Chapter 15 Explosives

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

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.

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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.

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.

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.

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.

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

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

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

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)

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.

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.

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.

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.

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.

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

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.