Laboratory Contamination Control and Explosives Evidence

Definition: The unintended transfer of material onto a crime scene and/or evidence.
Locard Exchange Principle: The scientific basis for contamination, stating that when two materials come into contact, there is a transfer of material between them.
Levels of Contamination: * Trace Contamination: Contact with residues at post-blast scenes or during search warrant executions. * Gross/Bulk Contamination: Direct exposure to raw/bulk explosives or firearms ammunition.

Sources of Residue: Firearms, explosive particles (smokeless powder, flash powder, and PETN in detonating cords), and explosive vapors.
Transfer Modes: * Particles: Common for Inorganic and Organic explosives like Black powder and Emulsions. * Particles and/or Vapour: Occurs over time with volatile substances such as Smokeless powder ( $NG$ ) and Dynamite ( $NG$ and $EGDN$ ).
Technological Impact: Modern instruments are approximately $1000 imes$ more sensitive than those used 20 years ago, requiring stricter protocols.

Prevention Steps: * Store new cans with lids on. * Use separate, dedicated clean tools for every sample. * Wear latex gloves consistently. * Segregation: Never store or transport live explosives, post-blast evidence, search warrant evidence, and suspects together.
Decontamination of Equipment: * Boots and Metal Tools: Clean with soap, water, and a brush. Do not wear crime scene boots at a shooting range. * Clothing: Normal machine laundering or dry cleaning is effective. * Prohibition: Do not reuse tools with wooden handles.

Incident: A Boeing 747 bound for Paris exploded at $13,000 ext{ ft}$ ( $ext{∼} 4 ext{ km}$ ) off Long Island, NY, killing all 230 people on board.
Investigation: Trace amounts of RDX, nitroglycerin, and PETN were detected on wreckage. However, no physical bomb damage (pitting/cratering) was found on the aircraft.
Contamination Conclusion: The NTSB attributed the residues to the plane's history as a military transport and use in bomb dog training. The crash was officially caused by static electricity igniting the center fuel tank.

Device: Improvised pressure cooker bombs ( $6 ext{ quart/6 liter}$ ) filled with TNT or other explosives and shrapnel (nails, ball bearings).
Mechanics: Primary charges created shock waves traveling at over $3,300 ext{ feet per second}$ ( $1,005 ext{ m/s}$ ).
Initiation: Digital watches, cell phones, or garage door openers used as triggers.

S.P.I.C.E.S. Component Identification: * S: Switch(es) * P: Power Source(s) * I: Initiation * C: Container fragments * E: Explosive residues (witness materials) * S: Shrapnel
Recovery Methods: Use clean shovels, brooms, or vacuums with special filter attachments. Sieving is crucial for recovering lower mass materials near the blast seat.

Containers: Rigid metal cans are the best standard; fill them no more than $2/3$ full.
Moisture Control: If evidence is wet or bloody, it must be dried before sealing in plastic or paper to prevent DNA decay, toolmark oxidation, or the disintegration of water-soluble explosives like black powder.
Swabbing: Used only as a method of last resort. Actual components are preferred. Always include a control swab for comparison.
Security: Use tamper-proof seals, initial and date the seal, and maintain formal chain of custody labeling.