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Fires and Explosions > Fires 9 > Flashcards

Fires 9 Flashcards

1
Q

How can you tell from a burn pattern that a liquid fuel has been used?

A
  • Very distinct pattern
  • Looks like sloshing
  • Localised burning and areas of high damage next to areas of low damage
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2
Q

What does it mean if there are spots of damage around a main area of damage?

A
  • Likely liquid fuel splashing when coming into contact with the floor
  • Called streamers or trailers
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3
Q

How do you reconstruct a fire scene?

A
  • Speak to witnesses
  • Look at pre-fire photos - can see the original placement of objects
  • Match up objects to burn patterns
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4
Q

What do asymmetric burn patterns show you?

A

What side of something was pointing towards a fire
* can see if its been disturbed or moved

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5
Q

What are smoke records and how are they used to know what happened at a scene?

A

Smoke will have left deposits on all open surfaces
* regions without smoke deposits must have been covered - physical barrier to the smoke

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6
Q

Give some examples of where smoke records can be found

A
  • 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’
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7
Q

Why might excavation be needed after a fire?

A
  • to find the bottom of V-patterns
  • to estabilish order which objects fell
  • to find remnants of ignition sources
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8
Q

How can we tell if electricity has caused a fire?

A
  • There will be beads at the end of wires if arcing has occured before the fire
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9
Q

How can electricity start fires?

A
  • counterfeit electronic goods
  • deign faults
  • incorrect use
  • excess current results in heating of the cable and melting the insulation
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10
Q

What are signs that a fire is due to arson?

A
  • 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
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11
Q

What are physical evdience of arson?

A
  • 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)
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12
Q

What are some circumstances that could mean arson was involved?

A
  • 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
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13
Q

Why is chemical analysis used for fire scene residues?

A

To determine whether accelerants are present at the scene - evidence but not proof of arson

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14
Q

Where are you likely to find unburned accelerants?

A
  • 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
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15
Q

What can be used to absorb traces of accelerant?

A

Absorbant materials (diatomaceous earth) can be sprinkled on concrete
* Need to know what is in the concrete for lab analysis

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16
Q

How can you detect accelerants?

A
  • GC-MS
  • Sniffer dog
  • ‘Sniffer’ device
17
Q

What is a sniffer device and how is it used?

A
  • Portable hydrocarbon detector
  • Accelerants are volatile so will evaporate and become vapours
  • Organic vapours affect the electrical conductivity of a semiconductor detector
18
Q

How does a ‘Sniffer’ work?

A
  • 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
19
Q

What are the disadvantages of sniffer dogs and ‘Sniffers’?

A

They can only indicate the possibility of accelerants so lab samples are still needed

20
Q

What samples must be taken from the fire scene?

A
  • 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
21
Q

What type of samples do you take at a fire scene?

A
  • Flooring where accelerant was used (burnt and unburnt)
  • Swab-able surface
  • Floorboards
  • Soil
22
Q

Sample packaging and containers for fire scene samples

A
  • 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
23
Q

Why is a lab negative control sample used?

A

To guard against accelerant trances arising from contamination of lab equipment
* swabs from the workbench/samples of pure solvents

24
Q

How is a positive control used in the lab analysis of samples?

A
  • 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
25
Q

Recovery of accelerants by passive headspace analysis

A
  • 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
26
Q

Recovery of accelerants by dynamic headspace analysis

A
  • 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
27
Q

Chemical analysis of fire scene residues (liquid sample)

A
  • 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
28
Q

Interpretation of GC analysis

A
  • Retention time mainly depends on molecular mass (No. of carbon atoms)
  • Depends on the structure of the molecule - linear/branched/aromatic
29
Q

What pattern do distillate fuels show in GC analysis?

A

Gaussian distribution

30
Q

What can change the GC spectrum from fire residue samples?

A
  • 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
31
Q

What other substances could be present in a GC spectrum from a fire scene?

A
  • Combustion and pyrolysis products of other materials - carpets, clothing, wood products
  • Fire retardants

Fires and Explosions (20 decks)

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