Assessing the utility of LIBS in the reconstruction of firearm related incidents

Assessing the shooting distance of lead-free ammunition regardless of composition using Laser Induced Breakdown Spectroscopy

At present, it is challenging to accurately determine firearm shooting distances in the case that lead-free ammunition is involved, largely because different manufacturers use different primer compositions. Laser-induced breakdown spectroscopy (LIBS) allows the simultaneous detection of multiple elements with high sensitivity and so may represent a solution to this problem. Previous studies have, in fact, demonstrated that LIBS can be used to determine shooting distances when working with gunshot residues from conventional ammunition based on scanning fabric surfaces. The present study confirms that the shooting distance can be ascertained using LIBS to detect copper originating from the ammunition casing and projectile but not the primer on fabric surfaces. This estimation can be performed regardless of the primer composition of lead-free ammunition.

Key points

Evaluation of gunshot residue from lead-free ammunition using scanning electron microscopy-energy dispersive X-ray analysis indicated that 40% of the particles contained copper.The iForenLIBS system allowed the detection of copper-containing particles on fabric surfaces after firing at different distances with high sensitivity.Laser-induced breakdown spectroscopy can determine the shooting distance of lead-free ammunition through copper detection even in ammunition that does not used this element in the primer.This technique can generate density maps allowing the evaluation of short, medium, and long-range shooting distances.

Identification and highly selective differentiation of organic gunshot residues utilizing their elemental and molecular signatures

Firearm related evidence is of great significance to forensic science. In recent years, many researchers have focused on exploring the probative value of organic gunshot residue (OGSR) evidence, which is often bolstered by many factors including recoverability. In addition, OGSR analysis has shown the potential to achieve differentiation between OGSRs generated from various ammunition brands and/or calibers. Raman spectroscopy is a vibrational spectroscopic technique which has been used in the past for gunshot residue analysis-including OGSR specifically. Raman spectroscopy is a nondestructive, highly-selective, simple, and rapid technique which provides molecular information about samples. LIBS or Laser-Induced Breakdown Spectroscopy is a simple, robust, and rapid analytical method which requires minimal to no sample preparation and a small amount of sample for analysis. LIBS provides information on the elemental compositions of samples. In this study, Raman spectroscopy and LIBS were used together in sequence in an attempt to achieve the specific identification and characterization of OGSR particles from ammunition types which were closely related. The main goal was to determine if this method had the potential to differentiate between various ammunition types of the same caliber and produced by the same manufacturer, and generated under identical firing conditions. High-resolution optical microscopy documented the OGSR particles' morphologies and Raman spectroscopy was used to identify particles as OGSRs. Finally, LIBS analysis of the OGSR particles was carried out. Advanced chemometric techniques were shown to allow for very successful differentiation between the OGSR samples analyzed.

The influence of a possible contamination of the victim's clothing on the estimation of shooting distance in mutual shooting incidents

One of the tools for estimating shooting distance is examination of smokeless powder particle distribution on the target area. Components of the powder that are utilized for this purpose are nitrite anions. The traditional method for detecting nitrite anions mainly involves applying the Griess Test. A known-distance test firing is performed with shootings done at several distances from different targets. The color pattern corresponding to nitrite anions from the crime scene is then compared to the patterns obtained from known distances of the test firing. When a mutual shooting takes place at the crime scene, and a shooter also becomes a shootout victim, it is possible that when examining the shooter as a victim (i.e. target), additional nitrite-containing particles, resulting from his/her shooting, will be present on the shooter-victim clothing. This kind of addition may affect the estimation and practically give a shorter-distance estimation comparing to the actual distance. In this paper, an experimental setup was designed in order to understand if nitrite-containing particles were added to a victim as a consequence of him/her being also a shooter. All of the experiments were predominantly designed to try and minimize the effects of other influencing factors and variables in order to examine if the additions resulting from the firing action affect distance estimation. The experiments involved various types of pistols and distances. The results show that in such a scenario, there are marginal additions of nitrite signals on the victim's shirt. Although the forensic expert's final assessment was within the tolerance interval at all distances, caution should be exercised when attempting to estimate shooting distances in scenarios where the victim also shoots.

How may analysis of an inner layer of clothing affect the scene reconstruction in a shooting incident?

Shooting distances are utilized, among other forensic input, to position shooters and victims at the crime scene. Shooting distance estimation is done, mainly under the knowledge or assumption, that no intervening object has been present between the target and the muzzle. In the analysis of clothing items of victims that have been involved in a shootout, it is assumed that most of the marks and materials are left on the outer layers of clothing. The outer layers, not the inner layers, are the first objects the bullet meets. Consequently, undergarments are often disregarded for shooting distance estimation as they are not assumed to contain sufficient information in the form of marks or materials. In light of the above, it is the standard operating procedure in some forensic laboratories not to examine underwear. In this case report, the forensic examiners started by the implementation of standard operating procedures, thus examining only the outer layers of clothing of the victims. A thorough visual examination of the various clothing items led a forensic examiner to decide to extend the standard operating procedures, and try and examine an undergarment of one of the victims as well. The interpretation of examination products led to different scene reconstruction, one that may have led to a different interpretation of evidence. In this case, simply sticking to standard operating procedures might have led to less accurate crime scene reconstruction. This case report comes to stress the importance of thorough inspection of forensic exhibits, and scientific and critical thinking about different scenarios. Moreover, it demonstrates how the case-by-case approach in shooting distance estimation may add meaningful information, and eventually-bring us even closer to the truth.

Analysis of primer gunshot residue particles by laser induced breakdown spectroscopy and laser ablation inductively coupled plasma mass spectrometry

This study reports novel approaches for the detection of gunshot residues (GSR) from the hands of individuals using Laser-Induced Breakdown Spectroscopy (LIBS) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). The methods' performance was evaluated using 159 GSR standard and authentic samples. Forty specimens generated from characterized microparticles were used as matrix-matched primer gunshot residue (pGSR) standards to examine the elemental profiles of leaded and lead-free residues, compared to SEM-EDS and solution-ICP-MS. Also, 119 authentic skin samples were analyzed to estimate error rates. Shooter samples were correctly classified into three categories based on their elemental composition (leaded, lead-free, or mixed pGSR). A total of 60 non-shooter samples were used to establish background thresholds and estimate specificity (93.4% for LA-ICP-MS and 100% for LIBS). All the authentic leaded items resulted in the detection of particle(s) with composition characteristic of pGSR (Pb-Ba-Sb), as observed by simultaneous elemental identification of target analytes at the exact ablation times and locations. When considering the pre-characterized elemental composition of these primers as the "ground truth", LA-ICP-MS resulted in 91.8% sensitivity (true positive rate), while LIBS resulted in 89.2% sensitivity. Particles containing Ba, Bi, Bi-Cu-K, and Cu-Ti-Zn were found in the lead-free residues. Identification of lead-free GSR proved more challenging as some of these elements are common in the environment, resulting in 85.2% sensitivity for LA-ICP-MS and 44.4% for LIBS. Overall accuracies of 94.9% and 88.2% were obtained for the LA-ICP-MS and LIBS sets, respectively. LA-ICP-MS provided an additional level of confidence in the results by its superior analytical capabilities, complementing the LIBS chemical profiles. The laser-based methods provide rapid chemical profiling and micro-spatial information of gunshot residue particles, with minimal destruction of the sample and high accuracy. Chemical mapping of 25 micro-regions per sample is possible in 2-10 minutes by LIBS and LA-ICP-MS, offering new tools for more comprehensive forensic case management and quick GSR screening in environmental and occupational sciences.