Explosive investigation Flashcards
What are the three types of scientific evidence?
Classification - are the explosives linked?
Identification - what type of explosive is it?
Quantification - how much explosive material has been used?
What 3 factors do we want to achieve in an explosive investigation?
Selectivity - the ability to determine an analyte from a mixture of compound without interferences/contamination
Specificity - being confident that nothing other than the desired analyte contributes to the result
Sensitivity - linear slope of the response/ limits of detection and quantification
What locations would we want to take explosive residues from?
suspect’s hands
clandestine labs or explosive storage dumps
explosion scenes
What would the scene of an explosion be like when starting an investigation?
- Any unexploded devices would be rendered safe by explosive ordinance technicians
- Does it contain a viable device?
- Does it resemble anything encountered before? if yes then it allows for easier evidence collection
if not, then additional steps need to be taken to determine the types of volatile material. In situ measurements become more important
What questions does the analyst need to ask themselves?
- Was a condensed phase (solid or liquid) explosive used?
- What was the explosive?
- How was it used?
- How was it initiated?
what do we need to consider when we collect evidence post an explosion?
- All personnel wear overalls, gloves, shoes covers etc to protect the scene
- The point of detonation is usually recognised by a crater
- Evidence may be scattered a considerable distance - appropriate placement of cordons
- Collect debris and loose soil from the scene and surroundings
- Residues may be found on surfaces on nearby metal objects and soft objects around the site
metal surfaces is quite good for collecting residues - Entire area is searched for remains of detonating mechanisms and explosive containers
Collection of explosives
High explosive typically leave very small quantities of residues
improvised explosive devices often don’t fully explode or have a detonator malfunction
rapid screening of bulk residues can be accompanied using a portable instruments like a suitcase GC or handheld Raman
Analytical instrument in explosive analysis labs
Spectroscopy - IR/raman , X-ray (SEM-EDX)
These are quick and non-destructive and are used to look for characteristic signatures and molecular bands
SEM - morphological structure of sample
EDX - gives the elemental composition of molecules
Separation - HPLC/GC, ion chromatography, electrophoresis
Detection - mass spectrometry (quadropole, TOF) - sensitive good for trace amounts
UV-VIS - complementary to MS
Flame ionisation
Ion mobility
Clean room
Techniques required for the trace explosive analysis
Clean room technology tightly control the background levels of explosive material
What are the control measures of a clean room?
Positive pressure = the contaminants diffuse out of the controlled environment
High efficiency particulate air (HEPA) filters = removal of contaminant particulates
Air particulate monitoring = air quality is constantly monitored to keep an eye on the background levels of different substances and collect readings to make sure of regular intervals
Disposable equipment = gloves, shoe covers, lab coats. These are double bagged on entry and exit
CLEAN ROOMS ENSURE EVIDENCE IN UNCONTAMINATED AND ADMISSIBLE IN COURT.
Regular monitoring is used to maintain explosive clean room strategy. What actions do we need to take when a particular level of contaminants are found when sampling regularly?
Below 5ng = none action taken.
5-10ng = confirm identity of explosive and thorougly clean area.
10-100ng = Pause all analyses, throughly clean and re-test until all samples test negative
more than 100ng =
conduct enquiry and identify contamination source.
Review all potentially affected case work
Clean and re-test until negative.
What do we mean by trace and bulk evidence?
Bulk evidence - refers to mg or more, quantitative evidence, spectroscopy paired with non-chemical evidence
trace evidence - refers to 10 to 100ng
often challenged in court due to higher uncertainties associated with type of evidence
needs to be water tight - clean room technology
How are trace explosives processed?
We need to extract the vapours for detection of volatile explosives and taggants
What are the main sampling techniques?
Static headspace sampling where the sample is placed in a sealed container, heated and headspace injection into GC
Dynamic headspace sampling where the headspace is drawn into a solid trap, retains explosive vapours, concentrates GC before injection.
Soild-phase micro extraction - SPME
pre concentration step before injection.
fused silica phase with stationary phase
desorb (by heating) from stationary phase directly into GC injector
SPME can be used for dual function of trace sample recovery and pre-concentration
How are particulates recovered?
Swabbing - smooth surfaces, solvent polar mixture such as water/acetone or water/methanol
Vacuuming - good for fabric to pull out particulates
Solvent wash - sonicate the whole sample in a suitable extraction solvent and agitates the sample.
How are samples cleaned or pre-concentrated?
SPE - solid phase extraction - small cartridge containing sorbet material
normal phase - adsorbs polar components strongly
reverse phase - adsorbs non-polar components strongly
ion exchange
size exclusion - adsorbs lower molecular weight strongly
How do we analyse an explosive?
Use a separatory system combined with a detection system
Standard analytical techniques such as GC/MS, LC/MS do not always provide sufficient evidence
In order to detect explosives, we require explosive-specific detection systems. What are these?
Chemiluminiscence = specific techniques of nitro groups (C-ONO2 and O-NO2) groups
This is done by a thermal energy analyser (TEA) controlled pyrolysis to form NO
What is TEA?
Thermal energy analyser
start with explosive compound
undergoes pyrolysis to generate NO radicals, pushing it into the excited state
reacts with ozone to produce a nitrogen dioxide radical, which is the excited state
this subsequently decays to the ground state and this emits light
This allows for the specific detection at a wavelength around 600nm of the NO2 groups
What are the advantages of TEA?
Picogram limit of detection (LOD) - very sensitive and competitive against MS
Simple - can be used and maintained easily
Fast
Reliable
Robust evidence - no false positives in decades of use
How does TEA work?
the same experiment is repeated 3 times using 3 columns of slightly different polarity
What are the three columns used in TEA?
BP1 - ‘non polar’ PDMS
BPS ‘slightly polar’ we change the secondary phase to have a small amount of phenyl groups
CPSIL19 - ‘polar’ introduces phenyls, CNs and alkenes
if the functionalities in the explosive materials preferentially bind to these new groups then, it will make the retention times slightly longer.
if the three columns indicate the presence of an explosive then analysis is considered trustworthy
What are reference compounds?
These are compounds containing nitro functionalities with different retention times to explosives/taggants
for example
- FNT
- MT
What is the equation for retention time calculation?
RRT = RT (peak) / RT (ref)
