Explosions 7 Flashcards
How do we analyse explosives?
- A separatory system combined with a detection system
What standard analytical techniques can we not use to analyse explosives and why?
- GC/MS and LC/MS
- Non-explosive compounds may have very similar molecular weights and retention times to explosives so it is hard to separate them out especially with complex mixtures
What is required in an explosive to use a explosive-specific detection system?
- Chemiluminescence - specific detection of NO2 groups
- Thermal energy analyser - controlled pyrolysis to form NO
What is pyrolysis?
Thermal decomposition in absence of air
What are the steps involved in a thermal energy analyser (TEA)?
- Start with explosive compounds that contain NO2 groups
- Pyrolysis to genrate a nitrosyl radical
- React with ozone (O3) to generate an excited nitrogen dioxide
- This then decays into NO2 and will emit light at around 600 nm
What are the advantages of the TEA system?
- Very sensitive limit of detection (picogram)
- Simple to use (dont need specialist training) and maintain
- Fast
- Reliable
- Robust evidence (no known false positives)
What are the three different columns used in the TEA?
- BP1 - non-polar
- BPS - slightly polar
- CPSIL19 - polar
All three must indicate the presence of an explosive if it wants to be trustworthy
What are the refernce compounds used in the TEA and why?
- TEA positive compounds with different retention times to explosives/taggents
What are the calculations that must be done to work out the retention time to validate explosive identity?
RRT = RT(peak)/RT(ref)
%RT(diff) = [RRT(sample) - RRT(std)/RRT(std)] x 100
What must be done/run for each column in the TEA to confirm explosive identity?
IS = internal standard
1. Standard (+IS)
2. Blank (+IS)
3. Sample (+IS)
4. Standard (+IS)
5. Calculate %RRT difference
6. Apply acceptance criterion
7. Confirm on two other systems
What % must the %RT(diff) be to confirm explosive identify?
<0.50%
Must be below this for all the columns
What is the equation for to work out the mass sample injected?
Mass sample injected = Mass standard injected x [peak area(sample)/peak area(standard)]
What is the equation to work out the total mass of explosive?
Total mass = mass injected x (total volume of sample/volume injected)
Not all explosives contain nitro groups so what analysis is done then?
FTIR and Raman
* Complementary and good at identifying specific functional groups to identify an explosive
* Useful for non-UV active, non-nitro organic based explosives
* Bad for complex mixtures
What technique can be used to look at inorganic species in explosives?
SEM-EDX
* Can look for inorganic fuels = Mg, S, Al
* Can look for inorganic oxidisers = KClO4, KNO4, NH4NO3
* Can look at specific regions for elemental analysis
* Can look at different shapes of particulates
