Chapter 15

1. Chapter 15 Explosives

2. Objectives • Students should gain an understanding of: • The classification of explosives • The composition of commercial explosives such as dynamite and ammonium nitrate/fuel oil (ANFO) • The composition of military explosives • How improvised explosive devices (IEDs) are made • Field tests for explosive residue • Confirmatory laboratory tests for explosive residue • The use of taggants to identify the manufacturer of an explosive

3. Introduction • Explosives are attractive weapons: • Can be made at home • Bring considerable attention to bombers’ issue • Bombings may be political in nature, grudges between motorcycle gang members, or the work of petty criminals. • Explosives can be sophisticated in nature or very crude pipe bombs.

4. Explosions • Explosion: a release of mechanical or chemical energy in a violent manner • Generates heat and the release of gas • Can create a rapid chemical reaction that produces large quantities of gas • Explosive: a substance capable of producing an explosion • Usually held in a metal container • May include nails or glass intended to cause more harm

5. Types of Explosives (1 of 6) • Deflagration • A chemical explosion in which the reaction moves through the explosive at less than the speed of sound • Low explosives = deflagration

6. Types of Explosives (2 of 6) • Detonation • A chemical explosion in which the reaction front moves through the explosive at greater than the speed of sound • High explosives = detonation

7. Types of Explosives (3 of 6) • Low explosives (black powder and smokeless powder) • Conversion of solid reactants into gaseous products provides pressure for explosion • Combustion requires two components: a fuel and an oxidant • Production of nitrogen and carbon dioxide is highly exothermic, which is why nitrogen is a main constituent of explosives

8. Types of Explosives (4 of 6) • Low explosives burn (rather than explode) if not confined in a container • Fuse for a low explosive: black powder wrapped around a fabric such as cotton • Condensed explosives: solid or liquid • Dispersed explosives: gas or aerosol

9. Types of Explosives (5 of 6) • Pipe bomb • Most common illegal explosive device in the United States • Usually contains black or smokeless powder or an improvised explosive mixture • Improvised explosive device (IED) • Any explosive material confined in a closed pipe that is fitted with a detonating device

10. Types of Explosives (6 of 6) • High explosives • Primary high explosives: extremely sensitive to shock and heat; detonate powerfully • Secondary high explosives: more stable • Primers: used to set off a bomb that contains a charge of secondary explosives

11. Commercial Explosives (1 of 4) • Nitroglycerine (NG) • First high explosive that was widely used in commercial application • Oily liquid that is extremely sensitive to shock

12. Commercial Explosives (2 of 4) • Dynamite • NG’s sensitivity is greatly reduced when it is absorbed into an inert material • The resulting product—dynamite—can be shipped and handled without danger • Dynamite’s invention made large-scale blasting possible

13. Commercial Explosives (3 of 4) • Dynamite • Original formula’s ignition was unreliable in cold climates • Problem was solved by adding ethylene glycol dinitrate to reduce the freezing point • Most common dynamite composition in North America is 80/20 mixture of EGDN and NG

14. Commercial Explosives (4 of 4) • Ammonium nitrate/fuel oil (ANFO) • AN fertilizer is widely used by farmers. • AN mixed with a source of carbon or fuel oil makes a cheap explosive. • The most common explosive in the world today is a 94% AN and 6% FO mixture. • ANFO requires a primer charge to initiate a blast, so it is classified as a blasting agent. • Large amounts have been used in terrorist bombings.

15. Military Explosives (1 of 7) • Military explosives are manufactured for specific purposes • Such explosives must be produced from cheap raw materials that are not strategic and are available in great quantity • Operations to manufacture them must be simple, cheap, and safe • Density of explosives needs to be as high as possible

16. Military Explosives (2 of 7) • TNT (2,4,5-trinitrotoluene) • Was dominant military explosive during World War I • Was conserved during World War II by mixing it with ammonium nitrate

17. Military Explosives (3 of 7) • Characteristics of TNT • Has a low melting point • Is not sensitive to shock • Will not spontaneously explode • Can include a detonator if the goal is to produce a pressure wave • Can quickly change from a solid to hot, expanding gas

18. Military Explosives (4 of 7) • RDX (cyclotrimethylenenitramine) • Remains stable in storage and at room temperature • Is the most powerful and brisant of the military high explosives • Reaches maximum pressure so rapidly that a shock wave forms • Reacts concentrated nitric acid with hexamine • Must be used in conjunction with a detonator to produce an explosion

19. Military Explosives (5 of 7) • C-4 • A popular explosive choice among terrorists • Military plastic explosive that is about 90% RDX; remainder is a plastic binder material and oil

20. Military Explosives (6 of 7) • PBX (plastic bonded explosive) • Plastic binder coats the explosive material, making it safer to handle • Plastic binder makes the explosive material easy to mold into different shapes

21. Military Explosives (7 of 7) • HMX (high-melting-point explosive) • Also known as octogen • About 30% more powerful than TNT • Powerful, yet shock-sensitive, nitroamine-based high explosive • Chemically related to RDX

22. Improvised Explosives (1 of 4) • 1990–1994: 64% of all bombings reported in the United States involved low explosives • Components of a low-explosive IED: • Container to confine the explosive • Fuse or primer to detonate it

23. Improvised Explosives (2 of 4) • Types of low-explosive IEDs • Commercially available products modified to act as explosives • Combinations of chemicals

24. Improvised Explosives (3 of 4) • Composition of low-explosive IEDs • Low-explosive IEDs: contents of road flares, match heads, powder from shotgun shells, or chemicals from fireworks • Pipe bombs: (1) potassium nitrate with sugar and aluminum; (2) sulfur/charcoal, potassium chlorate, and sugar; or (3) potassium perchlorate with sugar and aluminum • MacGyver bomb: toilet bowl cleaner, aluminum foil in a 2-L plastic soda bottle

25. Improvised Explosives (4 of 4) • High-explosive IEDs: TATP • New terrorist explosive used in the Middle East • More powerful than military-explosive analogs • Extremely sensitive to impact, temperature change, and friction • Similar operation to decomposition of an azide, which is used in automobile airbags to produce nitrogen gas

26. Initiators (1 of 5) • Initiator: a device used to start a detonation or deflagration • Most common IED in the United States is a pipe bomb filled with low explosive charge • Most popular initiator for those bombs are the safety fuse and the electric match • Detonator (blasting cap): a device used to set off a high explosive

27. Initiators (2 of 5) • Safety fuse: when ignited at one end, is intended to burn uniformly and to transmit the flame from one point of ignition to the IED • Application: fits into the opening at the end of a pyrotechnic or fuse detonator, where it triggers a fuse detonator • Fuse detonator: metal shell that is loaded with two or three types of explosive powder

28. Initiators (3 of 5) • Electric match: triggers an IED when it is set off by an electrical current • The resistance wire is coated with a pyrotechnic composition that ignites when the wire is heated. • Bombers typically attach the electric match to a timer that is rigged to close a switch in an electrical circuit at a specific time.

29. Initiators (4 of 5) • Detonators • Military mines use mechanical detonators, which are activated when someone steps or drives on them. • Commercial and military high explosives use electrical detonators.

30. Initiators (5 of 5) • Electrical detonators • Insulated wires are attached to a high-resistance bridge wire. • Pyrophoric material is pressed into place above the match head. • When electric current from a blasting machine passes through the wires, the bridge wire heats quickly, igniting the pyrophoric material, which then ignites the explosive.

31. Collection of Explosive-Related Evidence (1 of 4) • Precautions • Personnel involved in bomb scene or bomb threat searches should look for and report suspicious objects. • Post-blast, bomb disposal experts should first search for a secondary bomb. • Only then should investigators attempt to determine the point of detonation and type of blast effects.

32. Collection of Explosive-Related Evidence (2 of 4) • Searching the site • Begin the search for bomb evidence at the site of the crater and then proceed outward. • Sift, sort, and collect samples. • Use wire mesh screens to sort through the rubble. • Have explosives detection canines search the scene.

33. Collection of Explosive-Related Evidence (3 of 4) • Send all materials to the lab in sealed, labeled containers • Soil and loose debris: metal containers or plastic bags • Sharp objects: metal containers

34. Collection of Explosive-Related Evidence (4 of 4) • Date/shift code • Manufacturers must identify explosives with a date, shift, and place of manufacture • ATF can use the date/shift code to trace commercial and military explosives recovered by law enforcement

35. Field Tests for Explosive Residue • Ion mobility spectrometer • Presumptive test • Can detect a wide range of explosives • Portable hydrocarbon detector • Presumptive test • Can detect residue on objects or people • Specially trained dogs • Chemical reagents

36. Laboratory Analysis of Explosive and Explosive Residues (1 of 6) • Examine evidence microscopically • Search for fingerprints, tool marks, and other identifying marks • Run chemical tests on high explosives • Rinse recovered materials with acetone (dissolves many organic materials)

37. Laboratory Analysis of Explosive and Explosive Residues (2 of 6) • Thin-layer chromatography: a presumptive chemical test used to screen for the presence of an explosive • TLC separates compounds based on their size, shape, solubility in solvent, and interaction with the thin-layer plate. • A compound will travel a fixed distance relative to the distance traveled by the solvent front. • Multiple samples and standards can be spotted on the same TLC plate.

38. Laboratory Analysis of Explosive and Explosive Residues (3 of 6) • Gas chromatography: a confirmatory test for organic explosive residue • Each component of the residue has a unique retention time. • GC uses a thermal energy analyzer to detect volatile organic compounds. • Once the flow rate and temperature of the TEA furnace are set, a reference sample is injected and the GC determines its composition.

39. Laboratory Analysis of Explosive and Explosive Residues (4 of 6) • Infrared tests • Each explosive gives a characteristic IR spectrum. • The spectrum of the residue at a bomb scene can be compared to the standard IR spectrum of each explosive. • The investigator may need to chemically separate mixtures of explosive materials before performing IR analysis.

40. Laboratory Analysis of Explosive and Explosive Residues (5 of 6) • Analysis of inorganic explosive residues • Analysis of inorganic cations and anions in the debris may help investigators determine the composition of the explosive. • When ammonium nitrate or potassium perchlorate is suspected, the residue can be washed with water. • If spot tests indicate the presence of an inorganic substance, confirmatory ion chromatography should be performed.

41. Laboratory Analysis of Explosive and Explosive Residues (6 of 6) • Ion chromatography • Components of IC instrument: solvent reservoir, solvent pump, sample injector, separation column (containing an ion-exchange resin), detector, date system • Instrument must be set to handle anion separation. • Separate IC analyses are performed from cations and anions.

42. Taggants (1 of 2) • Identifiers may be put into batches of explosives that can help track down the manufacturer and purchaser of the material. • Types of taggants: • Tiny multicolored chips of plastic that could be recovered from crime scene • Addition of isotopes to compounds

43. Taggants (2 of 2) • Isotopes have not been shown to survive a severe blast or be recoverable using current evidence collection standards. • Less than 2% of criminal bombings involve commercial explosives, so the vast majority of bomb scene investigations would not be facilitated by the use of taggants.