A Study of Background Levels of Antimony,Barium, and Lead on Vehicle Surface Samples by Graphite Furnace Atomic Absorption

Law enforcement agencies routinely sample for gunshot residue (GSR) by bulk techniques and often submit swabs taken from other surfaces besides the hands of the suspect shooter. This study aims to establish the prevalence of antimony, barium, and lead on normally handled automobile surfaces by graphite furnace atomic absorption analysis. No positives were determined on 50 sampled automobile surfaces above cutoff (positive) from background levels. Transfers of GSR particles from shooter hands to automobile surfaces were found to potentially allow for positive GSR determinations, but such transfers seem to be dependent on the shooting conditions and length of GSR exposure. We determined that our bulk analysis method yields an overall 87.94 ± 5.52% extraction efficiency from cotton swabs, while the LOQ determinations strengthening the fact that bulk analysis methods are valid and valuable tools for GSR investigations.     

The Relationship Between the Surface Morphology and Chemical Composition of Gunshot Residue Particles

In this study, chemical composition and morphology of gunshot residue (GSR) of 9 × 19 mm Parabellum‐type MKE (Turkey)‐brand ammunition were analyzed by scanning electron microscope and energy dispersive X‐ray spectrometer. GSR samples were collected by “swab” technique from the shooter's right hand immediately after shooting. According to general principles of thermodynamics, it is likely that the structures will have a more regular (homogeneous) spherical form to minimize their surface area due to very high temperatures and pressures that occur during explosion. Studied samples were collected under the same conditions with the same original ammunition, from the same firearm and a single shooter. This is because many other variables may affect size, structure, and composition in addition to the concentrations of elements of the structure. Results indicated that the chemical compositions are effective in the formation of GSR morphological structures.     

Lead isotope measurement of primer gunshot residues and likelihood ratio predictions for forensic cartridge discrimination and individualization in China

Gunshot residues (GSR), cartridge projectiles, and casings are frequently encountered evidence in gun-related forensic investigations. However, in circumstances where the investigation of striation marks is impossible, such as unrecovered or deformed projectiles and cartridge casings, GSR deposited on the hands or clothes of the shooter and victim-related items can provide information to establish a link between the suspect, the firearms used, and the victim. Since the formula of primers used by all cartridge manufacturers in China is identical, links based on the conventional morphological and compositional analysis of GSR are difficult to establish. However, the abundance of lead isotopes in primer components of lead styphnate varies significantly, and a fundamental understanding of these differences may facilitate the validation of primer (p)GSR evidence in forensic investigations. Here, 44 pGSR samples were characterized by Pb isotope ratios of ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb using laser ablation multicollector inductively coupled plasma mass spectrometry. There was no obvious mass fractionation of the lead isotope ratios of the primers from individual cartridges analyzed before and after the shooting process, thereby establishing a basis for the comparison of pGSR and unfired cartridges. Evaluation of the results using univariate likelihood ratio (LR) computations revealed low rates of misleading evidence (<0.53%) The results demonstrated that lead isotope ratio analysis of pGSR and LR predictions can provide a practicable method for forensic cartridge discrimination and individualization.     

Comparison of GSR Composition Occurring at Different Locations Around the Firing Position

Abstract: Variations in gunshot residue (GSR) compositions are used in the reconstruction of shooting incidents. In this study, GSR samples taken from seven different locations around and in the firearm were collected and analyzed using scanning electron microscopy/energy dispersive X‐ray analysis. Four different types of ammunition were applied. Very low correlations were found when different ammunition were used. This clearly shows that it is possible to differentiate between ammunition types. When the same ammunition was used, high correlations were found between samples taken from external positions (such as hands of shooter, bullet‐entrance holes) but poor correlation was found between internal samples (such as firearm barrel, cartridge case) and external samples. A high degree of association was found between samples that simulated victim and shooter. These findings clearly demonstrate that GSR comparison studies are meaningful but care needs to be taken when choosing suitable exhibits. External samples (such as hands of shooter, bullet‐entrance holes) are more suitable candidates than internal samples (barrel of the firearm, cartridge case).     

Evaluation of the Simultaneous Analysis of Organic and Inorganic Gunshot Residues Within a Large Population Data Set Using Electrochemical Sensors*,†

The increasing demand for rapid methods to identify both inorganic and organic gunshot residues (IGSR and OGSR) makes electrochemical methods, an attractive screening tool to modernize current practice. Our research group has previously demonstrated that electrochemical screening of GSR samples delivers a simple, inexpensive, and sensitive analytical solution that is capable of detecting IGSR and OGSR in less than 10 min per sample. In this study, we expand our previous work by increasing the number of GSR markers and applying machine learning classifiers to the interpretation of a larger population data set. Utilizing bare screen-printed carbon electrodes, the detection and resolution of seven markers (IGSR; lead, antimony, and copper, and OGSR; nitroglycerin, 2,4-dinitrotoluene, diphenylamine, and ethyl centralite) was achieved with limits of detection (LODs) below 1 µg/mL. A large population data set was obtained from 395 authentic shooter samples and 350 background samples. Various statistical methods and machine learning algorithms, including critical thresholds (CT), naïve Bayes (NB), logistic regression (LR), and neural networks (NN), were utilized to calculate the performance and error rates. Neural networks proved to be the best predictor when assessing the dichotomous question of detection of GSR on the hands of shooter versus nonshooter groups. Accuracies for the studied population were 81.8 % (CT), 88.1% (NB), 94.7% (LR), and 95.4% (NN), respectively. The ability to detect both IGSR and OGSR simultaneously provides a selective testing platform for gunshot residues that can provide a powerful field-testing technique and assist with decisions in case management.     

A Mixture Model for the Number of Gunshot Residues Found on Suspects' Hands

Quantifying the strength of gunshot residue (GSR) evidence requires scientific knowledge about the number of particles expected to be found on individuals who were or were not involved in a shooting. However, controlled experiments demand expensive resources in terms of microscope time and labor, which restricts the data of most studies to only a small group of individuals. We suggest a novel method that exploits data collected routinely on suspects during the daily work of forensic laboratories. These observational data relate to both persons who were involved in a shooting and innocent individuals. We suggest a mixture approach with different models for the number of gunshot residue particles in each group and develop an iterative algorithm to estimate the probabilities of observing the evidence under the defense proposition that the suspect is innocent and under the prosecution assumption that he is not. The method is applied to data of more than 500 suspects collected by the Israel National Police Division of Identification and Forensic Science. The analysis shows that the probability of finding three or more GSR particles on the hands of innocent suspects is very small, less than 1.5 in 1000 cases. Our new method enables researchers to use data on real cases, possibly supplemented by experimental data, in order to estimate the probabilities of a given GSR finding under the defense and prosecution propositions.     

Identification of gunshot residue: a critical review

A review of the scientific papers published on inorganic gunshot residue (GSR) analysis permits to study how the particle analysis has shown its capability in detection and identification of gunshot residue. The scanning electron microscope can be the most powerful tool for forensic scientists to determine the proximity to a discharging firearm and/or the contact with a surface exposed to GSR. Particle analysis can identify individual gunshot residue particles through both morphological and elemental characteristics. When particles are detected on the collected sample, the analytical results can be interpreted following rules of a formal general interpretative system, to determine whether they come from the explosion of a primer or from other possible sources. The particles on the sample are compared with an abstract idea of "unique" GSR particle produced by the sole source of the explosion of a primer. "Uniqueness" is not the only problem related to GSR detection and identification for a forensic scientist. With "not-unique" particles interpretation of results is extremely important. The evidential strength of "not-unique" particles can increase with a more fruitful interpretative framework based on Bayes rule. For the assessment of the value of a GSR in linking a suspect and a crime, it is important to compare two hypothesis: the first can be that of the evidence if the suspect has been shooting in a specific situation, the second that of the evidence if the suspect was not involved in this shooting. This case specific or case-by-case approach is closer to what the court is interested in. The authors consider that a "case-by-case" approach should be followed whenever possible. Research of models and data such as those developed in other trace evidence material (fibres, glass, etc.) using a Bayesian approach is suggested in the interpretation of GSR.     

Grouping of ammunition types by means of frequencies of occurrence of GSR

An attempt was made to build a classification scheme for gunshot residues (GSR) samples originating from four types of ammunition, collected from shooters' hands immediately after shooting. The secured material was examined with the use of SEM-EDX method in the automatic manner. The obtained results were expressed as frequencies of occurrence of particles assigned to various chemical classes. In order to establish the most discriminative of these features the Mann-Whitney test was performed. Cluster analysis was performed for grouping the analysed samples according to their origin, i.e. the type of ammunition. It has been found that samples of GSR originating from Browning 7.65 mm and Luger 9 mm ammunition can be fairly easy differentiated from the remaining samples, whereas samples of GSR originating from of Makarov 9 mm and these of Tokarev 7.62 mm could not be differentiated using frequencies of occurrence of particles in the selected chemical classes.     

A study of dispersion of gunshot residue from a frequently used Serbian ammunition cal. 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.     

Statistical Challenges in the Quantification of Gunshot Residue Evidence

The discharging of a gun results in the formation of extremely small particles known as gunshot residues (GSR). These may be deposited on the skin and clothing of the shooter, on other persons present, and on nearby items or surfaces. Several factors and their complex interactions affect the number of detectable GSR particles, which can deeply influence the conclusions drawn from likelihood ratios or posterior probabilities for prosecution hypotheses of interest. We present Bayesian network models for casework examples and demonstrate that probabilistic quantification of GSR evidence can be very sensitive to the assumptions concerning the model structure, prior probabilities, and the likelihood components. This finding has considerable implications for the use of statistical quantification of GSR evidence in the legal process.     

Voltammetric analysis of luminescent markers in gunshot residues

Currently, SEM-EDS is used to detect gunshot residue (GSR) from the presence of Ba, Pb, and Sb in the sample. However, the development of new nontoxic ammunition (NTA) has prevented conventional metals from being found. In this work, we aim to determine the presence of an inorganic luminescent chemical marker based on rare earth in gunshot residues using the technique of squarewave voltammetry (SWV). After firing, the luminescent complex [(Eu₂Zr)(btc)₃(Hbtc)_0.5.6H₂O], which is used as a chemical marker, can be detected under a UV lamp. An aqueous solution with 0.1 mol L⁻¹ KCl as supporting electrolyte can be easily collected on carbon paste electrode surfaces for SWV analysis A = 100 mV, f = 10 Hz, and step potential of 5 mV are required. The luminescent marker incorporated into the carbon paste electrode showed two anodic peak currents in the region of 0.4 V (vs Ag/AgCl) and at 0.75 V (vs Ag/AgCl) and also a cathodic one in 0.4 V (vs Ag/AgCl). SEM-EDS was able to analyze the same voltammetric results for conventional and nontoxic ammunition containing the luminescent marker. Therefore, voltammetry and SEM-EDS are valid for detecting the new residue marker in GSR. Despite this, the electrochemical method is still more advantageous because of its low cost and lack of expensive equipment and supplies in forensic laboratories.     

Distribution of GSR particles in the surroundings of shooting pistol

Several series of experiments were performed to study the distribution of GSR particles in seven directions in the surroundings of shooting firearm (pistol CZ 85 caliber 9 mm Luger). External and internal conditions and two different primer types were used for the shooting experiments. The results showed that maximum number of GSR particles could be found in the right front quadrant at a distance of 2–4 m with respect to the shooting firearm position and shooting direction. GSR particles were even found in distance 10 m from shooting firearm. A significant influence of climatic conditions on GSR distribution was confirmed.     

Distribution of gunshot residues--the influence of weapon type

Gunshot residue (GSR) analysis is a frequently used forensic method for investigating shooting events. In this process, GSR-particles are detected on pertinent objects, often the hands of a suspect, with energy dispersive X-ray microanalysis in a scanning electron microscope. The question asked in court is usually the following: "… has or has not a given shot been fired by the suspect (at a certain place and time)?" Unfortunately, this question frequently cannot be answered by the analyst. Only the presence or absence of "characteristic" GSR-particles on the investigated sample can be stated with confidence. All other assumptions are inherently biased by many factors that are usually only known by the culprit or are generally unknown. In order to draw more profound conclusions from GSR analytical results, the dynamics of GSR particles have to be considered, i.e., how these particles reach a suspect. Consequently, the formation of the plume after firing was investigated for eleven different firearms with high speed-video analysis. The results show that a vast scope exists between revolvers - distributing many particles near the shooter - and e.g. shotguns that are quite sealed. These differences between the various weapon types should be considered for interpretation of the results of GSR-analysis.     

Gunshot residue in Chicago police vehicles and facilities: an empirical study

Suspects in shooting investigations in Chicago are routinely transported in department vehicles and detained in department facilities prior to gunshot residue (GSR) evidence collection. The GSR test results are used to associate the suspect with primary exposure to GSR. The potential for these vehicles and facilities being sources of secondary GSR contamination needed to be determined. A total of 201 samples were collected from randomly selected vehicles and detention facilities. The sampling collected trace materials from surfaces that suspects' hands may contact during the arrest process. These samples were examined for the presence of GSR particles using scanning electron microscopy. Upon completion of the automated analysis, those particles that met an initial GSR screening criterion were relocated and reanalyzed. The locations where GSR particles were recovered allowed us to make recommendations to the Chicago Police Department with regard to transporting and detaining these suspects. The low number of GSR particles recovered suggests that the potential for secondary contamination, although present, is relatively low.     

Variation of the chemical contents and morphology of gunshot residue in the surroundings of the shooting pistol as a potential contribution to a shooting incidence reconstruction

A study of the chemical contents and sizes of gunshot residue originating from 9×18mm PM ammunition, depositing in the vicinity of the shooting person was performed by means of scanning electron microscopy and energy dispersive X-ray spectrometry. Samples of the residue were collected from targets placed at various distances in the range 0-100cm as well as from hands and clothing of the shooting person. Targets were covered by fragments of white cotton fabric or black bovine leather. In the case of cotton targets microtraces were collected from circles of 5 and 10cm in radius. Results of the examinations in the form of numbers of particles, proportions of their chemical classes and dimensions revealed a dependence on the distance from the gun muzzle, both in the direction of shooting and in the opposite one, i.e., on the shooting person. The parameters describing gunshot residue differed also depending on the kind of the target substrate. The kind of obtained information gives rise to understanding the general rules of the dispersion of gunshot residue in the surroundings of the shooting gun. Thus, it may be utilised in the reconstruction of shooting incidences, especially in establishing the mutual positions of the shooter and other participants of the incident.     

Screening of gunshot residues using desorption electrospray ionisation-mass spectrometry (DESI-MS)

Several studies have indicated that there are potential environmental sources of particles resembling inorganic primer found in gunshot residues (GSR); as a consequence examiners are reluctant to unambiguously assign the origin of inorganic particles. If organic gunshot residues (OGSR) were found in combination with inorganic particles, the possibility of environmental sources could be potentially eliminated, thereby significantly enhancing the strength of the evidence. Methods have been previously described whereby GSR specimens can be analysed for the presence of OGSR or inorganic GRS (IGSR). However, no methods have been reported that allow the analysis of both OGSR and IGSR on the same specimen. Described in this article is a direct method using desorption electrospray ionisation-mass spectrometry (DESI-MS) for the detection of methyl centralite (MC), ethyl centralite (EC) and diphenylamine (DPA) on adhesive tape GSR stubs typically used for scanning electron microscopy-energy-dispersive X-ray (SEM-EDX) analysis. The optimisation of numerous parameters was conducted using an experimental design. The results indicate that direct analysis of these organic components of GSR is possible although some limitations were also identified. This initial investigation has also indicated that subjecting stubs to DESI analysis does not interfere with subsequent SEM-EDX analysis of primer residues; therefore the technique described herein allows a comprehensive examination of GSR that would be highly probative in the event that both OGSR and IGSR are detected in the same specimen.     

Blood-spatter patterns: hands hold clues for the forensic reconstruction of the sequence of events

Biologic and nonbiologic traces on the hands are of particular importance for the forensic reconstruction of shooting incidents; gunpowder residue analysis in particular helps determine whether the gunshot is close range or distant. In crime scene investigation, knowledge about the morphology of bloodstain patterns-including gunshot-related back spatter-has increased since various experimental examinations have been performed in the last years; nowadays, these traces are frequently used for forensic crime scene reconstruction. The goal of this study was to deduce the position and orientation of the hands, and therefore the firearm, according to the bloodstain patterns on the hands of the deceased. For this purpose blood-spatter stains on the hands were examined on site in 5 suicides caused by gunshot. In all cases, forensically relevant conclusions regarding forensic reconstruction were enabled through close examination of the spatter marks. Therefore, in shooting incidents, analysis and documentation of blood-spatter findings on the hands is recommended before hands are tested for gunpowder residue or wrapped for the transport of the body.     

Stubs Versus Swabs? A Comparison of Gunshot Residue Collection Techniques

Abstract: The collection efficiency of two widely used gunshot residue (GSR) collection techniques—carbon‐coated adhesive stubs and alcohol swabs—has been compared by counting the number of characteristic GSR particles collected from the firing hand of a shooter after firing one round. Samples were analyzed with both scanning electron microscopy and energy dispersive X‐rays by an experienced GSR analyst, and the number of particles on each sample containing Pb, Ba, and Sb counted. The adhesive stubs showed a greater collection efficiency as all 24 samples gave positive results for GSR particles whereas the swabs gave only positive results for half of the 24 samples. Results showed a statistically significant collection efficiency for the stub collection method and likely reasons for this are considered.     

Fate and Behavior of Gunshot Residue: Recreational Shooter Vehicle Distribution

The susceptibility for recreational shooters to transfer gunshot residue (GSR) to both the interior and exterior of a vehicle is investigated. A comprehensive sampling protocol was used to assess the most likely areas of GSR transfer from recreational shooter contact, such as the steering wheel and the area, the firearms were stored (the trunk). Up to 315 characteristic GSR, particles were found in several locations throughout the interior of a vehicle. As many as 876 characteristic particles were found throughout a single vehicle. The data indicate that vehicles frequently occupied by firearms users are a potential source for inadvertent transfer of GSR to persons unrelated to firearm activity. In criminal cases where vehicles have been used, such transfer processes for GSR need to be considered within the context of any case interpretation. The implications for subsequent contamination and transfer processes from such vehicles require further investigation.