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