Interpol review of the analysis and detection of explosives and explosives residues

A survey of explosive traces in public places

Interpretation and evaluation of trace explosives evidence requires practitioner understanding of factors including transfer, persistence, and environmental prevalence. This study builds on previous work and studies the contemporary prevalence of organic high explosives and inorganic ions of explosives significance in public places. 450 swab and vacuum samples were collected from across Great Britain. Analysis was conducted using liquid chromatography-high resolution mass spectrometry and ion chromatography-mass spectrometry to screen for a wider number of explosives analytes with a higher degree of selectivity and with lower limits of detection than previous studies. Analytes screened for included military high explosives, organic peroxide explosives, and inorganic ions of explosives significance. Only eight low nanogram level traces of organic explosives (HMX, NG, PETN, and RDX) were detected. The results indicate that high explosives traces remain uncommon in the public environment and transport network. Due to the low prevalence, these results strengthen the association between the detection of a trace and explosives activity, and assist the practitioner in assigning significance. Many inorganic ions (ammonium, calcium, chloride, magnesium, nitrate, nitrite, potassium, sodium, and sulfate) were detected at milligram or sub-milligram quantities. They are common in the environment, naturally occurring, and used commercially. Interpreting the general significance when detecting traces of common inorganic species is challenging. Barium, chlorate, perchlorate, strontium, and thiocyanate were not detected and are therefore more uncommon, strengthening the association between detection and explosives activity.

Systematic analysis of post-blast organic traces in soil, application of color tests, TLC, GC-MS, and ITMS

The post-blast trace analysis of explosives is inherently complex, requiring the separation and identification of multiple unknown compounds within contaminated matrices. The study explores modified methods for the extraction, analysis, and detection of post-explosion residues, of the three main organic explosives groups, focusing on key explosives such as NC (Nitrocellulose), NG (Nitroglycerin), Tetryl, TNT, RDX, PETN, as well as sheet and plastic explosives. A comprehensive protocol was developed, involving sample extraction, filtration, and clean-up processes, followed by analysis using a range of analytical techniques. The study introduces an optimized spot test scheme and proposes novel TLC mobile phases for improved separation and characterization of explosives. In addition, a new GC/EI-MS method was developed for the selective detection of PETN, overcoming common thermal degradation issues. The presented GC-MS findings of this study demonstrate the significant advancements made in overcoming these obstacles, providing a clearer understanding of the analytical improvements enabling its reliable detection. Ion Trap Mobility Spectrometry (ITMS) was employed for evaluating the concentration-time decay of explosive residues on hand swabs, revealing that NG exhibited higher persistence than TNT. The study determined the Minimum Detection Limits (MDL) to be 2 ng for TNT, 3 ng for RDX and NG, and 4 ng for PETN. The research underscores that the effectiveness of explosive detection methods depends on aligning techniques with the physical and chemical properties of the analytes. Results also emphasize the importance of using confirmatory techniques beyond TLC and spot tests, as these methods are presumptive analyses and insufficient for independent compound identification. The proposed TLC mobile phases achieved significant resolution of the studied explosives, achieving significantly improved Rf values for a wide range of explosives across different groups, confirming their suitability for forensic applications. This study establishes that GC/EI-MS with its basic set, can be used for PETN analysis, while ITMS offers a fast and reliable approach for field detection and persistence studies. Collectively, these findings contribute to the analytical protocols for forensic explosive residue analysis.

Detection of organic explosive residues from outdoor detonations using confocal Raman microscopy

The detection of post-blast residues in the aftermath of an explosion involving organic explosives with spectroscopic techniques is challenging as, typically, no microscopically visible unreacted particles remain after the explosion. However, some low-order explosions may leave visible particles behind, as well as the presence of significant amounts of unreacted material. In this study, four authentic open-air detonations using two simulated improvised explosive devices (IEDs) containing a mixture of military explosives (TNT and RDX), and two IEDs containing smokeless powder were conducted. The various materials they contained, including plastic, wood, and metal, were swabbed and extracted with acetone to create post-blast liquid extracts. The extracts were then dried and examined using confocal Raman microscopy, alongside a 50 ppm reference mixture of smokeless powder constituents, which was created to evaluate the effects of Raman scattering within the full smokeless powder mixture. Smokeless powder constituents, such as ethyl centralite, diphenylamine, nitroglycerin, and dibutyl phthalate, were successfully identified by comparison to the reference mixture on most substrates, with the exception of the paint stick (wood) substrate. TNT/RDX was also able to be identified in the extracts, with RDX crystals being observed in some dried extracts after solvent evaporation. However, the detection of TNT/RDX in the second detonation was unsuccessful, possibly due to an explosive chain reaction that was highly efficient. No trends were seen in substrate affinity for TNT/RDX. The challenges and benefits with the developed methodology for the detection of organic explosive residues from a variety of substrates are discussed in detail.

Characterization of 2-Butanone Peroxide Oligomeric Profiles and Their Associated Gas-Phase and Solution-Phase Rearrangement Products by Electrospray Ionization Mass Spectrometry for Forensic Applications

2-Butanone peroxide (also known as methyl ethyl ketone peroxide, MEKP) has applications as a cross-linker in the chemical industry and is also encountered as a homemade primary high explosive; therefore, it is of interest to both process chemists and forensic examiners. Specifically for forensic applications, we demonstrate that when traditional synthetic procedures, available to any hobbyist, are utilized to generate MEKP, oligomeric peroxide units (n ≤ 12), along with several other oligomeric byproduct distributions, are readily observed by liquid chromatography-mass spectrometry (LC-MS). These oligomeric byproducts correspond to the formation of methyl/ethyl ketone end group(s) at the oligomer end group (i.e., loss of ethanol(s) and/or methanol(s) from the oligomer termini). Based on the interpretation of the MS and MS/MS behavior along with the characterization of newly generated terminal alkyl ketone products, we propose that these byproducts are consistent with a Hock-like rearrangement of the primary MEKP distribution in the acidified reaction medium. Following a procedure for homemade preparation, triplicate lots were synthesized. Unique oligomeric and byproduct distributions provided discriminatory power between the synthetic lots. Furthermore, the distributions of MEKP oligomers and the various byproducts in the initiated MEKP match the intensity distributions observed in the intact material with remarkable accuracy. This observation suggests that the postinitiation residue of MEKP could be associated or dissociated from a separately collected intact material obtained during an investigation by examining these oligomeric and byproduct profiles.

Time-Efficient SNR Optimization of WMS-Based Gas Sensor Using a Genetic Algorithm

This paper presents the description of the wavelength modulation spectroscopy (WMS) experiment, the parameters of which were established by use of the Artificial Intelligence (AI) algorithm. As a result, a significant improvement in the signal power to noise power ratio (SNR) was achieved, ranging from 1.6 to 6.5 times, depending on the harmonic. Typically, optimizing the operation conditions of WMS-based gas sensors is based on long-term simulations, complex mathematical model analysis, and iterative experimental trials. An innovative approach based on a biological-inspired genetic algorithm (GA) and custom-made electronics for laser control is proposed. The experimental setup was equipped with a 31.23 m Heriott multipass cell, software lock-in, and algorithms to control the modulation process of the quantum cascade laser (QCL) operating in the long-wavelength-infrared (LWIR) spectral range. The research results show that the applied evolutionary approach can efficiently and precisely explore a wide range of WMS parameter combinations, enabling researchers to dramatically reduce the time needed to identify optimal settings. It took only 300 s to test approximately 1.39 × 1032 combinations of parameters for key system components. Moreover, because the system is able to check all possible component settings, it is possible to unquestionably determine the operating conditions of WMS-based gas sensors for which the limit of detection (LOD) is the most favorable.

Sensitive Detection of Trace Explosives by a Self-Assembled Monolayer Sensor

Fluorescence probe technology holds great promise in the application of trace explosive detection due to its high sensitivity, fast response speed, good selectivity, and low cost. In this work, a designed approach has been employed to prepare the TPE-PA-8 molecule, utilizing the classic aggregation-induced emission (AIE) property of 1,1,2,2-tetraphenylethene (TPE), for the development of self-assembled monolayers (SAMs) targeting the detection of trace nitroaromatic compound (NAC) explosives. The phosphoric acid acts as an anchoring unit, connecting to TPE through an alkyl chain of eight molecules, which has been found to play a crucial role in promoting the aggregation of TPE luminogens, leading to the enhanced light-emission property and sensing performance of SAMs. The SAMs assembled on Al2O3-deposited fiber film exhibit remarkable detection performances, with detection limits of 0.68 ppm, 1.68 ppm, and 2.5 ppm for trinitrotoluene, dinitrotoluene, and nitrobenzene, respectively. This work provides a candidate for the design and fabrication of flexible sensors possessing the high-performance and user-friendly detection of trace NACs.

Analytical Tools and Methods for Explosive Analysis in Forensics: A Critical Review

This review summarizes (i) compositions and types of improvised explosive devices; (ii) the process of collection, extraction and analysis of explosive evidence encountered in explosive and related cases; (iii) inter-comparison of analytical techniques; (iv) the challenges and prospects of explosive detection technology. The highlights of this study include extensive information regarding the National & International standards specified by USEPA, ASTM, and so on, for explosives detection. The holistic development of analytical tools for explosive analysis ranging from conventional methods to advanced analytical tools is also covered in this article. The most important aspect of this review is to make forensic scientists familiar with the challenges during explosive analysis and the steps to avoid them. The problems during analysis can be analyte-based, that is, interferences due to matrix or added molding/stabilizing agents, trace amount of parent explosives in post-blast samples and many more. Others are techniques-based challenges viz. specificity, selectivity, and sensitivity of the technique. Thus, it has become a primary concern to adopt rapid, field deployable, and highly sensitive techniques.

Stationary Explosive Trace Detection System Using Differential Ion Mobility Spectrometry (DMS)

Detecting trace amounts of explosives is important for maintaining national security due to the growing threat of terror attacks. Particularly challenging is the increasing use of homemade explosives. Therefore, there is a constant need to improve existing technologies for detecting trace amounts of explosives. This paper describes the design of a stationary device (a gate) for detecting trace amounts of explosives and explosive taggants and the design of differential ion mobility spectrometers with a focus on the gas system. Nitromethane (NM), trimeric acetone peroxide (TATP), hexamine peroxide (HMTD), and explosive taggants 2,3-dimethyl-2,3-dinitrobutane (DMDNB) and 4-nitrotoluene (4NT) were used in this study. Gate measurements were carried out by taking air from the hands, pocket area, and shoes of the tested person. Two differential ion mobility spectrometers operating in two different modes were used as explosive detectors: a mode with a semi-permeable membrane to detect explosives with high vapor pressures (such as TATP) and a mode without a semi-permeable membrane (using direct introduction of the sample into the measuring chamber) to detect explosives with low vapor pressures (such as HMTD). The device was able to detect trace amounts of selected explosives/explosive taggants in 5 s.