After a bombing or explosion, the investigative process is paramount in uncovering the truths of the event. For criminal justice officials, constructing a case and pinpointing culpability hinges on the scant evidence available at the scene. This evidence is vital for identifying, prosecuting, and ultimately securing a conviction of the perpetrator to thwart future incidents.

The inquiry into such events requires a collective effort across disciplines, involving law enforcement, scientists, and engineers. The process is delineated into three main phases: prevention, response, and reconstruction of the incident. Each phase is defined by its specific context—whether at security checkpoints, during search warrant executions, at the crime scene, or within forensic laboratories. At each stage, diverse testing methods are employed, from field tests and canine detection to portable devices and advanced laboratory analyses.

The National Institute of Justice (NIJ) has a longstanding commitment to advancing forensic technologies in the aftermath of terrorist attacks. This article presents findings from two NIJ-funded projects focusing on the reconstruction phase of explosives investigations. The first project, initiated in 2017, investigates the application of a novel analytical tool for explosives traces: gas chromatography-vacuum UV spectroscopy (GC-VUV). The second project, started in 2018, explores the potential of isotopic signatures to link an explosive charge to its manufacturing source. Both endeavors significantly extend the investigative toolkit for addressing the complexities of these crime scenes.

Chemical analysis plays a crucial role in explosives investigations, employing a range of physical, microscopical, chemical, and instrumental methods to examine evidence. Instrumental methods are fundamental to the forensic examination of controlled substances, ignitable liquids, explosives, and other physical evidence. These methods’ efficacy lies in their capacity to detect substances with high sensitivity, selectivity, and specificity, though forensic chemists must navigate the strengths and limitations of each instrument.For an analytical method to be applicable in explosives investigations, it must exhibit three primary characteristics:

– Sensitivity: This refers to the instrument’s ability to detect low levels of the substance under analysis (the analyte). It’s often quantified as the slope of the calibration curve for an analyte. A method with high sensitivity enhances the likelihood of detecting the analyte, even at minimal levels, thereby minimizing false negatives. Such sensitivity reduces the need for sample pre-concentration, which typically involves heating the sample under a flow of inert gas to increase the analyte concentration.

– Selectivity: This is the instrument’s capability to identify an analyte within a complex mixture, even when compounds with similar chemical structures are present. Achieving selectivity often requires chemically separating the mixture’s compounds so that each can be analyzed independently, without interference from others. High selectivity facilitates the analysis of complex samples without extensive preliminary cleanup, saving both time and resources. Moreover, it enables the detection of an analyte amidst interfering substances or compounds that could obscure the analyte.

– Specificity: This defines the instrument’s precision in unambiguously identifying the analyte. Various techniques can differentiate between similar analytes based on minor structural variations, increasing specificity and eliminating identification ambiguities. This is particularly crucial when evidence protocols depend on the definitive identification of specific compounds to formulate a scientific and legal conclusion. For instance, identifying nitroglycerin in post-blast debris is necessary to deduce the use of double-base smokeless powder, which is rich in nitroglycerin.

In forensic analysis, these three factors—sensitivity, selectivity, and specificity—are critical. Sensitivity is essential for detecting analytes present at trace levels in evidence types such as post-blast explosives. Selectivity is crucial due to the often messy nature of evidence, which can contain numerous interfering substances. Specificity is imperative to ensure laboratory results are reliable and legally significant.

A novel advancement in the instrumental analysis field is the coupling of vacuum UV (VUV) spectrometry with gas chromatography (GC). This method, recently applied to explosives analysis through NIJ-funded research, allows for the detailed examination of complex mixtures by separating and identifying compounds based on UV absorption.

The pursuit of linking post-blast explosives to their source is challenging yet innovative. Supported by the NIJ, research aims to establish whether isotopic and chemical signatures of explosive materials can be preserved post-detonation and thus traced back to their origins. Initial results from field detonations and subsequent analyses provide insight into the potential for source attribution, marking a significant advancement in forensic science.

As this field progresses, these projects underscore the promise and challenges of applying advanced analytical techniques to the aftermath of explosions. Continued research and development are crucial, aiming to enhance investigators’ capabilities to extract more information from crime scenes, develop leads, identify suspects, and ascertain the origins of explosive devices with greater certainty.

Journal Article:

John Goodpaster, “Advances in Detecting and Identifying Explosives After an Attack,” February 26, 2024, nij.ojp.gov: https://nij.ojp.gov/topics/articles/advances-detecting-and-identifying-explosives-after-attack