Forensic Investigation of Explosives Chapter 13 Forensics

The Chemistry of Explosions - 1 Explosions – combustion reaction that creates heat and gases ◦ Happens VERY quickly and creates a sudden buildup of gas pressure which is thrown out from the center of the explosion Explosions still need oxygen but happen so quickly that the oxygen in the air is not enough ◦ Most explosives must supply their own source of oxygen

The Chemistry of Explosions - 2 If an explosion happens in a relatively small, closed container (ie closed room or a car) ◦ The gases build up inside the container ◦ The heat causes the gases to expand, increasing the gas pressure ◦ The walls of the container stretch and then break, throwing debris and shrapnel everywhere, along with a ‘gust’ of gases which can cause further damage

2 Types of Explosives High explosives ◦ Speed of detonation – how fast a supersonic shock wave is thrown out from the explosion ◦ Detonate at m/s Low explosives ◦ Speed of deflagration (burning) – subsonic pressure wave pulses out from the explosion. Less damaging but still dangerous ◦ Decomposition happens at 1000 m/s ◦ Sometimes used to propel ammunition or skyrockets

Examples of Low Explosives - 1 Black Powder ◦ relatively stable mixture of potassium nitrate, sodium nitrate, charcoal and sulfur ◦ Unconfined – just burns, so frequently used for safety fuses (fuse on firecracker) ◦ Only dangerous when confined Smokeless Powder ◦ Nitrocellulose or nitroglycerin mixed with nitrocellulose These two are the most widely used low explosives because they are easily available to the public

Examples of Low Explosives - 2 Chlorate Mixtures ◦ Mixtures that provide oxygen through the chlorate and fuel through the rest of the molecule ◦ Ex: potassium chlorate and sugar

Examples of Low Explosives - 3 Gas-Air Mixtures ◦ Natural gas mixing with air ◦ Only burn within a set concentration range  If the mixture is ‘lean’ (almost too little gas) the mixture explodes but doesn’t ignite  If the mixture is ‘rich’ (almost too much gas) the mixture explodes and then burns violently

Two Types of High Explosives Primary explosives ◦ Ultrasensitive to heat, shock or friction ◦ Detonate violently instead of burning ◦ Frequently used in small amounts as blasting caps to detonate other explosives Secondary Explosives ◦ Relatively insensitive to heat, shock or friction ◦ Normally burn rather than detonate ◦ Most high explosives fall in this category

Examples of High Explosives - 1 Dynamite (or straight dynamite) ◦ Used when a quick shattering action is desired ◦ Usually contains nitroglycerine and pulp (makes the nitroglycerine less sensitive), sodium nitrate (provides oxygen) and a stabilizer ◦ Rated by strength according to percent of nitroglycerine it contains

Examples of High Explosives - 2 Ammonium Nitrate Explosives ◦ Replacing straight dynamite as the high explosive of choice because they are low cost and stable ◦ Water gels - Have the consistency of jell-o or gel toothpastes – can be used under wet conditions ◦ Emulsions – have an oil phase and a water phase that are mixed ◦ ANFO – ammonium nitrate soaked in fuel oil. Inexpensive and safe to handle. ◦ Ammonium nitrate is easily available in the form of fertilizer

Examples of High Explosives - 3 TATP – Triacetone triperoxide ◦ Homemade explosive made by mixing acetone with peroxide ◦ Friction and impact sensitive explosive. Frequently used in pipe bombs ◦ Used in the 2005 London transit bombings ◦ One of the few liquid explosives

Examples of High Explosives - 4 Military High Explosives ◦ RDX – or composition C-4 ◦ TNT – used frequently in WWII ◦ PETN – used for small-caliber projectiles and grenades. Used commercially as a detonating cord Detonators ◦ High explosives must have an initiating explosion ◦ Usually blasting caps – can be chemically or electrically activated

Detecting Evidence of Explosions Evidence can be found on many different surfaces near an explosion, ranging from chemical residue to pieces of the detonator itself Ion Mobility Spectrometer (IMS) – most frequently used to detect chemical residue ◦ Collects explosive residues with an attached vacuum ◦ Identifies chemicals by how long it takes them to move through the tube inside

Colleting & Packaging Evidence of Explosions Must be places in airtight sealed containers and labeled Items collected from different areas should be packaged separately.

Analyzing Evidence of Explosions - 1 May be viewed under telescope Rinse with acetone. Most explosives are dissolved in acetone and easily removed from the debris. Color tests can be done to identify the chemicals found in the residue. (p 462) TLC and GC/MS can be done ◦ If enough is found can do IR spectrometry

Analyzing Evidence of Explosions - 2 Taggants ◦ Proposed program that would make finding useful evidence MUCH easier at crime scenes ◦ Tiny color-coded chips would be added to commercial explosives ◦ Would identify where it was made and the chemical used ◦ Would be detected with magnets or UV light ◦ Currently only Switzerland is using this idea