Forensic Explosives Analysis Notes

Definition of an Explosive

• Explosive: Compounds or mixtures that undergo decomposition, producing gas and energy rapidly.
  • The process should be self-sustained throughout the explosive mass.
  • Should not explode spontaneously; ignition is required (via heat, impact, friction, etc.).

Chemical Requirements for an Explosive

• Must contain:
  • Fuel: Carbon (C), sulfur (S), phosphorus (P), hydrocarbons, metals (Al, Mg, Fe), etc.
  • Examples of hydrocarbons: fuel oil, diesel, brake fluid ($C<em>{12}H</em>{26}O<em>{7}$), sugar ($C</em>{12}H<em>{22}O</em>{11}$), glycerol ($C<em>{3}H</em>{8}O_{3}$).
  • Oxidizing agents: Nitrates (e.g., $KNO<em>{3}$, $NH</em>{4}NO_{3}$), hypochlorites, chlorates, peroxides, etc.

Mechanical Mixtures of Explosives

• Example mixtures:
  • Black powder: mixture of $KNO_{3}$, carbon, and sulfur.
  • Calcium hypochlorite ($Ca(OCl)_{2}$) with brake fluid.
  • Potassium permanganate ($KMnO<em>{4}$) with glycerol ($C</em>{3}H<em>{8}O</em>{3}$).
  • Ammonium Nitrate ($NH<em>{4}NO</em>{3}$) mixed with fuel oil.

Explosive Substance Structures

• Combines oxidizing agent with fuel to release energy.
• Oxygen linked to atoms other than carbon or hydrogen, via configurations like:
  • Nitro: $-NO_{2}$
  • Nitroso: $-NO$
  • Nitrate: $-O-NO_{2}$
  • Nitramine: $-NH-NO_{2}$
• Examples:
  • TNT, PETN, RDX.

General Classification of Explosives

• Low Explosives: Decomposition rate < speed of sound (deflagration).
  • Example: Smokeless powder (nitrocellulose).
  • Propellants (single-base with only NC).
• High Explosives: Decomposition rate > speed of sound (detonation).

Primary and Secondary Explosives

• Primary Explosives:
  • Highly sensitive; initiate secondary explosions.
  • Examples: Mercury Fulminate ($Hg(ONC)_{2}$), DDNP, Tetrazene.
• Secondary Explosives:
  • Less sensitive; act as main charge in devices.
  • Examples: TNT, PETN, RDX.
  • Classified as industrial or military explosives.

Examples of Industrial and Military Explosives

• Industrial Explosives:
  • Dynamite (NG), ANFO (Ammonium Nitrate + fuel oil).
• Military Explosives:
  • Semtex (mixture of RDX, PETN), HMX (used in plastics).

Chemical Classification of Explosives

• Nitroaromatic Compounds: Nitro groups bonded to benzene (e.g., TNT).
• Nitrate Esters: Contain $C-O-NO_2$ group (e.g., Nitroglycerin, PETN).
• Nitramines: Contain $C-N-NO_2$ group (e.g., RDX).
• Peroxides: Contain $C-O-O-C$ groups (e.g., TATP, DADP).
• Poly-nitro Cage Compounds: High-density, 3D structured molecules (e.g., 1,3-dinitroadamantane).

Sampling and Analysis of Explosives

• Submit whole device if unexploded, remains if exploded.
• Avoid swabs; they limit material available for testing.
• Keep samples away from water; many residues are soluble.
• Avoid friction lid tins, particularly for TATP which can re-crystallize.

Extraction Methods for Explosives

• Organic explosives: Extracted in acetone or acetonitrile.
• Inorganic residues: Extracted in deionized water.
• Sulfur: Extracted in chloroform ($CHCl_{3}$).
• Metals: Extracted in 5% HCl.
• Resins: Extracted in isopropanol.
• Fuel oils in ANFO: Extracted in hexane.

Chromatography for Screening Explosives

• Techniques: TLC, HPLC, GC, FTIR.
• FTIR Characteristic Bands for Detection:
  • C-NO$2$: 1320-1390 cm$^{-1}$; Ring-NO$2$: 1340-1370 cm$^{-1}$; C-O-NO$2$: 1270-1285 cm$^{-1}$.
• Important considerations:
  • Choice of column stationary phase affects elution.
  • High temperatures in GC may decompose thermally labile explosives.

Advanced Detection Techniques

• Gas Chromatography (GC):
  • Can be coupled with various detectors (e.g., GC-FID for comparative, GC-MS for structure elucidation).
• Thermal Energy Activation Detector (TEA):
  • Detects nitro compounds by generating nitric oxide for analysis.
• Ion Chromatography (IC): Separates inorganic ions such as $Cl^{-},~NO<em>{2}^{-},~NO</em>{3}^{-}$.
• Inductively Coupled Plasma / Atomic Emission Spectroscopy (ICP-AES): Detects metals, phosphorus, and sulfur in explosive mixtures.

Example Analysis Scheme

1. Examine supply information for analysis.
2. Conduct FTIR on gas phase inside container.
3. Perform microscopic examination.
4. Extract debris with deionized water; analyze extract via FTIR, IC, ICP-AES.
5. Re-extract with acetonitrile; analyze by TLC or GC-TEA.
6. If positive, confirm with GC-MS or LC-MS/MS.