11.4 Notes

1. 11.4 Notes

2. High Explosives • Classified into two groups • Primary explosives – detonate violently with no burning • Used as primers – detonate other explosives via chain reaction • Rarely the main ingredient in homemade bombs • Secondary explosives – normally burn rather than detonate • Comprise mostly of commercial and military blasting • Dynamite, TNT, PETN, RDX, tetryl

3. Types of High Explosives • ANFO is ammonium nitrate soaked in fuel oil • Widely used in mining • Readily available for homemade varieties due to abundance in fertilizer • TATP = triacetone triperoxide • Made with acetone, hydrogen peroxide and hydrochloric acid • Extremely potent when confined in a pipe • Used by terrorists groups in Israel and the Middle East

4. RDX – cyclotrimethylenetrinitramine • Most powerful military explosive • Often in the form of a plastic doughlike substance called a C-4 • PETN – Pentaerythritol tetranitrate • Used by the military for small-caliber projectiles and grenades • Commercial used as a detonating cord • Used to interconnect a series of explosive charges so that detonation is simultaneous

5. Industrial Explosive Market • Moved from nitroglycerin based dynamite to ammonium nitrate based • Mix oxygen-rich ammonium nitrate with a fuel to form low-cost, very stable explosive • High explosives must be detonated by an initiating explosion

6. Detonator • Most common type is a blasting cap • Composed of copper or aluminum cases filled with lead azide as an initiating charge and PETN or RDX as a detonating charge • Homemade bombs usually initiated by an electrical blasting cap

7. Collection and Analysis of Explosives • Most important is the collection of samples from the explosion scene • Determination of high or low explosive is the presence of a crater at the origin of the blast • Locate crater and remove loose soil and debris in the crater to be preserved • Wire mesh screens are used for sifting debris • Objects near the source must be collected and checked for penetration or residuals on the surfaces

8. Use of the IMS (ion mobility spectrometer) • A vacuum to collect explosive residue from surfaces with additional surface residue wiped down with a paper disc • The residues are vaporized and converted to electrically charged ions • Used as a screening test • All materials placed in sealed containers • Soil and soft loose materials stored in metal containers or plastic bags

9. In the lab • Microscopic examination for particulate recognition • Rinse in acetone and analysis with TLC and then GC/MS • If it is a low explosive then a rinse in water • Confirmation tests include IR Spec; X-ray diffraction

10. Proposed Taggant program • Color coded chips added to commercial explosives • Chips are fluorescent and magnetic sensitive • Color indicates where the explosive was made and produced • Allows for a trace through distribution chain • Only now used in Switzerland • ICAO (International Civil Aviation Organization) has mandated volatile taggants be added to plastic explosives

11. 11.4 Questions • What is a detonator? What is the composition of most detonators? • Name three military high explosives. Which is the most powerful and popular of these explosives? • What is the most obvious post-explosion characteristic of a high or contained low explosive? • What device is widely used to screen objects for the presence of explosive residues? • How are soil and other soft loose materials collected at the scene of an explosion best stored? • What is the first procedure typically used to analyze bomb-scene debris that arrives in the laboratory? • What does the forensic scientist do to explosive debris following microscopic examination? Why? • List three procedures commonly used as screening tests for explosive residues. • What is a taggant? What purpose do taggants serve?