Stand-off detection refers to the ability to detect, identify, and sometimes characterize potentially explosive materials from a distance or without being in direct contact.

Methods of Stand-Off Detection

Electromagnetic energy or sound pulses: Interacts with the sample, attenuating or changing the signal based on the amount and type of materials present. Examples: Tetrahertz (THz) and Raman spectroscopy.

High vapor pressure explosives detection: Explosives like TNT and TATP have appreciable amounts in the gas phase, detectable using various methods.

  • Factors: Concentration of explosive vapor and detection limits of instrumentation/sensor system.
  • Preconcentration methods: Solid-phase microextraction (SPME) or heated headspace.

Ion Mobility Spectrometry (IMS):

  • Usage: Widely used field detection instrument for explosives.
  • Modes: Senses vapors in a stand-off mode or as a direct sensor through thermal desorption of wipes.
  • Mechanism: Separates ion–molecule clusters based on size-to-charge ratio, operates in positive- or negative-ion mode.
  • Target: Most explosives detection in negative-ion mode targeting nitrates or nitrate groups.
  • Explosives: Nitroglycerin (NG), RDX (cyclonite), and pentaerythritol tetranitrate (PETN).
  • Doping agent: Methylene chloride increases selectivity and reduces background interference.

Raman Spectroscopy:

  • Advantages: Direct and stand-off detection, small portable configurations, ability to probe materials through many types of containers.
  • SORS (Spatially Offset Raman Spectroscopy):
    • Method: Collection of signal at locations physically offset from the point of incidence, unlike conventional Raman.
    • Benefit: Greater return signals from photons emerging from deeper parts of the sample, reducing the surface signal relative to deeper levels.
  • Examples: RaIDer–X (Rapid Identification Detector-Explosive) by HEMRL.

Reference: Forensic Chemistry by Suzanne Bell

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