Gunshot residue (GSR): Frequency of residue types encountered in case work and background levels on control samples

The evaluation of criminal cases involving the discharge of a firearm requires reliable and up to date information regarding the transfer and persistence of gunshot residue (GSR). Similarly, knowledge of background levels of GSR on relevant populations and awareness of the potential for contamination/secondary transfer is essential. In this paper we build on previous work published by this laboratory and provide an update on the frequency of gunshot residue types in discharged cartridge casings (DCC) encountered in casework within the Republic of Ireland. In conjunction, an examination of the types of firearms encountered in casework and the associated residue types is undertaken. Finally, a review of levels of GSR particles detected on control samples taken from members of An Garda Síochána, the Irish police is detailed. Control samples are taken before a police officer samples a detainee suspected of involvement in an incident where a firearm was discharged and/or subsequently handled.

7.65 mm to support selected aspects of casework in North Macedonia

The subject of this research was the inorganic gunshot residue component collected from shooting patterns obtained on woven cotton cloth using a Pietro Beretta model 70 pistol, cal. 7.65 mm and Serbian ammunition for the following muzzle-to-target distances: 25, 50, 75, 100 and 125 cm. For each distance, three rounds of shooting were performed. Particles were lifted within a 10 cm radius of the projectile entrance and automatically analyzed using a scanning electron microscope coupled with an energy dispersion X-ray spectrometry. The obtained data on the populations of particles were analyzed taking into account their numbers, chemical classes and sizes. The results showed an apparent maximum incidence within all particles containing barium at about 50 cm distance. Also, lead particles revealed a distinct behaviour, being dominant at a 25 cm distance, falling below the other chemical classes, and finally becoming dominant again at 125 cm. The analysis of the frequency of occurrence of particles sorted according to their sizes confirmed that the small particle population is the largest, and their distribution in function of the equivalent circle diameter is exponential-like. The obtained results provided knowledge on the distribution of particles in the vicinity of the tested firearm and ammunition cal. 7.65 mm which generally corroborates with similarly studied GSR distributions obtained for the use of pistols cal. 9 mm. This information, together with the examinations of gunshot damages and other types of residues such as soot or unburned propellant grains may support qualitative inferences on shooting distance estimation, especially in cases, when the firearm and cartridges are not available to perform test shooting. In such cases even roughly estimated shooting distance can be helpful, e.g. for confirming or excluding the possibility of self-inflicted injuries or suicide and infer on the mutual position of the shooting stage actors. An example of casework that illustrates intermediate shooting distance estimation is presented.

Comparison of four commercial solid-phase micro-extraction (SPME) fibres for the headspace characterisation and profiling of gunshot exhausts in spent cartridge casings

Headspace solid-phase micro-extraction (SPME) is a promising technique for the characterisation and profiling of gunshot exhausts in spent cartridge casings, especially for health and environmental risk assessments, as well as forensic purposes. To date, however, no comprehensive investigation has been carried out to objectively assess the kinds of compound released during a discharge that can be recovered by this approach, the selectivity of the main commercially available fibres, and their relative performances for the analysis of gunshot exhausts and the discrimination of different ammunition types. This study aimed to fill this gap. Gunshot exhausts in spent cartridge casings from four different ammunition types were analysed by GC–MS, after extraction with four different commercial fibres: 100 μm polydimethylsiloxane (PDMS), 85 μm polyacrylate (PA), 65 μm polydimethylsiloxane/divinylbenzene (DVB), and 85 μm carboxen/polydimethylsiloxane (CAR). Results showed that, overall, a total of 120 analytes could be observed across the cartridges, but the different tested fibres also displayed distinct performances, which were, to some extent, complementary for the characterisation of gunshot exhausts. DVB, in particular, recovered the most compounds simultaneously. On the other hand, the observed variability between measurements was also high, making it a poor candidate for (semi-)quantitative applications (e.g. estimation of time since discharge and/or source profiling). In this regard, PA demonstrated the highest potential for broad use and implementation in multi-purpose methods.

Objectifying evidence evaluation for gunshot residue comparisons using machine learning on criminal case data

Comparative gunshot residue analysis addresses relevant forensic questions such as 'did suspect X fire shot Y?'. More formally, it weighs the evidence for hypotheses of the form H₁: gunshot residue particles found on suspect's hands are from the same source as the gunshot residue particles found on the crime scene and H₂: two sets of particles are from different sources. Currently, experts perform this analysis by evaluating the elemental composition of the particles using their knowledge and experience. The aim of this study is to construct a likelihood-ratio (LR) system based on representative data. Such an LR system can support the expert by making the interpretation of the results of electron microscopy analysis more empirically grounded. In this study we chose statistical models from the machine learning literature as candidates to construct this system, as these models have been shown to work well for large and high-dimensional datasets. Using a subsequent calibration step ensured that the system outputs well-calibrated LRs. The system is developed and validated on casework data and an additional validation step is performed on an independent dataset of cartridge data. The results show that the system performs well on both datasets. We discuss future work needed before the method can be implemented in casework.

The use and understanding of forensic reports by judicial actors-The field of gunshot residue expertise as an example

A comprehensive review was conducted on 42 closed court cases for which at least one gunshot residue (GSR) expertise had been requested. The aim of this study is to examine the use and understanding of the GSR findings by criminal justice professionals, the contribution to/relationship with other (forensic) elements of the case, and ultimately to assess the place of the GSR expertise in judicial decisions. The study shows that, in the vast majority of cases, the court appeared to interpret and use the GSR findings correctly, although some minor misuses were identified, mainly when the court incorrectly mentioned the expert's words, using the prosecutor's fallacy. In the end, a higher percentage of conviction rate (+23% points) was observed when incriminating GSR findings were obtained. Surprisingly, in half of the cases examined in which a guilty verdict was reached, GSR evidence was the only forensic evidence in the file.

Simple gunshot residue analyses for estimating firing distance: Investigation with four types of fabrics

This work presents two simple methods for estimating the firing distance from the gunshot residues (GSRs) on fabric targets. Four types of fabric targets, namely twill weave denim cotton-polyester (80/20), jersey knitting 100% cotton, plain weave cotton-polyester (80/20) and plain weave cotton-polyester (60/40), were employed. The firing tests were carried out using these white fabrics as targets at distances of 5-100 cm, respectively. In the first method, digital images of the black GSRs on fabric materials were recorded inside an illuminated box and the inverted gray intensity values were plotted against the firing distances. Since the plots of all fabrics are not significantly different, the estimation of firing distance employs the same exponential curve for all test fabrics. Although simple, the imaging method is not suitable for dark-colored materials. A chemical-based method was therefore developed as an alternative method. In the second method, a small disposable microfluidic paper-based analytical device (μPAD) was employed for detecting Pb(II) extracted from the GSRs. The μPAD method uses the measurement of the length of a narrow band of a pink color resulting from reaction between rhodizonate reagent and the Pb(II) extract. The plots indicated that the data of thick denim material are significantly different to other test fabrics which are much thinner. These three fabrics share the same estimation curve. However, it is recommended that the separate estimation curve for denim materials must be used. Both methods are suitable for short range firing distance, no further than 60 cm, since at greater distances the inverted gray intensity and the 'band-length' methods are unable to detect the GSRs.

Case work experience and experimental evidences

According to the active ASTM E1588-20 Standard Practice for Gunshot Residue (GSR) Analysis, particles from lead-based primers classified as "characteristic of GSR" will have the chemical composition lead/antimony/barium. Further elements allowed to be incorporated into GSR are explicitly listed in the ASTM guideline. Fluorine is not considered a possible additional element as no common sources of F in shooting related activities have ever been documented. Moreover, presence of fluorine was demonstrated in GSR-similar particles produced by airbag deployments and the possibility to use F as a chemical marker to exclude any discharging of a firearm was consequently suggested. In authors' case work experience, fluorine containing particles were found on stubs collected from victims' clothes, discharged firearms and shooters' hands. Adopting a "case by case" approach, a firearm-related fluorine origin was then sought. Fluorine-based protective lubricants, used both for guns and ammunition components, were experimentally confirmed as a possible source of F in GSR.

Lead toxicity in Lucilia cuprina and electrochemical analysis: a simple and low-cost alternative for forensic investigation

Entomotoxicology allows the detection and analysis of substances such as poisons, drugs, and metals in necrophagous insects using analytical protocols. In a forensic situation related to death by gunshot, the gunshot residue (GSR) is dispersed at the crime scene and may be consumed by necrophagous insects. Lead (Pb) is the most abundant metal in GSR samples and it can be determined using non-portable methods. However, the toxicity effects of GSR samples on Lucilia cuprina (Diptera: Calliphoridae) and the detection of Pb via portable electrochemical methods have not been investigated. This study describes for the first time the toxicity analysis of Pb on immature L. cuprina through their survival rate and influence of Pb on immature development. In addition, the bioaccumulation of Pb in the larvae samples was determined based on square wave anodic stripping voltammetry (SWASV) measurements. The results revealed a low limit of detection to Pb (6.5 μg L^-1) and the analytical performance was satisfactory because it measures Pb levels in larvae exposed to a diet containing 50 μg Pb g^-1. Furthermore, the levels of Pb influenced the survival rate and development time of the immature L. cuprina. Larvae exposed to a high concentration of the metal (50 μg Pb g ^-1) showed statistically significant changes (p < 0.05). The presence of Pb in immature L. cuprina can be used to estimate the post-mortem interval; thus, the present study provides important information in forensic entomology.

Advances and limitations in the determination and assessment of gunshot residue in the environment

Gunshot residue (GSR) stemming from the discharge of firearms has been essential to advancements in the field of forensic science however the human and environmental health impacts from GSR are far less researched. GSR represents a multifaceted concern: it contains a complex mixture of inorganic and organic components and produces airborne particles with variable sizes, depositions, and fates. Herein we evaluate studies in the literature examining GSR collection, deposition, composition, environmental contamination, and potential remediation techniques within the last two decades (2000 - 2020). Throughout we reflect upon key findings and weaknesses in relation to environmental characterization of GSR and associated firearm contaminants. Research focused on techniques to analyze both inorganic and organic GSR simultaneously has begun, but requires additional effort. A vast majority of the available environmental characterization literature focuses on soil contamination at outdoor firing ranges for a select number of elements (Cu, Pb, Sb) with comparisons between ranges or at different collection distances and depths. There is limited ability for between study comparisons due to collection and analysis differences as well as a lack of background soil sampling. Notably, these studies lack direct quantification of the contribution of contaminants from GSR as well as analysis of organic compounds. Currently, there is a need for air monitoring to determine the composition, deposition, and fate of GSR, particularly in outdoor settings. This review summarizes the collection, characterization, and environmental studies related to GSR and highlights areas of research needed to establish the environmental health impacts.

Interpol review of gunshot residue 2016-2019

This review paper covers the forensic-relevant literature in gunshot residue analysis from 2016 to 2019 as a part of the 19th Interpol International Forensic Science Managers Symposium. The review papers are also available at the Interpol website at: https://www.interpol.int/content/download/14458/file/Interpol%20Review%20Papers%202019.pdf.

Simultaneous determination of lead and antimony in gunshot residue using a 3D-printed platform working as sampler and sensor

3D-printing is an emerging technique that enables the fast prototyping of multiple-use devices. Herein we report the fabrication of a 3D-printed graphene/polylactic acid (G-PLA) conductive electrode that works as a sampler and a voltammetric sensor of metals in gunshot residue (GSR) using a commercially-available G/-PLA filament. The 3D-printed surface was used as swab to collect GSR and next submitted to a square-wave voltammetric scan for the simultaneous detection of Pb²⁺ and Sb³⁺. The proposed sensor presented excellent analytical performance, with limit of detection values of 0.5 and 1.8 μg L^-1 to Pb²⁺ and Sb³⁺, respectively, and linear ranges between 50 and 1500 μg L^-1. Sampling was performed through the direct contact of G-PLA electrode in hands and clothes of shooters, followed by immersion in the electrochemical cell in the presence of supporting electrolyte for the SWASV scan. The proposed method showed a great performance in the recovery, identification and semi-quantification of Pb²⁺ and Sb³⁺ in the evaluated samples without the need for sample preparation. Moreover, the device can be reused as sampler and sensor (until three times without loss of electrochemical performance) and the fabrication is reproducible (RSD = 7%, for three different devices). Hence, this 3D-printed material is an excellent candidate for the analysis of GSR, an indispensable analysis in the forensic field.

Technical note: presence of gunshot residue in and around a police station

Previous studies on the transference of gunshot residue (GSR) have shown that GSR can be transferred to surfaces through everyday activities and can persist on surfaces. Being that all police departments operate differently and have different spaces, GSR can be transferred and accumulates in different areas. Samples were collected from persons and surfaces in and around the Scranton Police Department and tested by scanning electron microscopy to identify GSR. Surfaces included police car seats, gun holsters, clothing around holsters, and belts around holsters. The results of the study showed that of the 25 samples collected, 40% contained at least one particle that was "characteristic of primer GSR", 64% contained at least one particle that was "consistent with primer GSR", and 92% contained at least one particle considered "commonly associated with primer GSR". This research characterizes where GSR is transferred within and around the police department. This data can be used to implement cleaning procedures or methods for decontamination. This study continues to strengthen the body of knowledge surrounding transferring of GSR.

Time since last discharge of firearms and spent ammunition elements: state of the art and perspectives

The estimation of the time since last discharge of firearms or spent ammunition elements (e.g., casings) may provide crucial information in the investigation of a shooting incident and, eventually, the following trial. Herein, an exhaustive review of the methods described in the literature is reported, with the aim to evaluate their potential and limitations from a forensic perspective. This work, in particular, highlighted the fact that a number of investigations have been carried out in the field during the last century (with an especially high rate in the last 30 years), but the implementation of related procedures in forensic laboratories is still rare. The situation has been discussed and a series of propositions have been forwarded, in order to overcome challenges and facilitate the implementation of dating approaches in real casework.

Examination of gunshot residue arising from shotgun cartridges containing steel, bismuth or tungsten pellets

In Australia, shotguns feature in criminal activity and suicide shootings relatively frequently. However, compared to studies dealing with GSR originating from rifled firearms, those dealing with residues originating from shotguns are rare. The purpose of the research described in this article was to specifically characterize particle populations ejected from shotguns, and especially populations produced when ammunition containing steel shot was used, as it was expected that the high levels of iron likely to be produced during discharge may complicate GSR evidence evaluation. The expected high numbers of iron particles in GSR ejected from the muzzle of a shotgun during discharge of cartridges loaded with steel shot were not detected. The inevitable conclusion of that finding - that metal from the shot does not contribute to GSR ejected from the muzzle of a shotgun - was confirmed when practically no particles containing zinc were detected when cartridges loaded with zinc plated shot were fired. It is suggested that the shot-cup in the ammunition was very effective at preventing the formation of GSR particles from the shot. In contrast to those findings, high numbers of iron particles were detected in GSR ejected from the breech of a shotgun when ammunition loaded with steel shot was fired. However, when ammunition loaded with bismuth or tungsten shot was fired, the levels of tungsten and bismuth particles ejected were very low but high numbers of iron particles were still detected. It is suggested that the firing mechanism in the gun itself, rather than the ammunition used, is responsible for the abundance of iron detected in GSR ejected from the breech. Despite the presence of iron in shotgun GSR, typical primer-derived residues (containing any or all of the elements lead, barium and antimony) were detected in residues ejected from the muzzle and the breech therefore evidence evaluation is relatively straightforward.

Application of total X-Ray fluorescence to gunshot residue determination

Currently the great majority of the criminal acts have involved the use of firearms, for these reasons the evidences generate from these are one of the fundamental pillars of a forensic investigation. The firearm leaves evidence known as gunshot residue (GSR), which is principally composed of burnt and unburnt particles from the detonation, as well as fragments of the bullet, cartridge case, and the firearm. Gunshot residue (GSR) is produced when a firearm is discharged and large quantities of it can be transferred to an individual who has fired. SEM-EDX is the common technique used in the forensic laboratories, the analysis consists in detecting the particles and its elements. In this work we propose the use of X-ray Spectrometry by Total Reflection (TXRF) for the analysis of metals present in related samples in ballistic cases. The analysis was focused in the relationship of three elements present in GSR. A series of experiments with different persons firing gun of 9 mm was performed in a shooting range. Analytical XRFS signals corresponding to K line of Copper and L lines Barium and Lead were employed as the best discriminating variables. Machine Learning techniques, such as discriminant analysis, supported vector machines and partial least squares - discriminant analysis, enable the correct classification of all samples analyzed. A hundred samples were analyzed so far, this method has demonstrated a very high classification performance for detecting gunpowder residues in the skin.

Laser-Induced Breakdown Spectroscopy for the Rapid Characterization of Lead-Free Gunshot Residues

This study investigated the use of laser-induced breakdown spectroscopy (LIBS) and scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDX) as means of characterizing gunshot residue (GSR) originating from commercially available lead-free rounds. Data from two experiments are presented in this work. One experiment focused on identifying prominent analytical markers present in lead-free GSR by LIBS while the other applied SEM-EDX to determine the degree of evidence preservation after LIBS analysis. Samples of GSR were collected via tape-lift method from the hands of volunteer shooters and instrumental analyses were conducted in triplicate. As a result, the lead-free ammunition analyzed in this work generated GSRs comprising primarily Ba, Al, Si, and/or K. Trace amounts of Ti, Fe, and S were also apparent in some compositions. Through SEM-EDX analysis, a spheroidal geometry consistent with traditional lead-containing GSR was observed. Additionally, it was determined that evidence is preserved after LIBS analysis which supports the implementation of LIBS as a rapid preliminary screening method followed by confirmatory testing via SEM-EDX on the preserved evidence.

Gunshot residue and brakepads: Compositional and morphological considerations for forensic casework

Previous research has raised the possibility that automotive brake pads can produce particles that are both compositionally and morphologically similar to gunshot residue (GSR). These studies, published in the early 2000s, coincided with the reduction or removal of unnecessary sources of lead from the automotive industry. The question therefore arises whether modern brake pads might still be a relevant source of 'GSR-like' particles. In the present study, a total of 75 brake pads taken from a range of cars currently on the road in Adelaide, South Australia, were collected from service centres. These pads were screened by XRF, and 12 were selected as representatives of the types of pads available on the market. Some pads generated XRF results for elements particularly relevant to GSR analysis. Signals for barium and antimony were commonly observed, with some pads showing results possibly attributable to lead. The surfaces of these 12 pads were directly sampled using aluminium stubs equipped with carbon-filled adhesive tape. Following this, they were screened using SEM-EDS and an automated GSR particle search in order to detect particles containing lead, barium or antimony. No particles containing all of these elements were found, although a large number of particles containing both barium and antimony were located. Other particulate samples were collected using stubs from persons or objects associated with brakes in order to examine whether particles similar to GSR might be present on them. No three-component particles were detected in samples collected from the wheel rims or the hands of those exposed to automobiles. From this study of common, contemporary cars, brake pads and brake pad technicians, it was determined that the possibility of obtaining three-component 'GSR-like' particles from brake pads appears to be much lower now than when this issue was first raised in the early 2000s. While some brake pads do produce particles containing barium and antimony, they are often angular particles that contain sulphur. Furthermore, these 'GSR-like' particles are commonly found together with an abundance of particles containing iron. The particle evidence evaluated in total allows clear differentiation between residues originating from brakes and residues originating from firearms.

Gunshot residue contamination of the hands of police officers following start-of-shift handling of their firearm

If police officers are contaminated with gunshot residue (GSR) through the normal receiving, checking, loading, and securing of their issued firearm, there is the potential for secondary transfer of GSR to anyone those officers arrest during their shift. This 3-part study examined the level of GSR contamination of police officers following the start-of-shift handling of their standard issue firearm, the impact that hand-washing or the use of a self-drying hand-wash had on the level of GSR contamination, and the likelihood of officers re-contaminating their hands through contact with the exposed hand-grip of their holstered hand-gun. Almost 85% (28/33) of officers sampled had 3-component GSR particles on their hands immediately following the start-of-shift handling of their firearm. There was an average of 64 such particles over the 33 officers sampled. Of the 17 officers who washed their hands after securing their firearm, a single 3-component particle was recovered from the hands of one officer. GSR particles (maximum of 4) were recovered from 3 of the 14 officers who used self-drying hand gel following firearm handling. 3-component particles (maximum of 7) were recovered from the hand-grips of 12 of the 34 unissued handguns sampled.

Quantifying gunshot residues in cases of suicide: Implications for evaluation of suicides and criminal shootings

Gunshot residue (GSR) is a valuable form of forensic trace evidence in the investigation of firearms-related suicide and crime. In order to ensure that such evidence is given appropriate evidential weighting when it comes to case investigations, the dynamics of the deposition and distribution of GSR must be understood. As the activity of the subject after firing is a major factor in the retention of GSR, cases of suicide involving a firearm provide an opportunity to assess GSR distribution where post-shooting activity is not an issue, assuming the subject had not been moved. Seventy-one cases of suicide by gunshot in South Australia between 1998 and 2014 were examined to collect data on firearm type, calibre, and gunshot residue test results. Overall, 47% of cases were found to produce GSR results that were not of high probative value, even though it was known that the shooter had fired a firearm. Of particular interest were cases involving 0.22 rimfire ammunition, for which characteristic particles are not expected due to lack of antimony present in the primer. Despite this, 64% of these cases resulted in the detection of antimony-containing, three-component particles.

An assessment of firework particle persistence on the hands and related police force practices in relation to GSR evidence

In a previous study by Grima et al. Sci. Justice 52 (1) (2012) 49, it was shown that background particles can aid in the exclusion of firework particles which are indistinguishable from GSR. Issues relating to the persistence of such particle populations were presented. The scope of this project was to examine persistence on the hands in the context of possible post-display scene contamination and how this can affect GSR evidence, especially in light of possible GSR/firework mixtures. Persistence was investigated by recovering firework residues eight hours post-display following contact of the hands with bedding. In addition, particle profiling was carried out using SEM-EDX. Firework particle populations exhibited strong persistence in all displays, with not less than 667 particles persisting in each scenario. This factor challenges GSR evidence, especially if personnel stationed at display sites enter scenes of crime or come into contact with suspects after a display. A survey of UK police force practices following firework displays showed that authorities are not aware of the impact particle transfer may have on GSR evidence. Recommendations for the implementation of basic hygiene practices for particle transfer control have also been made.

Analysis of macroscopic gunshot residues by Raman spectroscopy to assess the weapon memory effect

Gunshot residues (GSR) are valuable evidence which provide the forensic analyst with useful information about a crime scene when proper analytical methods are used. Nowadays, the method of choice for analyzing GSR is scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM/EDX). However, SEM/EDX presents limitations when the GSR identification of "non-toxic" ammunition types is performed. To overcome this drawback, Raman spectroscopy has been recently proposed as a complementary technique to SEM/EDX. However, for the time being, it can only be used in a limited number of casework (e.g. examining the macroscopic GSR produced at short distance over victim's clothes) and further research to know when this technique could support SEM-EDX results is required. In the present work, the memory effect of the weapon, which plays an important role to link the GSR found and the ammunition fired, is studied. Twenty shots were fired at close distance (~30 cm) at paper targets using the same weapon with two different types of ammunition. The first, third, ninth, and twentieth shots were fired with the first ammunition and the shots among them using the second ammunition. The macroscopic GSR produced by the first ammunition were analyzed using Raman spectroscopy. First, the spectra obtained were visually differentiated by taking into account the band at about 1342 cm(-1). This first approach shown that in the first shot were no GSR particles from the second ammunition, but 1.5-7.5% of analyzed particles corresponded to the second ammunition in the third, ninth, and twentieth shots. Additionally, the same differentiation was then performed by discriminant analysis using the spectral range from 1800 to 800 cm(-1). Although using this second approach only one GSR was identified as the second ammunition, was remarkable that after the shots with different ammunition the GSR obtained shows greater variability. The results obtained suggest that the memory effect of the weapon has not a significant influence when the organic analysis of macroscopic GSR on targets by Raman spectroscopy is performed.