GSR finding must be taken. To assist this, a framework assessing the numerous factors related to the deposition and persistence of GSR may serve as a useful tool to aid interpretation. The research performed in this thesis assesses a number of factors pertinent to GSR evidence evaluation in South Australia, and explores incorporating them into an evidential assessment framework.

To inform this framework, the type and nature of firearms frequently encountered in forensic casework in the region was ascertained through a comprehensive case-review. The frequency with which 0.22LR ammunition was encountered prompted further investigation of the impact of the weapon memory effect, and how two-component primed ammunition, such as many 0.22LR ammunitions, can create three-component GSR particles. Additional investigation of GSR generated from heavy metal free (HMF) ammunitions also provides valuable context for GSR assessments in a changing ammunition market. This information may be used to better inform GSR analysts of what may be expected in different case circumstances.

As a further means of assessing the significance of a particular GSR finding, one particularly important factor is the possibility that GSR present on a suspect is due to contamination from an unrelated incident or source, rather than the incident under investigation. Contributing to this are factors such as the likelihood of cross-contamination from police and the background level of GSR in the random population. These factors inform the possibility that an individual may be mistakenly included in an investigation as a result of a false positive error. To investigate this, surveys of the background prevalence of GSR in the random Australian population were performed. Similarly, the prevalence of GSR particles present on the hands of police officers, and the possibility that these may be transferred to a suspect during the process of arrest was also surveyed.The results of these surveys were then assessed in the context of their contribution to an evidential framework that would allow for a greater understanding of the significance of a GSR test result. The calculated probabilities of observing GSR in each of the surveys was used to inform a Bayesian Network (BN) style approach to the assessment of GSR evidence. A BN approach is particularly useful in the assessment of complex evidence, as the network structure allows for the model to be adapted as case conditions changed. The experimental data was used to inform a section of this framework, targeted at assessing the probability that an individual unrelated to an investigation would return a positive GSR test result as a consequence of the GSR background, or transfer from police. The findings of this assessment may be combined with relevant case information to better inform evaluations of the significance of GSR casework results in South Australia.

Keywords: Forensic Science, Gunshot Residue, SEM-EDS, Bayesian Networks

Subject: Forensic & Analytical Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2019
School: College of Science and Engineering
Supervisor: Paul Kirkbride