Fires 9 Flashcards
How can you tell from a burn pattern that a liquid fuel has been used?
- Very distinct pattern
- Looks like sloshing
- Localised burning and areas of high damage next to areas of low damage
What does it mean if there are spots of damage around a main area of damage?
- Likely liquid fuel splashing when coming into contact with the floor
- Called streamers or trailers
How do you reconstruct a fire scene?
- Speak to witnesses
- Look at pre-fire photos - can see the original placement of objects
- Match up objects to burn patterns
What do asymmetric burn patterns show you?
What side of something was pointing towards a fire
* can see if its been disturbed or moved
What are smoke records and how are they used to know what happened at a scene?
Smoke will have left deposits on all open surfaces
* regions without smoke deposits must have been covered - physical barrier to the smoke
Give some examples of where smoke records can be found
- door frames show position of door
- Glass will show whether it was broken before or after fire
- Objects on desks and walls will leave a ‘shadow’
Why might excavation be needed after a fire?
- to find the bottom of V-patterns
- to estabilish order which objects fell
- to find remnants of ignition sources
How can we tell if electricity has caused a fire?
- There will be beads at the end of wires if arcing has occured before the fire
How can electricity start fires?
- counterfeit electronic goods
- deign faults
- incorrect use
- excess current results in heating of the cable and melting the insulation
What are signs that a fire is due to arson?
- Materials have been used to spread fire (paper, furnishings, fluids) may leave characteristic burn patterns
- Multiple seats of a fire
- Tell-tale signs of accelerant use
- Some synthetic furnishings can look like arson when they melt - misleading
- Samples should be taken
What are physical evdience of arson?
- Several seats of fire or in unusual place
- Accidental causes eliminted or highly unlikely
- Previous fires in building or area
- Repeat involvement of individual
- Unnatural spread of fire, spread trailers
- Seat near expensive equipment
- Improbable time for accident
- Alarms deactivated
- Incendiary devices (things that are flammable)
What are some circumstances that could mean arson was involved?
- Records destroyed
- Financial difficulties
- Contents removed prior to fire
- Insurance claim not filed once questions asked
- Audit/stock taking just about to happen
- Forced entry, evidence of search
- Interested parties know a lot/pay close attention
Why is chemical analysis used for fire scene residues?
To determine whether accelerants are present at the scene - evidence but not proof of arson
Where are you likely to find unburned accelerants?
- Liquids seep into porous materials
- Carpets
- Floorboards
- Upholstery
- Plaster
- Rags
- Floor cracks
- Concrete
- If there has been a flashover, unburned accelerants are less likely but the floor underneath furniture may have been sheltered from the fire
What can be used to absorb traces of accelerant?
Absorbant materials (diatomaceous earth) can be sprinkled on concrete
* Need to know what is in the concrete for lab analysis
How can you detect accelerants?
- GC-MS
- Sniffer dog
- ‘Sniffer’ device
What is a sniffer device and how is it used?
- Portable hydrocarbon detector
- Accelerants are volatile so will evaporate and become vapours
- Organic vapours affect the electrical conductivity of a semiconductor detector
How does a ‘Sniffer’ work?
- Flammables have low conductivity because they arent particularly polar molecules
- When they are present, you get a drop in conductivity if the vapours are in the air
- This gets picked up by the detector
- Gives you an indicator but isnt specific so isnt conclusive
What are the disadvantages of sniffer dogs and ‘Sniffers’?
They can only indicate the possibility of accelerants so lab samples are still needed
What samples must be taken from the fire scene?
- Fire sample - a sample as near as possible to the seat of the fire to be tested for accelerants
- Comparative control sample - same material the fire sample but from elsewhere in the room and therefore uncontaminated with accelerant
- Negative control samples - tools and empty containers of the same type used in the sampling process
- Other samples from the suspect if one is present
What type of samples do you take at a fire scene?
- Flooring where accelerant was used (burnt and unburnt)
- Swab-able surface
- Floorboards
- Soil
Sample packaging and containers for fire scene samples
- Must be airtight
- Retain volatile long chain hydrocarbons and aromatic molecules
- Glass jars with metal lids
- Lids must be able to withstand vapour pressure of volatile liquids
- Must be free of plasticisers
- Metal cans - not lined becuse plastic linings contain hydrocarbons
- Gloves
- Swan neck seal on plastic bags - no gases escape
Why is a lab negative control sample used?
To guard against accelerant trances arising from contamination of lab equipment
* swabs from the workbench/samples of pure solvents
How is a positive control used in the lab analysis of samples?
- prepared with a volatile flammable liquid on it
- Packaged and treated in the same manner as the fire scene samples
- analysed at the end of the analytical sequence to reduce the risk of contamination
Recovery of accelerants by passive headspace analysis
- Used for accelerant revovery by vapour conc
- The vapour in the sealed container is exposed to activated charcoal (chemical absorbent) where it is trapped for later analysis
- Will ususally heat to assist in vaporisation
Recovery of accelerants by dynamic headspace analysis
- vapour is drawn from the sample through an absorbant using a flow of carrier gas (dry nitrogen)
- absorbant is activated charcoal which traps organic volatiles but has a low affinity for water
- Blowing gas over the sample to evaporate it rather than using heat
- Greater control of sample flow
Chemical analysis of fire scene residues (liquid sample)
- gas chromatography
- sample is carried into a separating column by a stream of chemically inert carrier gas (helium, nitrogen)
- column temp can be controlled
- GC column separates the solutes by absorption on an active column material
- each solute has its own retention timee which depends on the setup of the experiment
Interpretation of GC analysis
- Retention time mainly depends on molecular mass (No. of carbon atoms)
- Depends on the structure of the molecule - linear/branched/aromatic
What pattern do distillate fuels show in GC analysis?
Gaussian distribution
What can change the GC spectrum from fire residue samples?
- Weathering of the sampling due to heating and aging at the scene
- Loss of most volatile components means that weathered fuel no longer matches a fresh standard/reference sample
- Main components that are more likely to evaporate are low molecular weight species
What other substances could be present in a GC spectrum from a fire scene?
- Combustion and pyrolysis products of other materials - carpets, clothing, wood products
- Fire retardants
