Differential pulse anodic stripping voltammetry of barium and lead in gunshot residues

Differential pulse anodic stripping voltammetry (DPASV) has been applied for characterization and quantitative detection of barium and lead from gunshot residues (GSR). Previous electrochemical techniques have detected antimony and lead from GSR, however barium has never been detected. This technique allows for simultaneous detection of Ba and Pb that is simple, fast, and nondestructive.     

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.     

A simple method for detection of gunshot residue particles from hands, hair, face, and clothing using scanning electron microscopy/wavelength dispersive X-ray (SEM/WDX).

We devised a simple and rapid method for detection of gunshot residue (GSR) particles, using scanning electron microscopy/wavelength dispersive X-ray (SEM/WDX) analysis. Experiments were done on samples containing GSR particles obtained from hands, hair, face, and clothing, using double-sided adhesive coated aluminum stubs (tape-lift method). SEM/WDX analyses for GSR were carried out in three steps: the first step was map analysis for barium (Ba) to search for GSR particles from lead styphnate primed ammunition, or tin (Sn) to search for GSR particles from mercury fulminate primed ammunition. The second step was determination of the location of GSR particles by X-ray imaging of Ba or Sn at a magnification of x 1000-2000 in the SEM, using data of map analysis, and the third step was identification of GSR particles, using WDX spectrometers. Analysis of samples from each primer of a stub took about 3 h. Practical applications were shown for utility of this method.     

GSR2005—Continuity of the ENFSI Proficiency Test on Identification of GSR by SEM/EDX

Abstract:  Within the framework of the ENFSI Expert Working Group "Firearms" a proficiency test on the detection and identification of GSR by energy-dispersive scanning electron microanalysis (SEM/EDX) is carried out in a 2 years term. The latest test was performed in 2005/2006 and was denoted as GSR2005 . Seventy-five laboratories from 28 countries participated in this proficiency test and submitted in total 83 independent data-sets. The participating laboratories were requested to determine the total number of PbSbBa containing particles on a synthetic test sample following their own laboratory specific methods of automated GSR particle search and detection by SEM/EDX. Furthermore size and position of the detected particles had to be reported by the laboratories and were evaluated statistically. The results were compiled by means of z -scores according to the IUPAC and EURACHEM guidelines—assessing individual laboratory achievements (inter-laboratory) as well as intra-laboratory performance—and were compared to the results of the previous proficiency test run GSR2003 ( 1 ). The comparison shows that there is a noticeable improvement in the method's detection capability.     

The Effect of Skin Debris on Gunshot Residue Sampling and Detection

These experiments were designed to determine whether skin debris (desquamated epithelial cells and apparent skin oils) affects gunshot residue (GSR) particle detection on the sticky tape lift samples prepared for scanning electron microscopy (SEM). A dabbing experiment showed that GSR particles accumulate not only on the adhesive surface of the sampler, but also on the epithelial cell surfaces. Samplers were loaded with target GSR followed by dabbing 30 times on the back of a hand. Backscatter electron images were taken at 20 kV and for some at 30 kV of the same areas. The samplers were then treated with a sodium/calcium hypochlorite solution (bleach) to remove skin debris and again imaged in the SEM. Comparison of these images shows more GSR particles will likely be revealed at 30 kV than 20 kV and more particles revealed by the bleach treatment in an automated SEM system.     

Maintenance of the ENFSI proficiency test program on identification of GSR by SEM/EDX (GSR2003)

Within the framework of the ENFSI Expert Working Group "Firearms," every second year, a proficiency test on the detection and identification of GSR by SEM/EDX is carried out. This proficiency test is a development and extension of the previous proficiency test GSR2001. The test material was again designed by the organization panel and manufactured by an external company for SEM accessories. This time the participating laboratories were requested to determine the total number of PbSbBa containing particles on a test sample following their own laboratory specific methods of automated GSR particle search and detection by SEM/EDX. One synthetic particle sample (SPS) with artificial GSR particles was dispatched to all participants. This paper summarizes the results of the study and assesses the overall performance of the participating laboratories. Furthermore an extended statistical evaluation and a comparison with previous studies was carried out.     

Automated gunshot residue particle search and characterization

The main disadvantage to gunshot residue (GSR) particle analysis utilizing scanning electron microscope/energy dispersive X-ray (SEM/EDX) instrumentation has been the excessive operator time required for search and identification. This study uses an automated particle search and characterization program for unattended GSR search and identification. This system allows for automatic matrix search, particle sizing, chemical typing, and spectral aquisition with subsequent storage of data to disk for later operator review and verification. This work describes various aspects of the program, determines appropriate parameters adequate for both unique and characteristic GSR particle identification, and evaluates the reliability of data obtained. Samples are collected via the tape lift method from test-firings of .38, .32, .25, and .22 caliber handguns at time after firing intervals of 0 to 6 h. Unique GSR particles are consistently and correctly identified by this method on tape lift samples taken up to 4 h after firing. False positive results of unique GSR particles are not encountered on control handblank samples. This technique appears to provide the forensic science community with an operator-free method of reliable GSR particle search and an improved analyst-time-per-case ratio.     

GSR2005--continuity of the ENFSI proficiency test on identification of GSR by SEM/EDX.

Within the framework of the ENFSI Expert Working Group "Firearms" a proficiency test on the detection and identification of GSR by energy-dispersive scanning electron microanalysis (SEM/EDX) is carried out in a 2 years term. The latest test was performed in 2005/2006 and was denoted as GSR2005. Seventy-five laboratories from 28 countries participated in this proficiency test and submitted in total 83 independent data-sets. The participating laboratories were requested to determine the total number of PbSbBa containing particles on a synthetic test sample following their own laboratory specific methods of automated GSR particle search and detection by SEM/EDX. Furthermore size and position of the detected particles had to be reported by the laboratories and were evaluated statistically. The results were compiled by means of z-scores according to the IUPAC and EURACHEM guidelines-assessing individual laboratory achievements (inter-laboratory) as well as intra-laboratory performance-and were compared to the results of the previous proficiency test run GSR2003 (1). The comparison shows that there is a noticeable improvement in the method's detection capability.     

A new method for collection and identification of gunshot residues from the hands of shooters

This work presents a novel collection method for gunshot residues (GSR) using a sampling procedure based on ethylenediaminetetraacetic acid (EDTA) solution as a complexing agent on moistened swabs. Detection was via a sector-field inductively coupled plasma mass spectrometry (HRICP-MS). The proposed collection and analytical method allowed detection of antimony (Sb), barium (Ba) and lead (Pb) after .38 shot tests. at detection limits of less than 1 microg L(-1) in four different areas of the hands of volunteers. This paper includes a discussion concerning hand areas near the thumb and forefinger as being more suitable for GSR collection as well as a comparison between differences observed using 2% diluted EDTA. 2% nitric acid solution, and simple deionized water as collecting solutions, proving the superior efficiency of EDTA in GSR recoveries.     

Combined Collection and Analysis of Inorganic and Organic Gunshot Residues

Gunshot residue (GSR) analysis and their interpretation provide crucial information on a criminal investigation involving the use of firearms. To date, several approaches have been proposed for the implementation of a combined sampling and analysis of inorganic (IGSR) and organic GSR (OGSR). However, it is not clear at this stage if concurrent analyses of both types of residue might be detrimental to the analysis of IGSR currently applied in forensic laboratories. Thus, this work aims to compare and evaluate three different protocols for the combined collection and analysis of IGSR and OGSR. These methods, respectively, involve the use of a modified stub (with two halves, one for the detection of IGSR and the other for the analysis of OGSR); the sequential recovery of GSR with two stubs mounted with different adhesives (double-sided carbon tape and Tesa® TACK) and the sequential analysis of IGSR and OGSR from a single carbon stub following carbon deposition. The detection of IGSR was carried out using SEM-EDX, while OGSR analysis was performed using ultra-high-performance liquid chromatography–tandem mass spectrometry (UHPLC-MS/MS). Obtained results for experiments performed using Geco Sinoxid^® ammunition indicated that sequential analysis was the most suitable protocol for the combined collection and analysis of both IGSR and OGSR. A higher number of inorganic (characteristic and consistent) particles and higher concentrations of ethylcentralite, N-nitrosodiphenylamine, diphenylamine, and nitroglycerin were recovered with this method.     

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.     

Unusual sources of Sn in GSR. An experimental study by SEM and IBA

Gunshot Residue (GSR) produced by the discharge of a firearm often provides very useful information in criminal investigations in cases involving the use of firearms. Scanning Electron Microscopy equipped with an Energy Dispersive X-ray Spectrometer (SEM-EDS) is typically used worldwide to visualize micrometric particles constituting GSR and to analyse their elemental composition. The 2017 ASTM Standard guide for gunshot residue analysis by scanning electron microscopy/energy dispersive X-ray spectroscopy specifies that “Particles classified as characteristic of GSR will have one of the following elemental compositions: Lead, antimony, barium; Lead, barium, calcium, silicon, tin”. For the first time, the presence of an additional element, such as Sn, plays a key role in ASTM particle classification. It is known that some ammunitions, used for pistols, revolvers and rifles, contain tin foil discs for sealing the primer mixture into the cup, resulting in GSR particles containing Sn. The authors faced some cases in which Sn was unexpectedly found in GSR particles from a 0.22 Long Rifle derringer and from some 12 gauge shotguns. No tin foil discs are used in rimfire ammunitions and there is no published evidence of tin foil discs in shotshell ammunitions. Following a “case by case” approach, experimental research has been carried out to explain how Sn can be present in GSR particles when the last discharged cartridge also does not contain any Sn either in components and in the explosive charges.Moreover, the use of Particle Induced X-ray Emission (PIXE) showed the capability to overcome overlap ambiguity of Sb and Sn peaks in the X-ray spectra, being a possible key issue in real shooting cases if Sn quantities are below the lower limit of SEM detection, especially when Sb is also present.     

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.     

Characterisation of 0.22 caliber rimfire gunshot residues by time-of-flight secondary ion mass spectrometry (TOF-SIMS): a preliminary study

The application of time-of-flight secondary ion mass spectrometry (TOF-SIMS) for the characterisation of gunshot residue (GSR) from 0.22 caliber rimfire ammunition is reported. Results obtained by TOF-SIMS were compared with conventional scanning electron microscopy (SEM) studies. As could be expected, TOF-SIMS exhibited greater elemental sensitivity than SEM equipped with energy dispersive X-ray detection (SEM-EDX), and was also capable of detecting fragments characteristic of inorganic compounds. This preliminary study indicates that TOF-SIMS offers substantial potential for forensic GSR examinations as a complementary technique to SEM-EDX. In addition TOF-SIMS is applicable to the analysis of individual particles in the typical size range encountered in GSR casework.     

Evaluation of shooting distance by AFM and FTIR/ATR analysis of GSR

The techniques of atomic force microscopy (AFM) and Fourier transform infrared attenuated total reflectance (FTIR/ATR) spectroscopy are applied to the analysis of gun-shot residue (GSR) to test their ability to determine shooting distance and discrimination of the powder manufacturers. AFM is a nondestructive technique that is capable of characterizing the shapes and size distributions of GSR particles with resolution down to less than a nanometer. This may be useful for estimation of the shooting distance. Our AFM images of GSR show that the size distribution of the particles is inversely proportional to the shooting distance. Discrimination of powder manufacturers is tested by FTIR/ATR investigation of GSR. Identifying the specific compounds in the GSR by FTIR/ATR was not possible because it is a mixture of the debris of several compounds that compose the residue. However, it is shown that the GSR from different cartridges has characteristic FTIR/ATR bands that may be useful in differentiating the powder manufacturers. It appears promising that the development of AFM and FTIR/ATR databases for various powder manufacturers may be useful in analysis and identification of GSR.     

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.     

A Comparison Between Digital Radiography,Computed Tomography,and Magnetic Resonance in the Detection of Gunshot Residues in Burnt Tissues and Bone

The radiological search for GSR is crucial in burnt material although it has been rarely tested. In this study, thirty‐one bovine ribs were shot at near‐contact range and burnt to calcination in an oven simulating a real combustion. Computed tomography (CT) and magnetic resonance (MR) were performed before and after carbonization and compared with former analyses with DR (digital radiography); thus comparing the assistance, the radiological methods can provide in the search for GSR in fresh and burnt bone. DR demonstrated the greatest ability in the detection of metallic residues, CT showed lower abilities, while MR showed a high sensitivity only in soft tissues. Thus, DR can be considered as the most sensitive method in the detection of GSR in charred bones, whereas CT and MR demonstrated much less reliability. Nonetheless, the MR ameliorates the analysis of gunshot wounds in other types of remains with large quantities of soft tissues.     

Gunshot residue testing in suicides: Part II: Analysis by inductive coupled plasma-atomic emission spectrometry

Several different methods can be employed to test for gunshot residue (GSR) on a decedent's hands, including scanning electron microscopy with energy dispersive x-ray (SEM/EDX) and inductive coupled plasma-atomic emission spectrometry (ICP-AES). In part I of this 2-part series, GSR results performed by SEM/EDX in undisputed cases of suicidal handgun wounds were studied. In part II, the same population was studied, deceased persons with undisputed suicidal handgun wounds, but GSR testing was performed using ICP-AES. A total of 102 cases were studied and analyzed for caliber of weapon, proximity of wound, and the results of the GSR testing. This study found that 50% of cases where the deceased was known to have fired a handgun immediately prior to death had positive GSR results by ICP/AES, which did not differ from the results of GSR testing by SEM/EDX. Since only 50% of cases where the person is known to have fired a weapon were positive for GSR by either method, this test should not be relied upon to determine whether someone has discharged a firearm and is not useful as a determining factor of whether or not a wound is self-inflicted or non-self-inflicted. While a positive GSR result may be of use, a negative result is not helpful in the medical examiner setting as a negative result indicates that either a person fired a weapon prior to death or a person did not fire a weapon prior to death.     

Environmental assessment of gunshot residue particles in the public domain of the United Kingdom

Only limited data currently exists on the inadvertent transfer of gunshot residue (GSR), or GSR-like particles through contact with public places. In this study, an assessment occurrence of GSR in public environments in England, UK was undertaken. Utilizing a stubbing sampling technique over 260 samples were collected from areas accessible to the public, including buses, trains, taxis, and train stations. Stub analysis was performed by Scanning Electron Microscopy with Energy Dispersive X-ray Analysis (SEM-EDX). The results showed no characteristic GSR particles were detected on any of the 262 samples taken. From these samples, a total of four indicative/consistent particles were identified on one train seat (2× BaAl, 2× PbSb). Although geographical location and firearm association is likely to influence GSR occurrence, the data suggests that the potential for inadvertent GSR transfer through contact with public transport and associated communal areas is insignificant. Further research assessing environmental background levels of GSR in additional geographical locations is critical in an evaluation of the potential for GSR transfer from the environment.     

Discharge of a Pistol Out a Car Window with the Breech Within the Interior of the Car: Analysis of Gunshot Residue on a Car's Interior Surfaces

The defendant, the driver of the questioned car, allegedly extended his right arm over the passenger seat and fired a single shot from a 380 pistol out the passenger window with the pistol's breech within the car. A simulation of this shooting scenario using the same model car, but different year, was conducted to quantitate gunshot residue (GSR) contamination of interior surfaces within the car. The test car's dash and headliner/window frame above the pistol had the heaviest GSR contamination. The dash GSR from airborne deposition documents a firearm discharge within the vehicle. Transfer from GSR‐contaminated hands or clothing to the dash is unlikely. The heavy GSR contamination of the headliner/window frame above the pistol likely documents the window from which the pistol was fired, but additional experiments are needed to verify.