Recent developments in methods of chemical analysis in investigations of firearm-related events

Chemical Analysis of Gunpowder and Gunshot Residues

The identification of firearms is of paramount importance for investigating crimes involving firearms, as it establishes the link between a particular firearm and firearm-related elements found at a crime scene, such as projectiles and cartridge cases. This identification relies on the visual comparison of such elements against reference samples from suspect firearms or those existing in databases. Whenever this approach is not possible, the chemical analysis of the gunpowder and gunshot residue can provide additional information that may assist in establishing a link between samples retrieved at a crime scene and those from a suspect or in the identification of the corresponding model and manufacturer of the ammunition used. The most commonly used method for the chemical analysis of gunshot residue is scanning electron microscopy with energy dispersive X-ray, which focuses on the inorganic elements present in ammunition formulation, particularly heavy metals. However, a change in the legal paradigm is pushing changes in these formulations to remove heavy metals due to their potential for environmental contamination and the health hazards they represent. For this reason, the importance of the analysis of organic compounds is leading to the adoption of a different set of analytical methodologies, mostly based on spectroscopy and chromatography. This manuscript reviews the constitution of primer and gunpowder formulations and the analytical methods currently used for detecting, characterising, and identifying their compounds. In addition, this contribution also explores how the information provided by these methodologies can be used in ammunition identification and how it is driving the development of novel applications within forensic ballistics.

Should inorganic or organic gunshot residues be analysed first?

Gunshot residues (GSR) collected during the investigation of firearm-related incidents can provide useful information for the reconstruction of the events. Two main types of GSR traces can be targeted by forensic scientists, the inorganic (IGSR) and the organic GSR (OGSR). Up to now, forensic laboratories have mainly focused on the detection of inorganic particles on the hands and clothes of a person of interest using carbon stubs analysed by scanning electron microscopy coupled with energy dispersive X-ray spectrometry (SEM/EDS). Several approaches have been proposed to also analyse the organic compounds since they might bring additional information for the investigation. However, implementing such approaches might disrupt the detection of IGSR (and vice versa depending on the applied sequence of analysis). In this work, two sequences were compared for the combined detection of both types of residues. One carbon stub was used for collection, and the analysis was performed either by targeting the IGSR or the OGSR first. The aim was to evaluate which one allows maximum recovery of both types of GSR while minimising losses that might occur at different stages of the analysis process. SEM/EDS was used for the detection of IGSR particles while an ultra-high performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) was used for the analysis of OGSR compounds. Extracting OGSR first required the implementation of an extraction protocol that did not interfere with the IGSR particles present on the stub. Both sequences allowed good recovery of the inorganic particles since no significant difference was observed in the detected concentrations. However, OGSR concentrations were lower after IGSR analysis than before for two compounds (ethyl and methylcentralite). Thus, it is advised to extract rapidly the OGSR before or after IGSR analysis to avoid losses during the storage and analysis processes. The data also indicated that there was a low correlation between IGSR and OGSR highlighting the potential of a combined detection and analysis of both types of GSR.

Spectroscopic (analytical) approach to gunshot residue analysis for shooting distance estimation: a systematic review

Background

The determination of the shooting distance using gunshot residue (GSR) analysis is crucial in the investigation and reconstruction of firearm-related crimes. However, the conventional chemographic method for GSR analysis is destructive and has limited sensitivity and selectivity. While the spectroscopic method has potential in GSR analysis for crime investigation, there is a current lack of consistency in the spectroscopic results obtained for shooting distance estimation via GSR analysis. Addressing such limitations will enhance the forensic capabilities of law enforcement and provide an added advantage to crime laboratories during an investigation. It will also reinforce the use of such spectroscopic data in a criminal investigation.

Main text

We obtained all peer-reviewed articles relevant to shooting distance estimation from searching Scopus, Web of Science, PubMed, and Google Scholar databases. We specifically searched the databases using the keywords “shooting distance,” “range of fire,” “gunshot residue,” “firearm discharge residue,” and “firearm-related crime” and obtained 3811 records. We further filtered these records using a combination of two basic keywords “gunshot residue” and “shooting distance estimations” yielding 108 papers. Following a careful evaluation of the titles, abstracts, and full texts, 40 original peer-reviewed articles on shooting distance estimation via GSR analysis were included in the study. The forgoing included additional sources (n = 5) we obtained from looking through the reference lists of the forensic articles we found.

Short conclusion

This paper discusses the current scope of research concerning the chemographic and spectroscopic analysis of GSR for shooting distance estimation. It also examines the challenges of these techniques and provides recommendations for future research.

Finding GSR evidence on used towels

When a firearm is discharged, gunshot residue (GSR) is produced and may be deposited on a shooter's body and his close vicinity. The tendency of GSR to drop off easily from the shooter may hinder forensic detection; this well-known phenomenon is accelerated by various common physical activities so that the number of particles detected on a suspect decreases over time. After shooting incidents, suspects will often try to cover their tracks by taking a shower. In these cases, it was assumed that no GSR will be detected upon examining the suspect's hand and hair. In the present study, we provide a way to overcome this loss of evidence by taking advantage of another occurrence, namely secondary transfer. Our participants were asked to take a shower after shooting a firearm. Samples were thereafter collected from the used bath towels and were found to contain up to a few dozen particles characteristic of GSR, including very large particles (>45 µm). The detection of GSR on a suspect's towel may provide significant forensic evidence aiding an investigation. When a shooter tries to remove evidence by taking a shower, sampling the towels that he may have used can preserve important evidence and connect a suspect to a shooting incident.

Vibrational spectroscopy and chemometrics in GSR: review and current trend

Background

This review represents many significant methods of chemometrics applied as data assessment methods originated by many hyphenated analytical techniques containing their application since its origin to today.

Main body of the abstract

The study has been divided into many parts, which contain many multivariate regression methods. The main aim of this study is to investigate the chemometrics tools used in GSR (gunshot residue) or forensic ballistics.

Short conclusion

As a final point, the end of part of this review deals with the applicability of chemometric methods in forensic ballistics. We select to give an elaborate description of many significant tools established with their algorithm in admire of utilizing and accepting them by researchers not very aware with chemometrics.

Ear as an alternative sampling site for GSR analysis following shotgun discharge

Detection of GSR particles potentially indicates that a person fired a gun or somehow involved to a shooting event. GSR on the shooter's hand, face, and clothing may disappear within hours and with sweat secretion, washing or cleaning to remove evidences. Due to its anatomical properties, ears are relatively protected; therefore, we aimed to identify GSR particles on ears, to compare its anatomical parts of ears, and compare ears with common GSR sampling sites, based on firing frequency. A 12-gauge semi-automatic shotgun was used. In the 4-week study, one shot in the first week, two consecutive shots in second week, three shots in third week, and five shots in fourth week were fired by six participants. Samples were taken from MAE, CA, and AAECA of both ears and common GSR sampling sites. The characteristic 3-component structure (Pb/Sb/Ba) of the samples was analyzed by SEM/EDX. Right CA was the most suitable area for sampling, which might be attributed to posture of body during targeting. Right ear was the most suitable area to take samples from CA or MAE in 3-shot group. Besides, left AAECA in 1- and 2-shot groups and the left MAE in 5-shot group were the most suitable areas for GSR sampling. In conclusion, ear seems to be a valuable alternative for detection of GSR particles, due to its complex anatomical structure potentially preventing loss of GSR with daily cleaning. Findings suggested that crime scene investigation teams and criminal laboratory staff should consider ear as a valuable alternative for GSR detection.

Prevalence of organic gunshot residues in police vehicles

The present study investigated the organic gunshot residue (OGSR) background level of police vehicles in Switzerland. Specimens from 64 vehicles belonging to two regional police services were collected and analysed by LC-MS in positive mode. The driver's and back seats were sampled separately to monitor potential differences between locations and to assess the risks of a suspect being contaminated by OGSR during transportation to a police station. The results showed that most of the 64 vehicles were uncontaminated (44 driver's seats and 38 back seats respectively). Up to six of the seven targeted compounds were detected in a single sample, once on a driver's seat and twice on back seats. The contamination frequency generally decreased as the number of compounds detected together increased. The amounts detected were in the low ng range and less than amounts generally detected just after discharge on a shooter. Our data indicated that detecting a combination of four or more compounds on a police vehicle seat appears to be a relatively rare occurrence. The background contamination observed was most probably due to secondary transfer from police officers (e.g. through recent participation in a shooting session or firearm manipulation) or from firearms stored in the vehicles. The present results might be used as a recommendation to minimize contact of a suspect with contaminated surfaces if OGSR is implemented in routine work in parallel to IGSR analysis.

Reviewing Research Trends—A Scientometric Approach Using Gunshot Residue (GSR) Literature as an Example

The ability to manage, distil and disseminate the significant amount of information that is available from published literature is fast becoming a core and critical skill across all research domains, including that of forensic science. In this study, a simplified scientometric approach has been applied to available literature on gunshot residue (GSR) as a test evidence type aiming to evaluate publication trends and explore the interconnectivity between authors. A total of 731 publications were retrieved using the search engine ‘Scopus’ and come from 1589 known authors, of whom 401 contributed to more than one research output on this subject. Out of the total number of publications, only 35 (4.8%) were found to be Open Access (OA). The Compound Annual Growth Rate (CAGR) for years 2006 and 2016 reveals a much higher growth in publications relating to GSR (8.0%) than the benchmark annual growth rate of 3.9%. The distribution of a broad spectrum of keywords generated from the publications confirms a historical trend, in particular regarding the use of analytical techniques, in the study of gunshot residue. The results inform how relevant information extracted from a bibliometric search can be used to explore, analyse and define new research areas.

Comparison of three collection methods for the sodium rhodizonate detection of gunshot residues on hands

The aim of this study was to compare three gunshot residue (GSR) collection methods used in conjunction with chemographic detection applied by different regional Swiss police services. The specimens were collected from the hands of a shooter with either filter paper (Filter method) or adhesive foil. The adhesive foil was then either applied against photographic paper during visualisation (AF Photo method) or coated with a layer of polyvinyl alcohol (AF PVAL method). The experiments involved two conditions of the examined hands, i.e. dry and humidified. The residues were revealed using the sodium rhodizonate test (SRT). Preliminary tests assessing the possibility of conducting a confirmatory Scanning Electron Microscopy coupled to Energy Dispersive X-ray spectroscopy (SEM/EDX) analysis after the chemographic test were performed on a number of specimens by cutting positive spots and mounting them on stubs. Obtained results were compared in terms of effectiveness - number of positive spots, time requirements, quality of subsequent SEM-EDX analysis, ease of use and cost. The Filter method generally yielded a high-quality detection with both dry and humidified hands, as well as a simple, quick and efficient confirmation by SEM/EDX. The AF Photo performed well on dry hands, but not on humidified hands. The AF PVAL method performance was lower compared to the other methods in both examined conditions of the hands. The SEM/EDX analysis showed that the Filter and AF PVAL method provided satisfactory results when a sufficient carbon coating thickness was applied to the cuttings. It was also observed that the thinner the PVAL layer, the better the quality of the spectra and obtained images in SEM/EDX. Furthermore, the surface of the photographic paper did not seem to be conductive, even after the application of a thick layer of carbon. In conclusion, the Filter method gave the best overall results, but its application required slightly more time and expertise than the two other methods.

Source apportionment and health risk assessment of organic constituents in fine ambient aerosols (PM2.5): A complete year study over National Capital Region of India

Fine ambient aerosols (PM_2.5) levels in the atmosphere are continuously worsening over Delhi and National Capital Region (NCR) of India. Complete source profiles are required to be assessed for implementation of proper mitigation measures over the NCR. In this study, emission sources of PM_2.5 are reported for the NCR of India for samples collected during December 2016 to December 2017 at three sampling sites in Delhi, Uttar Pradesh and Haryana. Organic constituents (n-alkanes, isoprenoid hydrocarbons, polycyclic aromatic hydrocarbons, phthalates, levoglucosan and n-alkanoic acids) in PM_2.5 were measured to apportion the sources over the study area. Source apportionment of PM_2.5 was performed using organic constituents by Positive Matrix Factorization (PMF) and Principal Component Analysis (PCA). Health risk associated with organic pollutants [PAHs and carcinogen BEHP bis(2-ethylhexyl) phthalate] demonstrated the threat of PM_2.5 exposure via inhalation. Transport pathways of air masses were evaluated using 3-day backward trajectories and observed that some air masses originated from local sources along with long-range transport which influenced the PAHs concentration during most of the study period over the NCR. PMF and PCA resulted in the five major emission sources [vehicular emissions (32.2%), biomass burning (30%), cooking emissions (16.8%), plastic burning (13.4%), mixed sources (7.6%) including biogenic and industrial emissions] for PM_2.5 over the sampling sites. The present study reveals that transport sector is a major source to be targeted to reduce the vehicular emissions and consequent health risks associated with organic pollutants especially PAHs.

Short range shooting distance estimation using variable pressure SEM images of the surroundings of bullet holes in textiles

Modifications of cotton and polyester textiles due to shots fired at short range were analyzed with a variable pressure scanning electron microscope (VP-SEM). Different mechanisms of fiber rupture as a function of fiber type and shooting distance were detected, namely fusing, melting, scorching, and mechanical breakage. To estimate the firing distance, the approximately exponential decay of GSR coverage as a function of radial distance from the entrance hole was determined from image analysis, instead of relying on chemical analysis with EDX, which is problematic in the VP-SEM. A set of backscattered electron images, with sufficient magnification to discriminate micrometer wide GSR particles, was acquired at different radial distances from the entrance hole. The atomic number contrast between the GSR particles and the organic fibers allowed to find a robust procedure to segment the micrographs into binary images, in which the white pixel count was attributed to GSR coverage. The decrease of the white pixel count followed an exponential decay, and it was found that the reciprocal of the decay constant, obtained from the least-square fitting of the coverage data, showed a linear dependence on the shooting distance.

Analysis of gunshot residues as trace in nasal mucus by GFAAS

When a gun is fired, the majority of gunshot residues are deposited on the shooter's hands. But these residues disappear through contact with surfaces or washing. Therefore, the maximum time frame to find GSR on a suspect's hands is 8h. The mucus, inside of a nostril, forms a surface layer where they are trapped foreign particles. In this way, mucus inside of a gunshot suspect's nostrils could act like an adhesive medium to stick on it gaseous particles from a gunshot. In this study, the presence of GSR in nasal mucus and its residence time is examined. A new procedure for the sampling of possible gunshot residue accumulated in the nasal mucus is designed. Samples are taken with cotton swabs moistened with a solution of EDTA and, after an acid digestion, are analysed by graphite furnace atomic absorption spectrometry. In addition, samples of hands are taken for comparison purposes. GSR recovery has been successful. The concentration of GSR in nasal mucus is found to be lower than on the hands, but with a longer residence time. Thus, it is possible to expand the sampling time of a suspect also, as nasal mucus cannot be contaminated by handling weapons.

A FT-NIR spectroscopy methodology to estimate firing distance based on the direct analysis of the bullet impact surface

Fourier transform near-infrared spectroscopy is proposed as a new methodology to estimate firing distance based on the direct analysis of organic components of gunshot residues.

Detection of Pb, Ba, and Sb in Blowfly Larvae of Porcine Tissue Contaminated with Gunshot Residue by ICP OES

Lead (Pb), barium (Ba), and antimony (Sb) concentrations were monitored in flies larvae (immatureChrysomya albiceps) contaminated with gunshot residue (GSR) from .40 caliber Taurus pistols during the period of 2 to 12 days after the death of a female pig in decomposition, during the winter, under the influence of rain and high relative humidity. The samples were also analyzed by the colorimetric test using sodium rhodizonate (Feigl-Suter reaction). It was possible to detect and quantify the three metals of interest by inductively coupled plasma-optical emission spectrometry (ICP OES), where the concentrations of all three elements kept practically constant during the putrefaction stage. Minimum ([Pb] = 382.26 μg L−1; [Ba] = 140.50 μg L−1; [Sb] = 39.18 μg L−1) and maximum ([Pb] = 522.66 μg L−1; [Ba] = 190.30 μg L−1; [Sb] = 56.14 μg L−1) concentrations were found during the third and fifth days after death, respectively. ICP OES presented higher sensitivity in metals determination when compared to the conventional colorimetric test, which showed negative result for the GSR extracts obtained from the immatureChrysomya albiceps.

Skin permeation of organic gunshot residue: implications for sampling and analysis

Traditional gunshot residue (GSR) analysis is based on detection of particulates formed from metals found in the primer. Recent concerns regarding the interpretation of GSR evidence has led to interest in alternatives such as the organic constituents (organic gunshot residue, OGSR) found in propellants. Previous work has shown OGSR to be detectable on hands for several hours after a firing event, and given the lipophilic nature of these compounds, it was expected that losses due to secondary transfer (an issue with GSR particulates) would be negligible. However, other loss mechanisms have been identified, specifically skin permeation and evaporation. This paper describes experimental and modeling studies used to elucidate characteristics of skin permeation of 5 compounds present in OGSR. Pharmaceutical methods were adapted to characterize skin permeation using a skin surrogate and Franz diffusion cells. The amount of compounds deposited on skin after an authentic firing event (1 and 2 shots) was experimentally determined and applied for the permeation experiments. A fully validated selected ion monitoring GC/MS method was developed for quantitative analysis, and easily accessible online tools were employed for modeling. Results showed that OGSR residues should be detectable on skin for many hours after a firing event of as few as one or two shots, with detection capability being a function of the efficacy of sampling and sample preparation and the instrumental method employed. The permeation rates of the OGSR compounds were sufficiently different to suggest the potential to develop methods to approximate time-since-deposition.

Detection of gunshot residues using mass spectrometry

In recent years, forensic scientists have become increasingly interested in the detection and interpretation of organic gunshot residues (OGSR) due to the increasing use of lead- and heavy metal-free ammunition. This has also been prompted by the identification of gunshot residue- (GSR-) like particles in environmental and occupational samples. Various techniques have been investigated for their ability to detect OGSR. Mass spectrometry (MS) coupled to a chromatographic system is a powerful tool due to its high selectivity and sensitivity. Further, modern MS instruments can detect and identify a number of explosives and additives which may require different ionization techniques. Finally, MS has been applied to the analysis of both OGSR and inorganic gunshot residue (IGSR), although the "gold standard" for analysis is scanning electron microscopy with energy dispersive X-ray microscopy (SEM-EDX). This review presents an overview of the technical attributes of currently available MS and ionization techniques and their reported applications to GSR analysis.

A novel method for the identification of inorganic and organic gunshot residue particles of lead-free ammunitions from the hands of shooters using scanning laser ablation-ICPMS and Raman micro-spectroscopy

SLA-ICPMS and Raman micro-spectroscopy has been applied to the characterization of GSR using modified tape lifts.

Chemical and morphological study of gunshot residue persisting on the shooter by means of scanning electron microscopy and energy dispersive X-ray spectrometry

Persistence of gunshot residue (GSR) simultaneously collected from hands, face and hair, and clothing of the shooting person was examined. Samples were collected from five shooters in nine time intervals after a single shoot with a Luger 9 mm pistol, in the range of 0-4 h and examined with scanning electron microscopy and energy dispersive X-ray spectrometry. Numbers of particles, frequencies of occurrence of certain compositions of particles, and their sizes in function of the time intervals were inspected. The greatest numbers of particles were observed in samples collected from hands right after shooting, but they decrease quickly with time. In samples collected from the face smaller initial numbers of particles were found, but they lasted at a similar level longer. The estimated half-life times of particles were less than 1 h for samples taken from the hands, over 1 h for clothing and about 2-3 h for the face. In samples collected at longer intervals after shooting, there were particles present of small sizes and irregular shapes. The results demonstrate that including evidence collected from the suspect's face and hair may increase the probability of detection of GSR in cases when the suspect has not been apprehended immediately after the investigated incident.