Lecture 6 - Explosions 6 Flashcards
what are the three types of scientific evidence used in explosive investigation
classification - are explosives linked
identification - what is it
quantification - how much is there
what are the three S’s used in explosive investigation
Selectivity - determine the analyte from a mixture
Specificity - only the analyte contributing to the end result
Sensitivity - the calibration curve - limit of detection and limit of quantification
what can the selectivity in explosive investigation be affected by
contamination of samples from the lab or surrounding environment
When is the evidence you present likely to be question (associated with the amount of it that you have)
if the amount if close to the LOD or LOQ of the instrument you are using
what may co-elution in the chromatogram mean
we can’t be 100% sure the peak seen is from the analyte and not another source
to do with the specificity of the method
what are the two main purposes if an explosive investigation
the initial investigation = what happened - think what evidence may be needed
presenting evidence at trial - what can you prove happened
what are some locations explosive residue may be detected (5)
- a suspect’s hands, clothing, body, face, hair
- scenes where an explosion have occurred
- Clandestine labs or explosive storage dumps
- consider how an explosive may have been stored/transported
- consider household/accessible items that may have been used to make an explosive
what is the first priority when attending a scene with an expected explosion/explosive
safety of everyone - the explosive and surrounding area needs to be deemed safe before investigation proceeds
if the material at the scene have been encountered before what three things could be estimated
the explosive power of the explosive
the materials that have been used
if the explosive could be a viable explosive
if the material used to create the explosive have not been seen before why do we need to be more careful when dealing with it
we would be unaware of the chemistry and potential damage it can cause
how sensitive it is
so can be a danger
what are 4 questions as the investigator you should ask yourself when dealing with an explosive scene
was it a solid or liquid?
what was it?
how was it used?
how was it initiated?
what may suggest a liquid explosive has been used
scattered droplets around the point where the explosive was
what can indicate the type of detonator used to set off an explosive
strands and fuses that may have been left behind
can estimate if there was a blasting cap and what this was made of
how is the point of detonation often recognised
by a crater - especially on soft ground
but if none are seen the explosive is likely to not have been set off on the ground
are residues more likely or unlikely to be easily found at an explosive scene
therefore what is it important to do when arriving at the scene
unlikely as the explosion causes material to be blown away from the point of detonation- evidence is likely to be scattered a considerable distance
make sure to cordon is set to an appropriate distance
what do all personnel entering the scene need to wear
PPE - gloves, shoe covers, overalls, gloves
to protect the scene
what type of evidence should be collected from explosive scene
collect all loose soil and debris package in sealed labelled containers
things that may indicate the type of detonation = blasting cap, electronic timer, phones
things that might indicate the storage/container of the explosive = pipes, plastic tubs (melted or deformed)
give two places residues are likely to be found
embedded in nearby soft objects such as wood, rubber or bodies
on the surface of nearby metal objects e.g cars, road signs
remember crime scenes are 3D
what affects the distance residue is scattered from an explosion
the amount of gas produced
do high or low explosives generally leave more residues at the scene
low explosives - not as powerful
what can be useful for scene investigators with the use of impr0vised explosive devices
they often don’t fully explode or the detonator malfunctions so can be easily identified
name two analytical methods that can be used for rapid screening of bulk residues at explosive crime scenes
portable instruments such as suitcase GC or handheld Raman
for the identification of materials used in the explosive material
name three types of spectroscopy used in the lab for analytical analysis of explosive evidence
IR spectroscopy
raman spectroscopy
X-Ray - SEM-EDX
name four types of separation techniques used in the lab for analytical analysis of explosive evidence
HPLC
GC
Ion chromatography
Electrophoresis
name four types of detector used in the lab for analytical analysis of explosive evidence
mass spectrometer
UV/Vis
Flame ionisation
Ion Mobility
what are the benefits of using IR or raman spectrometers for analysis here (3)
quick
non destructive
give characteristic bands of particular elements/bonds
how is SEM used here
to look at the physical characteristics of particulates in detail
considering size and arrangement
how is EDX useful here but what is the disadvantage of this method
elemental identification of the metallic components - scan across fragments and see if the element if above normal levels
time consuming
what is GC particularly good for identifying
volatile compounds
what is ion chromatography good for separating here
charged organic compound e.g nitrates and chlorates
what is electrophoresis good for separating
ionic and charged compounds due to the electrodes having +ve and -ve charges
when are UV-Vis detectors normally used
with HPLC
when is using mass spectrometry as the detector method good
for trace analysis as it is very sensitive
what is the name of the technique needed for analysis of trace explosives
clean room techniques
what in clean rooms needs to be tightly controlled and why
the background levels of explosive materials
to prevent sample contamination to ensure the evidence presented is robust so is admissible in court
name 4 control measures used in clean rooms
positive air pressure - pressure is higher inside the lab than out so air diffuses out not in
HEPA (high efficiency particulate air) filters - remove particulates that contaminate
monitor of air particulates - regularly checking background levels as analysts can bring things into lab from outside
use of disposable equipment - gloves, shoe covers, lab coats - these are double bagged on entry and exit
what features do clean rooms have to control contamination even further (2)
separate examination and instrument benches
a separate kit room to get changed
when environment samples of the clean room are taken, what action is done if the explosive residue in the background is below 5ng
no action
when environment samples of the clean room are taken, what action is done if the explosive residue in the background is between 5 and 10 ng
confirm the identity of the explosive and clean the area
when environment samples of the clean room are taken, what action is done if the explosive residue in the background is between 10 and 100ng
clean the area and re-test until the sample tests negative
pause all other analyses until the levels drop down
when environment samples of the clean room are taken, what action is done if the explosive residue in the background is above 100ng
identify the contamination source, look at casework that may have been affected and repeat it, clean and re-test until sample is negative
where is contamination entering the lab likely to come from (2)
from the analyst
from the evidence collected by the police force - they can’t always and don’t always do this in a controlled way
what is the difference between trace and bulk analysis of explosive evidence
bulk = mg or more
- quantitative analysis
- paired with non chemical evidence
- can be done on site
trace = 10 to 100ng
- tends to be challenged in court
- needs to use of clean rooms
- often has higher uncertainties
name three types of sampling methods used to process trace and briefly explain them
static headspace sampling:
- sample heated in a sealed container
- headspace injection into GC
dynamic headspace sampling:
- inert gas is pushed into the sample vial
- the headspace is drawn into a solid phase trap which retains explosive vapours
- sample is concentrated before it injected into GC
SPME (solid phase microextraction):
- preconcentrate sample before injection
- fused silica fibre coated with stationary phase (e.g PDMS) injected into tube
- sample adheres to the fibre and desorbs by heating from stationary phase into the GC injector
what type of sampling is best for trace analysis
dynamic headspace sample as the sample is concentrated before it is injected into the GC
how is SPME a dual function process
recovers trace samples and pre-concentrated the sample before injection
what is extracted from the headspace of vials in the sampling process
the volatile components
name three ways explosive particulates can be recovered
swabbing
- good for smooth surfaces
vacuuming
- good for fabrics
- little vacuums with filters
solvent wash
- sonicate whole sample in extraction solvent
sonicate = agitate samples to get more out of it
what types of solvents are used when swabbing explosive residues
polar mixtures
e.g water/acetone
water/MeOH
how are samples cleaned/pre-concentrated in sampling explosives
SPE - solid phase extraction
- the use of a small cartridge containing a sorbent material to adhere the sample to
what are the 4 types of SPE used when pre concentrating and cleaning explosive samples
normal phase = absorbs polar compounds (has polar groupd e.g OH)
reverse phase = absorbs non polar compounds (has hydrocarbon chains)
ion exchange = good for inorganic components
size exclusion = absorbs lower molecular weight molecules
can do a combination of these e.g reverse phase followed by size exclusion
what type of SPE technique would be suitable for separating picric acid and TNT
reversed phase - this will adsorb picric acid and allow TNT to elute through
explain the process of SPE conditioning for the analysis of an inorganic species
clean the sorbent e.g with ethyl acetate
condition the sorbent e.g with ethano/water mix
apply the sample
wash the sorbent e.g with ethanol/water then MTBE/pentane
elute the explosive/analyte using ethyl acetate
inorganic components wash through with polar or non polar solutions
what are the two main steps involved in explosive analysis
a separation followed by a detection system
name two explosive specific detection systems and what they are specific for
chemiluminescence = specific detection of C-NO2, N-NO2 and O-NO2 groups
thermal energy analysers = controlled pyrolysis to form NO
why are the standard analytical techniques GC-MS LC-MS not always good at providing sufficient evidence in explosive analysis
many non explosive compounds may have very similar molecular weights or retention times to explosive ones so can get confused
