The Detection and Analysis of Explosives Flashcards
What is an explosion, and what categories is it divided into?
Occurs when a large amount of energy is suddenly released e.g.,
** Nuclear explosion
Physical explosion
Chemical explosion **
What is a chemical explosion?
o One mole a solid goes to several moles of gas
o Extremely rapid exothermic reactions (roughly 1/100th second)
o Generally, contain Oxygen, nitrogen, and a fuel (C or H)
Give a reaction of a chemical explosion
What causes the damage of the chemical explosions?
o Shock wave and wave front which causes physical damage (glass shatter e.g.,)
o Injury to people due to flying debris
What are the two types of explosive material? What is meant in each circumstance and give an example where necessary
o Conventional- manufactured through industrial process for a purpose e.g., military explosives (level of quality control and batch control)
Plastic explosives
Military munitions
Commercial explosives
o Improvised- made in an improvised fashion from starting materials which can be bought (unknown for quality, depends on skill of manufacture, starting materials, what synthetic route was taken)
Inorganic explosives- firework composition, gunpowder
Tell me about TNT- Trinitrotoluene
- Yellow cast of flaked crystalline solid
- Used for military munitions and industrial blasting
- Can be heated or melted
- Colour can be yellow to off white depending on purity
- Cheaper explosives (e.g., 25kg is a few hundred $)
With commerical explosives, what are the various explosive compositions and the names it can go by?
*** Varied explosive composition **
o Nitro-glycerine (NG)
o Ethyleneglycoldinitrate (EGDN), ammonium nitrate, sodium nitrate, aluminium
o NG and EGDN are liquid in their raw form- soaked onto support to make solid like ammonium nitrate or an inert support like clay
* Names it goes by:
o Energel
o Gelamex
o Dynamite
- Shelf life is about 6 months
What are some legitimate uses of explosives?
- Military and law enforcement users
- Legitimate production and decommissioning
- Blasting and demolition
- Medication
o Angina treated with NG based preparations- can be an issue for trace detection even if small amounts
o Vasodilator
What is detection in ‘search’ applications?
- NOT ‘forensic’
- NOT ‘evidential’
- NOT ‘accredited’
- Used for ‘search’- before event, in real world
- Equipment and techniques used as ‘aid to search’
- Direct other search resources
- Action taken on confirmed ‘find’
- Several complimentary ‘layers’ to search operations e.g., trace, manual, dog in same area
Compare Bulk vs Trace explosive detection
*** ‘Bulk’ means **
o Anything visible to naked eye
o Usually, a milligram upwards
*** ‘Trace’ means **
o Invisible to naked eye
o Less than a milligram
- Trace particle detectors can detect a billionth of a gram
o Trace includes explosive vapor
How are trace explosives detected?
- Many different detection systems commercially available
- Two sample collection techniques
o Particulate sampling ‘swabbing’
o Vapour sampling ‘sniffing’ - Which is most suitable depends on vapour pressure of analyte
What are explosive particles/ particulates used in trace explosive detection?
- Solid explosives are made up of millions of small particles
- Handling explosives distributes these particles
- Unconscious actions lead to contamination and transfer
- MPS exploit secondary and tertiary transfer to see if explosive activity
How do trace particle contamination spread?
- Touched surfaces “Every contact leaves a trace- Locard exchange principle
- If the hand is contaminated with explosive particles a few of these will be left behind
- As the surface is reused this contamination will gradually be knocked off or picked up on hand
- Contamination may result in secondary transfer (cross-contamination)
What are some examples of COTS trace explosives detectors?
- GC-chemiluminescence
- Mass spectrometry (MS)
*** Ion mobility spectrometry (IMS) ** - Amplified fluorescent polymer (AFP)
- Quartz crystal microbalance (QCM)- tried but not used
- Surface acoustic wave (SAW)- tried but not used
Tell me some general information about the IMS detector?
- Vapour and particular trace detection
o Bench top
o Handheld- more useful for vapour detection
o Portal- similar to airport by look, fires jets of air to loosen particles on individual and sucks them into machine to analyse, however, not used as not the most practical - Usually part of a wider search ‘comb’
o X-ray
o Walk-through metal detector
o Detailed physical search
How does IMS trace detection work?
- Representative trace sample on swab
- Placed into instrument
- Desorber heats swab rapidly
- Absorption of sample in vapour phase
- Ionised In detector
- Down drift tube
- Small ions travel faster, large slower
- Flow against drift gas (e.g., air)
- Acts as a separator (like a column)
- Amplitude vs time generated (two peaks, one for small ion and one for large ion)
- Know exact drift times, so can set up system so if things fall in certain window and have reached amplitude threshold then identifies explosives
- Can get Lab IMS (ion mobility spectrometry)- these have longer drift tube= better separation however MPS after rapid techniques so need to be easy to use, robust
- Therefore, they are modified and developed for field use
- If using nitrogen rather than air, that would give better separation, shock mounted (lab based)
- MPS just wants confirmatory results whereas lab wants quantifications
Smiths detection Ionscan 500DT- trace
- Particulate sampling using a swab
- Detection of several target explosives
- Suitable for screening at points of entry
**o False alarm rate quite high- false positives **
With the Alarms that these trace detectors can give like IMS, what are the types of alarms? What can be done if an alarm goes off?
‘Alarm’ is only an indication not a certainty
o False alarm?
o Nuisance alarm?- Not a threat but legitimate source e.g., NG from angina medication
- Resample
- Physical search
- Resolve by questioning- medications, job etc.
- Robust procedures in place
What is the Griess reaction?
- Colorimetric test- colour is different dependent on type of explosives
- Perfect for use as a field test
- Coloured compound formed in the presence of explosives
Step 1- Formation of nitrite ion (treat with sodium hydroxide, OH-)
Step 2- Formation of diazo dye
(A) step
(B) step
Stable magenta colour as product** (nitramine)**- explosive acts as a catalyst to make the colour change instantaneous. Can lead to false positives if leave result before looking
- Different response to nitro-aromatic compounds
- Formation of coloured complexes after step 1
- Meisenheimer complex (after step 1)
- **Nitroaromatic **goes a dark brown/black colour change
With the Griess reaction, the meisenheimer complex can form after step 1, what is this?
Meisenheimer complexes are important intermediates in Nucleophilic Aromatic Substitution Reactions (SNAr). They are formed by the addition of electron rich species to polynitro aromatic compounds or aromatic compounds with strong electron withdrawing groups.
What are some types of spectroscopic equipment that can be used for bulk explosive detection? What are some limitations to these methods?
*** Infra-red and Raman spectroscopy **
o Match spectrum against library
o Sometimes match will be a partial match against several things- would then look at similarity score (%) to see what’s more likely
*** Limitations **
o Not all chemicals are IR or Raman ‘active’
o Some chemicals fluorescence strongly masking other peaks
o Technologies struggle with mixtures
- Key for drug identification
What is thermal imagery?
*** A visual displace of IR energy **
- Aid to search that requires a level of skill and understanding to utilise effectively
- It is not
o X-ray vision
o Able to see electricity - Dependent on the properties of what you’re looking at
What are the thermal imagery principles?
- Transmitted energy is that which passes through the subject from another thermal source
o Transmitted energy (e.g., energy can come through a material to see behind) + **reflective energy **(i.e., through glass can see what’s in front) + emitted energy (materials ability to emit IR radiation)
o These 3 elements come together for thermal imagery
What are the limitations to thermal imagery?
o Thick fog! Consider atmospherics and your transmitted path
o Defeated by a bed sheet, foliage, wet clothing
