Contributed by Natasha Neel
The analysis of explosives involves identifying the type of explosive(s) used in a device, or recovered in bulk. Explosives fall into two categories – high explosives and low explosives. The main difference between these two categories is the velocity of detonation. High explosives detonate at a rate greater than the speed of sound whereas low explosives deflagrate. Deflagration involves particle to particle burning. Low explosives usually require confinement to function properly, as opposed to high explosives which do not need to be confined. Low explosives include materials such as: black powder, smokeless powder, flash powder, and black powder substitutes such as Hodgdon® Pyrodex® and Hodgdon® Triple Seven®. High explosives include materials such as: RDX, PETN, TNT, HMX, TATP, HMTD, emulsions, and water gels.
The analysis of explosives depends on whether or not a sample is received intact or post-blast. More information can usually be derived from a sample that is submitted intact versus a post-blast sample. For example, if an intact sample of smokeless powder is received, an examiner can report morphological and chemical characteristics, for example, that a disc shaped double base smokeless powder was identified. However, if no intact particles are received or recovered during analysis, one may only be able to report that nitroglycerin, a component of double base smokeless powder and some dynamites, was identified.
Analysis of explosives evidence starts by examining the evidence visually and with a stereomicroscope to determine the physical characteristics of the material. As the examiner proceeds with their analysis, the instrumentation and analysis scheme used depends on whether the evidence received is intact or post-blast, and whether the explosives are organic or inorganic. If an intact sample is recovered, the morphology of the material will provide information as to what it may be. An ignition susceptibility test (IST) will also give useful burn properties of the material. With post-blast debris, microscopic analysis is a very important step in the analytical scheme. If the filler used was a low explosive, in most instances intact explosive particles will have survived the blast. The materials that may be available for analysis are dependent on the thoroughness of the evidence collection at the scene, as well as on the manner in which the original device functioned. If no intact particles are recovered, analyzing organic and/or aqueous extracts of the remaining post-blast components and debris will be required. The techniques used for the analysis of explosives and their residues may include FTIR (Fourier Transform Infrared Spectrometry), XRF (X-ray Fluorescence), XRD (X-ray Diffractometry), GC/MS (Gas Chromatography/ Mass Spectrometry), IC/MS (Ion Chromatography/ Mass Spectrometry) , HPLC (High Performance Liquid Chromatography) , LC/MS (Liquid Chromatography/ Mass Spectrometry), and PLM (Polarized Light Microscopy).
In addition to identifying the type of explosive(s) used in a device, another key part in the analysis of explosives evidence is the identification of components involved in the construction/manufacturing of a device. This includes not only the container and shrapnel used, but also any components involved in the firing train such as: fuses, detonators, wires, batteries, switches, and timing mechanisms. Information gathered from component identification is critical to determine where items may have been purchased, to help link devices together, or to aid the investigator during subsequent searches.
1. Akhavan, J. (2011). The Chemistry of Explosives, (3rd edition). Cambridge, UK: The Royal Society of Chemistry.
2. Beveridge, A. (Editor). (2012). Forensic Investigation of Explosions, (2nd edition). Boca Raton, FL: CRC Press.
3.Conkling, J. and Mocella, C. (2011). Chemistry of Pyrotechnics: Basic Principles and Theories (2nd edition). Boca Raton, FL: CRC Press.
4. Davis, T. The Chemistry of Powder and Explosives, (reprint edition). Las Vegas, NV: Angriff Press.