Electrochemical Detection of Gunshot Residue for Forensic Analysis: A Review

Strategies for assessing the limit of detection in voltammetric methods: comparison and evaluation of approaches

The realm of analytical chemistry continues to struggle with defining and evaluating the limit of detection in analytical methods in the sense that a multitude of definitions, criteria, caveats, and methods have been proposed, developed, and adopted across disciplines. The last decade has seen a surge in the growth of electrochemical methods and studies in the field of forensic science and forensic chemistry. While many disciplines within forensic science have established method validation guidelines, the historical and current lack of electrochemical methods within forensic laboratories throughout the United States has left a major gap in knowledge, inhibiting the adoption and utilization of electrochemistry, which may serve as a powerful tool in many subdisciplines of forensics. As such, this work begins this discussion by focusing first on the limit of detection (LOD), with application toward both qualitative and quantitative methods. Both inorganic (ferrocyanide and lead) and organic (diphenylamine, naltrexone, and acetaminophen) target analytes were analyzed via two common voltammetry methods: cyclic voltammetry and square-wave voltammetry. The LOD for each analyte was estimated and/or calculated following a variety of literature-described methods and compared. The accuracy and reliability of these LOD characteristics based on the experimental data is described herein along with suggestions and recommendations. This manuscript is intended to compare the resulting LOD values from various methods and provide a starting point for the incorporation of electrochemistry into the forensic science laboratory, beginning a focused discussion on the development of validation guidelines and parameters needed for the adoption of this technology in forensic laboratories in order to meet the standards required by the criminal justice system.

Voltammetric co-determination of lead and copper in gunshot residue based on iron oxide particle/spent coffee grounds-modified electrode

A novel electrochemical sensor was developed for the detection of lead (Pb) and copper (Cu) ions using spent coffee grounds decorated with iron oxide particles (FeO/SCG). The FeO-decorated SCG was used to modify a glassy carbon electrode (GCE). FeO, SCG, and FeO/SCG were characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The electrochemical properties of the modified electrode were characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The electrode modifications increased the active surface area and electron transfer and enhanced the accumulation of the target analyte. In the optimal condition, the developed sensor showed linear ranges of 1.0 µg L-1-0.05 mg L-1 and 0.05 mg L-1-0.8 mg L-1 for Pb2+ and 5.0 µg L-1-0.1 mg L-1 and 0.1 mg L-1-0.8 mg L-1 for Cu2+. The limit of detection (LOD) was 1.0 µg L-1 for Pb2+ and 2.4 µg L-1 for Cu2+. The developed sensor was successfully applied to determine Pb2+ and Cu2+ in bullet holes. The results were in good agreement with those obtained by inductively coupled plasma optical emission spectrometry (ICP/OES).

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.

Comparison of portable and benchtop electrochemical instruments for detection of inorganic and organic gunshot residues in authentic shooter samples

Analysis of gunshot residue currently lacks effective screening methods that can be implemented in real time at the crime scene. Historically, SEM-EDS has been the standard for analysis; however, advances in technology have brought portable instrumentation to the forefront of forensic science disciplines, including the screening of GSR. This study proposes electrochemical methods with disposable screen-printed carbon electrodes for GSR screening at the laboratory and points of care due to their rapid, cost-efficient, and compact platform. GSR residues were extracted from typical aluminum/carbon adhesive collection stubs and analyzed via square-wave anodic stripping voltammetry. Benchtop and portable electrochemical instruments were compared for the assessment and classification of authentic shooter samples by monitoring a panel of inorganic and organic GSR elements and compounds including lead, antimony, copper, 2,4-dinitrotoluene, diphenylamine, nitroglycerin, and ethyl centralite. The evaluation included the assessment of figures of merit and performance measures from quality controls, nonshooter, and shooter data sets. Samples collected from the hands of 200 background individuals (nonshooters), and shooters who fired leaded ammunition (100) and lead-free ammunition (50) were analyzed by the benchtop and portable systems with accuracies of 95.7% and 96.5%, respectively. The findings indicate that electrochemical methods are fast, sensitive, and specific for the identification of inorganic and organic gunshot residues. The portable potentiostat provided results comparable with the benchtop system, serving as a proof-of-concept to transition this methodology to crime scenes for a practical and inexpensive GSR screening that could reduce backlogs, improve investigative leads, and increase the impact of gunshot residues in forensic science.

Magnetic nanoparticle modified electrodes for voltammetric determination of propellant stabiliser diphenylamine

The overall aim of the work was to advance electrochemical devices capable of analysis of forensically relevant residues using rapid electrochemical sensor technology. In order to achieve this, electrochemical detection of the propellant stabiliser diphenylamine (DPA) was achieved via voltammetry with signal enhancement realised in the presence of iron oxide nanoparticle modified transducers. This allowed both mechanistic and analytical evaluation with the aim to achieve the required selectivity and sensitivity for reliable detection. DPA electrochemistry was examined at glassy carbon electrodes in aqueous (3:7 methanol: sodium acetate pH 4.3) electrolyte via potential sweeping, with an irreversible wave at E_p = 0.67 V vs. Ag/AgCl. The diffusion coefficient (D) for the oxidation process was calculated as 1.43 × 10^-6 cm² s^-1 with αn_a = 0.7. DPA electrochemistry in a non aqueous methanol/acetonitrile electrolyte resulted in a D value of 5.47 × 10^-8 cm² s^-1 with αn_a = 0.5. Electrochemical preparation of magnetic iron oxide nanoparticles was achieved via electrooxidation of an iron anode in the presence of an amine surfactant followed by characterisation with SEM/EDX, XRD, FTIR and thermal analysis. A surface confined layer of these magnetic nanoparticles served to positively influence the response to DPA while impeding formation of surface confined oxidation products, with generation of an improved analytical signal - sensitivity 1.13× 10^-3 A cm^-2 mM^-1 relative to bare electrode response (9.80 × 10^-4 A cm^-2 mM^-1) over the range 0.5-50 μM DPA using differential pulse voltammetry, with LOD 3.51 × 10^-6 M and LOQ 1.17 × 10^-5 M. Real sample analysis involved recovery and differential pulse voltammetry of unburnt and burnt gunshot residue with DPA qualitative and quantitative analysis.

Electrochemical profiling and liquid chromatography-mass spectrometry characterization of synthetic cathinones: From methodology to detection in forensic samples

The emergence of new psychoactive drugs in the market demands rapid and accurate tools for the on-site classification of illegal and legal compounds with similar structures. Herein, a novel method for the classification of synthetic cathinones (SCs) is presented based on their electrochemical profile. First, the electrochemical profile of five common SC (i.e., mephedrone, ethcathinone, methylone, butylone, and 4-chloro-alpha-pyrrolidinovalerophenone) is collected to build calibration curves using square wave voltammetry on graphite screen-printed electrodes (SPEs). Second, the elucidation of the oxidation pathways, obtained by liquid chromatography-high-resolution mass spectrometry, allows the pairing of the oxidation products to the SC electrochemical profile, providing a selective and robust classification. Additionally, the effect of common adulterants and illicit drugs on the electrochemical profile of the SC is explored. Interestingly, a cathodic pretreatment of the SPE allows the selective detection of each SC in presence of electroactive adulterants. Finally, the electrochemical approach is validated with gas chromatography-mass spectrometry by analyzing 26 confiscated samples from seizures and illegal webshops. Overall, the electrochemical method exhibits a successful classification of SC including structural derivatives, a crucial attribute in an ever-diversifying drug market.

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.

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.

Particle Analysis for the Detection of Gunshot Residue (GSR) in Nasal Samples Using Scanning Laser Ablation and Inductively Coupled Plasma-Mass Spectrometry (SLA-ICPMS)

Currently, aluminum stub with carbon adhesive devices are used to collect inorganic gunshot residues (GSR) from the hands of a shooter. In an ideal shooting case, the gunshot particles do not persist for more than 2 h in the hands of the shooter, provided that the hands have not been washed. However, for forensic analysis and inference, the extended persistence of GSR would be desirable. This study investigates a novel GSR sampling and detection protocol. Sampling was performed in the nostrils using swab devices impregnated in ethylenediaminetetraacetic acid (EDTA). The GSRs persisted for longer periods in nasal mucus than on the hands, and particles were detected 6 h after shooting occurred. The analytical determination was conducted by scanning laser ablation-inductively coupled plasma-mass spectrometry (SLA-ICPMS) which enable the identification of the number of particles and their elemental composition. Seventeen isotope signals corresponding to ¹³ C, ²⁰⁵ Tl and 15 analytes that are usually associated with the composition of GSR residues were monitored: ²⁷ Al, ²⁹ Si, ³¹ P, ³³ S, ³⁵ Cl, ³⁹ K, ⁴⁴ Ca, ⁵⁷ Fe, ⁶⁰ Ni, ⁶³ Cu, ⁶⁶ Zn, ¹¹⁸ Sn, ¹²¹ Sb, ¹³⁷ Ba, and ²⁰⁸ Pb. The SLA technique enabled the reduction of the swab analysis time to 40 min. The effectiveness of this methodology was evaluated with two types of firearms: a pistol and a shotgun. The results indicated that the methodology proposed for the analysis of the nasal GSR was effective and that it can improve or complement the forensic analyses and inferences presented in a court.

Development of an In-Field Method for the Detection of Barium in Various Water Samples Using Differential Pulse Anodic Stripping Voltammetry

This work presents a reliable, cost-effective, rapid and in-field voltammetric method for the detection of barium. The optimized method consists of an ultrathin mercury film deposited in situ on a glassy carbon electrode in dilute potassium chloride without deoxygenation, using differential pulse anodic stripping voltammetry (DP-ASV). Application of the method allowed for the quantitative determination of barium concentration in a variety of waters and brake pad dust samples. Comparative analysis of sample results from DP-ASV with inductively coupled plasma mass spectroscopy (ICP-MS) showed a mean percent difference of 1.8%.

Bio-Integrated Wearable Systems: A Comprehensive Review

Bio-integrated wearable systems can measure a broad range of biophysical, biochemical, and environmental signals to provide critical insights into overall health status and to quantify human performance. Recent advances in material science, chemical analysis techniques, device designs, and assembly methods form the foundations for a uniquely differentiated type of wearable technology, characterized by noninvasive, intimate integration with the soft, curved, time-dynamic surfaces of the body. This review summarizes the latest advances in this emerging field of "bio-integrated" technologies in a comprehensive manner that connects fundamental developments in chemistry, material science, and engineering with sensing technologies that have the potential for widespread deployment and societal benefit in human health care. An introduction to the chemistries and materials for the active components of these systems contextualizes essential design considerations for sensors and associated platforms that appear in following sections. The subsequent content highlights the most advanced biosensors, classified according to their ability to capture biophysical, biochemical, and environmental information. Additional sections feature schemes for electrically powering these sensors and strategies for achieving fully integrated, wireless systems. The review concludes with an overview of key remaining challenges and a summary of opportunities where advances in materials chemistry will be critically important for continued progress.

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.