A multivariate extension to the standard 4σ criterion for comparison of forensic glass evidence.

This manuscript presents a more accurate and easy to implement multivariate generalisation of the international standard 4σ forensic glass comparison technique. Many crimes result in glass breaking, and the broken glass found at a crime scene can be important forensic evidence. The chemical composition of this glass can be measured to establish whether it can be distinguished from glass fragments found on a suspect's clothing. The chemical composition can be measured using laser ablation-inductively coupled plasma mass spectrometry. A commonly used method to compare fragments of glass is the 4σ interval criterion. This method, however, compares each element individually and does not take advantage of the multivariate nature of this data. We introduce a multivariate extension to this method, which makes use of the correlation structure between the elements. We demonstrate that this method results in an improvement in the false positive rate, with only a small compromise in the false negative rate. The improvement in false positive rate is desirable as false positives translate to misleading evidence against a potentially innocent defendant. The multivariate generalisation improves accuracy while retaining a similar interpretation, and so is suitable to present in court.

Tandem detection of organic and inorganic gunshot residues using LC-MS and SEM-EDS.

Gunshot residue (GSR) is a valuable form of forensic trace evidence in the investigation of firearms crime. The current gold-standard approach does not include the analysis of organic components of the residues, which may be a deficiency, particularly in cases where there is little to no inorganic gunshot residue (IGSR) present or its attribution to a firearm source is ambiguous. A solvent extraction method was used for the extraction of organic GSR (OGSR) from the most common sampling device used to collect IGSR (i.e., SEM stubs with double-sided carbon adhesive tape). It was found that extraction did not significantly disturb inorganic GSR present on stubs, which raises the possibility that a valuable, comprehensive tandem analysis of both organic and inorganic GSR may be implemented using a single and commonly used residue collection device. The organic extract was analysed using Ultra High Performance Liquid Chromatography coupled to an Ion Trap Mass Spectrometer using Electrospray Ionisation (UHPLC-ESI-MS/MS), with preliminary results indicating that organic components can be extracted and detected at levels appropriate to casework GSR analysis. Testing of traces collected from the hands of recent shooters showed detection of stabiliser compounds typical of OGSR, which were confirmed to be present in the test ammunition's propellant. Total analysis time is approximately 30min per specimen, including preparation, instrumental analysis and data review. As the first step in the examination of GSR stubs in relation to a shooting case, extraction of organics and analysing them for the presence of OGSR may bring two operational benefits. First, that approach may be a useful way to determine which stubs warrant priority examination for IGSR, and second, it offers the possibility of providing relatively rapid case information to investigators.

A risk-based approach to cognitive bias in forensic science.

Over the past decade, the potential impact of cognitive bias in forensic science has instigated much discussion and debate between academics, scientists and those in the justice sector. Evidence of bias influencing subjective decision-making across a range of forensic disciplines has been described in the literature. Forensic service organisations are being urged to address cognitive bias in subjective decision-making by designing processes or procedures to limit access to (irrelevant) contextual information or reduce dependence on cognitive functions. Although some laboratories have implemented bias mitigating strategies, with varying impact on operational efficiency, there has been no systematic assessment of the risk posed by cognitive bias. Forensic Science SA assessed the potential impact of bias on forensic interpretations across multiple disciplines, using a risk management framework. This process proved useful in assessing the effectiveness of existing bias mitigating strategies and identified the latent level of risk posed. While all forensic organisations should seek to implement bias limiting measures that are simple, cost-effective and do not adversely impact efficiency, using a risk-based approach has contextualised the limited benefit of introducing resource hungry measures, as postulated in the literature. That is not to suggest that forensic organisations should dismiss the potential influence of cognitive bias but they need to strike an appropriate balance between risk and return, as they do with any business risk.

Methods for analysis of glass in glass-containing gunshot residue (gGSR) particles.

When lead, barium and antimony, or lead, barium, calcium, silicon and tin are found together in particles associated with a shooting investigation they are considered characteristic of gunshot residue (GSR). Antimony and tin are often absent from the primer of many low calibre rimfire ammunitions, which are the type most commonly used in Australia. Therefore, the likelihood of characteristic particles forming during the firing process of such rimfire ammunition is significantly less than the likelihood of these particles arising from higher calibre ammunition. The majority of rimfire ammunition examined in this research contains ground glass in the primer, which functions as a frictionator. These ammunitions produce a small number of gunshot residue particles containing glass coated with other primer components, which we refer to as glass-containing GSR (gGSR). If these particles are observed in an investigation, they have the potential to add a new dimension to gunshot residue analysis because they are not common in the environment. Furthermore, the composition of glass frictionator is stable during firing, which raises the possibility that chemical testing of the glass in gGSR may be used to identify the ammunition from which the residue was derived or to link deposits of GSR. This paper examines the application of scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS), focussed ion beam (FIB) techniques and time of flight-secondary ion mass spectrometry (ToF-SIMS) to the semi-quantitative analysis and comparisons of gGSR and frictionator extracted from unfired cartridges. SEM-EDS is effective for comparing gGSR with unfired frictionator, but the use of FIB to expose clean glass from the centre of gGSR followed by ToF-SIMS, or ToF-SIMS using ion sputtering to expose clean glass, offers more power for comparisons due to their capability for higher discrimination between frictionators from different sources.

Glass-containing gunshot residues and particles of industrial and occupational origins: Considerations for evaluating GSR traces.

In an ideal case, the value of traces would be determined numerically and presented through the use of likelihood ratios or verbal-equivalent scales. A problem in the evaluation of gunshot residue (GSR) evidence using these models is that in many shooting scenarios insufficient data exist to support a quantitative model of interpretation. The complex relationship that exists between ammunition composition and post-firing residues makes quantitative interpretation more difficult for GSR than for other traces such as glass. When evaluating the significance of traces in a quantitative model, the value of a trace is reduced as the number of random sources that could produce the trace increases. Previously published works have suggested that glass-containing GSR (gGSR), which is glass encrusted with lead (Pb) and barium (Ba) residues, are a new type of GSR not already classified under ASTM E1588 - 17. If random sources of particles resembling gGSR are rare, then gGSR may be valuable evidentiary traces. In order to potentially incorporate these particles into a future model, the general background prevalence of gGSR and specific sources capable of producing similar particles must be understood. Therefore, particles from fireworks, matches, and cartridge actuated nail guns were assessed on an individual basis and at a population level. These sources, known to produce particles resembling GSR, were assessed using backscattered electron - scanning electron microscopy - energy dispersive X-ray spectroscopy analysis (BSE-SEM-EDS) for the presence of glass-containing particles that resemble gGSR. In the experiments described in this article the nail gun produced particles compositionally indistinguishable from gGSR, due to the primer in the brand of nail gun cartridges used containing glass as the frictionator in addition to Pb and Ba compounds. In this study, no particles were located from fireworks or matches that were indistinguishable from gGSR, nor was any evidence observed or found in the literature that would suggest that such particles could be formed.

Analysis of elemental and isotopic variation in glass frictionators from 0.22 rimfire primers.

The majority of 0.22 calibre rimfire ammunition available in Australia, and overseas, tends to use glass powder rather than antimony sulfide frictionator in the primer. This glass can be the nucleus of a GSR particle, with other primer components condensing around and onto the glass structure. As the composition of glass frictionator remains largely unaltered during ammunition discharge [1] there is the possibility that frictionator composition could be used in GSR examinations to either correlate or discriminate between samples, thereby providing valuable information to an investigation. In this study, the composition of glass frictionator from a wide variety of ammunition was analysed by time-of-flight - secondary ion mass spectrometry (ToF-SIMS), sensitive high-resolution ion microprobe (SHRIMP) and scanning electron microscopy - energy dispersive X-ray spectrometry (SEM-EDS). Refractive index (RI) was measured using glass refractive index measurement (GRIM). Across the population of ammunition studied, it was found that the elemental and isotopic composition of frictionator varied. ToF-SIMS was able to discriminate 94.1% of brands in a pairwise comparison and SEM-EDS achieved a pairwise discrimination power of 79.4%. If SHRIMP was combined with the other two techniques, 95.6% of brands could be discriminated. Refractive index measurements supported the elemental data showing that there appeared, in most cases, to be only one population of glass within a cartridge. The results suggest that there is scope for frictionator analysis to contribute valuable, new capability to forensic GSR examinations.

A study into the distribution of gunshot residue particles in the random population.

When considering the impact and value of gunshot residues (GSR) as forensic trace evidence, the likelihood of a suspect producing a positive GSR analysis result without having direct exposure to a firearm is a major consideration. Therefore, the random prevalence of GSR and 'GSR-like' residues in the wider population is a highly pertinent question when considering the probative value of such evidence. The random prevalence of GSR in two Australian jurisdictions - Victoria and South Australia - was assessed through the collection and analysis of GSR samples obtained from randomly selected members of the public. Volunteers were asked to declare any firearms use, hobbies or potential firearms exposure before allowing their hands to be sampled using aluminium GSR sample stubs coated in adhesive tape. A total of 289 samples, 120 from Victoria and 169 from South Australia were collected and analysed using scanning electron microscopy coupled with energy dispersive X-ray microanalysis (SEM-EDS). Across all samples, three 'characteristic' three-component Pb/Ba/Sb particles were detected from a single subject in South Australia, corresponding to an overall prevalence of 0.3%. Two-component 'consistent' particles were more prevalent, with Pb/Sb particles being the most frequently occurring, in 8% of samples, and in South Australia only. A number of samples, approximately 7%, showed populations of single element particles of Pb, Ba and Sb, which has the potential to generate a false positive for GSR if using a bulk analysis technique such as NAA or AAS. The prevalence of GSR or 'GSR like' particles in this study matches closely with similar surveys conducted in other jurisdictions. Such surveys are a useful foundation for the creation of a probabilistic method for the assessment of GSR evidence.

Glass fragments from portable electronic devices: Implications for forensic examinations.

Personal electronic devices (PEDs) are now widespread in the community. Many such devices have glass display screens that, despite being a relatively strong and specialised material, are vulnerable to breakage. Unlike other glass objects that are usually thrown away when they break, PEDs can still function with a broken or cracked screen and it is not uncommon for their owners to keep using them in this condition. Broken PED screens, therefore, might represent a new and significant source of glass fragments that are present on the clothing and belongings of the general public and individuals suspected of offences involving the breaking of glass. The forensic implications of this new source of glass fragments in the community were investigated. PED glass is easily recognised using scanning electron microscopy-energy dispersive X-ray analysis and refractive index measurement and is easily distinguished from domestic and automotive soda-lime glass using these methods; as a consequence there should be no confusion of soda-lime glass fragments and PED glass fragments in forensic glass casework. In cases where the objective is to compare recovered glass fragments to a putative PED source, comparison using refractive index measurement and elemental analysis achieves good discrimination between sources.