Fires 9 - Investigation and Analysis Flashcards
What does a burn pattern look like if a liquid fuel is used?
sloshing
What does a burn pattern look like it petrol is used?
On carpet?
splats and maintains the shape when burning
non-uniform and super localised
What three things does firefighting involve?
- high pressure hoses
- windows smashed for ventilation
- items cleared out
What three things are used to reconstruct scene?
- speak to witnesses
- look a pre-fire photos or if it is available camera footage/cloud photos (made easier nowadays with digital records, cloud storage/social media. this can help return objects to original locations or tell you if something was out of place when burnt (give potential motive if burned something they moved intentionally))
- match up objects to burn patterns
What must movements be in reconstruction?
justified and fully recorded
What can be said about smoke record left at fire scene?
- smoke will have left deposits on all open surfaces (anything without smoke deposits must have been covered)
- it is a great indicator for depicting what happened at scene
- door frames show position of door
- glass show whether broken before or after fire
- objects on desks and walls will leave a shadow
- physical barrier to the smoke makes scene reconstruction easier
Why might excavation be needed?
debris (burnt objects, walls, ceilings, coverings…) may have fallen around the scene and could cover the seat of the fire so excavation may be needed:
- to find bottom of V-patterns
- to establish order which objects fell
- to find remnants of ignition sources
What can be said about fire and electrical faults?
How can you identify the differences?
electrical faults can cause fires but fire can also cause electrical faults
e.g. can expose wires and cause short circuits as the fire spreads and damages plugs
- excess current results in heating of cable
- this melts insulation which means wires come into contact
- arcing wires develop beads
- beads - imply wires been arcing for while and this is cause of fire
How can electrical fires start (3 ways)
1 - counterfeit electronic goods
2 - design faults
3 - incorrect use
What four questions should be asked surrounding electrical fires?
was it plugged in and turned on?
- if no electricity running through appliance = didn’t start fire
- smoke record and position of switches will tell you if an appliance was on when the fire started
was it at the point of origin?
- is it near the origin?
was the fault there before the fire? and were there fuels nearby?
- need to find fault and find nearby fuel that was ignited by fault
- if these are not there then the item probably didn’t start the fire
What can be said about people who have had previous accidents with fire?
What questions must be asked about lifestyle of these individuals?
when people have an accident with fire, it is not usually the first time they have acted that way
- are there carelessly discarded cigarettes around or cigarette burns in other areas?
- any evidence of prior minor burning from cooking?
- chip pan on stove?
- unsafe electricals around?
- candle use? metal disks from tea light candles?
What can be said about materials that are used to spread a fire?
paper, furnishings and fluids may leave characteristic burn patterns and leave multiple seats of a fire
What must be noted if visualise tell-tale signs of accelerant use at fire?
synthetic furnishings may melt to produce similar patterns and this can be misleading hence why sampling is important so can analyse samples
What can be said about the ignition source of an arson fire?
- if a match - usually no evidence left of it as burnt but sometimes some phosphorous left behind
- unlikely to leave lighters/box of match behind
- suspicious burn patterns or residues are more reliable than trying to find these ignition sources
How can make timing devices?
- a candle burning until it reaches fuel source
- ignition of a timer switch with soldered connections - can specify time as short will occur when timer hits zero
What are nine suspicious physical signs of arson?
- several seats of fire, or in unusual place e.g. in middle of room not near a power outlet
- accidental causes eliminated or highly unlikely - when no accidental cause = start thinking about arson
- previous fires in building or area (indicative of criminal activity)
- repeat involvement of individual
- unnatural spread of fire, spread trailers – evidence of accelerants
- seat near expensive equipment = insurance fraud
- improbable time for accident (cooking fires in the evening, electrical fires less likely at night)
- alarms etc. deactivated
- incendiary devices
What are seven suspicious circumstantial signs of arson?
- records destroyed
- financial difficulties
- contents removed prior to fire
- insurance claim not filed once questions asked (someone who gets cold feet after police enquire about possible fraud)
- audit/stock taking just about to happen
- forced entry, evidence of search
- interested parties know a lot, pay close attention
What does testing in labs do?
- need to form hypothesis and perform the needed tests
- testing in labs should help validate or disprove hypotheses e.g. trails = accelerants - need a test to validate or disprove
- most tests are excludatory in forensics (eliminates possibilities but can rarely definitively prove something)
- can say that theory is most likely hypothesis based on your tests
what does lab testing rely upon?
relies upon good reasoning in hypothesis:
- if hypothesis means look for particular compound
- if pyrolysis products of other plastics can also be the same then hypothesis is faulty and lab experiments will not help you
What is the most common reason to do test and samples collection?
- to provide information concerning ignition sources and accelerants
- particularly in the search for chemical evidence of flammable liquid residues in a fire which is the primary sort of lab based analysis in fire investigation
what is the prime aim of chemical analysis of fire scene residues?
- to determine whether accelerants are present at the fire scene as this is evidence (not proof) of arson
- unburnt accelerant is most likely to remain in carpets, floorboard timers, upholstery, plaster, rags, floor cracks concrete as liquids seep into porous materials
What can be said about unburned accelerant if there has been a flashover?
unburned accelerant is less likely to remain if there has been a flashover, but floor underneath furniture may have been sheltered from the fire
What can be done if unburned accelerant has seeped into some concrete?
- absorbent mineral powders such as “diatomaceous earth” / “fuller’s earth” / “Celite” can be sprinkled over the concrete surface to absorb traces of accelerant
- even disposable nappies have been used, where nothing else was available in time
How might you detect accelerants at a scene?
- GC-MS
- sniffer dogs
- portable hydrocarbon detector
Why is time important in fire investigations?
common accelerants are volatile liquids so need to test before the residues evaporate
however can exploit evaporation to detect their presence as they will be in the air around you
How can a portable hydrocarbon detector (sniffer) be used in fire investigations?
to detect flammables?
is it substance specific?
- sniffer is a tube that houses a semi-conductor and this will measure the electrical conductivity of the air
- organic vapours in air affect electrical conductivity
- flammables have low conductivity because they are not particularly polar molecules so when they are present you get drop in conductivity if vapours in air
- it is not substance specific - gives you an indicator but it is not specific - just telling you if volatile non-polar molecules are in the air
- so not conclusive evidence tool
how can sniffer dogs be used in fire investigations?
what are their pros and cons
- they can be trained to detect 0.1ppm of petrol in an ideal clean sample
- they are temperamental
- can only indicate possibility of accelerants so lab samples still needed (as lab samples meet court requirements)
Why must take more than one sample at a fire scene?
in any location, there will be a characteristic set of molecules that will be around for one reason or another:
- cooking habits
- lifestyle
- furniture
- etc.
so will need to be able to compare any fire sample you take with appropriate controls
define the three samples must take at fire scene?
what other samples could be taken from suspect?
fire sample - sample from as near as possible to seat of fire to be tested for presence of accelerant
comparative control sample - same material as fire sample but taken from elsewhere in room and therefore uncontaminated with accelerant
- aka substrate control
- to remove variability of the material from the measurement
negative control sample - tools and empty containers of the same type used in sampling process to ensure containers aren’t contributing to evidence
if there is one:
- clothing from suspect
- material from suspect’s premises
What are the four sample locations to take?
flooring where accelerant use suspected
- easily removed e.g. floorboard, laminate, carpet
- at least 1 m^2
- both burnt and unburnt for comparison
- include underlay which may have unburnt material
swab-able surfaces
- tiles and light bulbs are frequently coated with condensation products
- things that did not fully combust might sit on surfaces as residues and can scrape them off
- sterile swabbing of tiles with control swabs from clean areas
- package lightbulbs
floorboards
- tongue and groove burning patterns can indicate seepage of fuel between cracks
- control samples
- be careful if petrol-powered tools needed to obtain sample
soil
- soil under completely burnt floors may have soaked up fuel
- take top few cm - and control
What type of compounds will we be packaging from fire scene?
What are needed properties for sample packaging?
why can’t we use ordinary polyethylene bags?
- housing volatile long chain hydrocarbons and aromatic molecules e.g. toluene and xylene
- need a container that will retain compounds, that is airtight and that doesn’t absorb compounds
- ordinary polyethylene bags are too porous to hydrocarbons as they can leach through and escape
What is the material used for packaging?
What is reason for this choice of material?
How are these sealed?
What are other methods of sample packaging from fire scene?
- dual bagging in nylon (inner) and polypropylene/polyethylene (outer)
- must be free of plasticisers
- nylon - polar and hydrocarbons have low affinity for it so they leach into it less. it is stronger
- polypropylene - protects outside of nylon from elements
- sealed by knotting the neck and sealing with cable tie (not adhesive tape)
- glass jars with metal lids (lid must be able to withstand vapour pressure of volatile liquids)
- metal cans not lined as plastic linings have hydrocarbons - not suitable for corrosive samples as corrosives will eat metals
What must contamination be avoided from in sample packaging?
avoid contamination from gloves and tools as this will spoil samples
What are general rules for sample packaging?
1 - accurate labelling of all samples is necessary.
2 - documentary evidence of sample provenance and history is vital - record original location, name of person taking sample and date
3 - good record keeping is essential to ensure continuity of evidence.
- samples for analysis should be signed in and out of lab - continuity tracking or lab chain of custody using electronic records (bar coding)
4 - tamper evident seals on sample containers to increase security during storage
what are positive and negative control samples?
positive control sample - prepared with volatile flammable on control sample
- any chemistry that happens to the actual samples would happen to the control too
- packaged and treated in same manner as fire scene samples
- analysed at the end of analytical sequence to reduce the risk of contamination
negative control sample - from within analysis laboratory should be monitored to guard against accelerant traces arising from contamination of lab equipment e.g. swab from workbench, samples of pure solvents etc.
What is passive headspace analysis?
apparatus for accelerant recovery by vapour concentration
- hang a charcoal strip in a jar
- it is super adsorbent so the headspace vapours volatilise
- they get trapped in charcoal for later analysis
- 60 - 80 degrees for 2 hours
What is dynamic headspace analysis?
Describe the advantage over passive headspace analysis
- vapour is drawn from sample through an adsorbent (charcoal or TENAX - a porous polymer based on diphenylene oxide which traps organic volatiles but has a low affinity for water) using a flow of carrier gas such as dry nitrogen
- detection limit as low as 1 microlitre of accelerant
- applicable to a wide range of accelerants including alcohols and ketones
advantage: have greater control of sample flow rate and have less worries about the heat used (which can be a consideration in passive headspace analysis)
How are adsorbents collected in passive or dynamic headspace analysis analysed?
- eluted from adsorbent using a solvent such as pentane, diethyl ether or carbon dioxide
- samples analysed by gas chromatography
- samples containing volatile solutes are injected into high temperature port and vapourised
- sample is carried into separating column by a stream of chemically inert carrier gas (helium/nitrogen)
- column temperature can be increased in a controlled way - this controls extent to which vapours stay in gas phase
- GC column separates solutes by adsorption on active column material or partition in liquid coating on an inert packing material
- each solute has its own characteristic retention time which depends on the set up of the experiment
How to interpret gas chromatography?
- retention time depends primarily on molecular mass i.e. number of carbon atoms in molecular skeleton and secondarily on the structure of the molecule (linear, branched, cyclic, aromatic)
- for samples with broad boiling point range, programmed temp increase of column is used
How are components with different number of carbon atoms spread in distillate fuels?
What does this pattern allow?
- the spread of components with different numbers of carbon atoms (Gaussian distribution) is a characteristic pattern of distillate fuels (i.e. fractionated by distillation over a temp range)
- for example petrol has peaks that come out at a lower retention time than diesel which makes sense as petrol is a lighter mix of hydrocarbons than diesel so diesel will have components that get retained for longer
- this pattern allows these fuels to be distinguished from mixtures of unrelated single substances that might be present in a sample - such as pyrolysis products
What is missing from chromatogram of fire sample of gasoline compared to fresh unevaporated sample?
- the most volatile components are missing from the fire samples because they have evaporated before the samples could be collected
- this means weathered fuel no longer matches a fresh standard/reference sample therefore shows how important age of the sample is when looking at GC
- most volatile components = ones with lower molecular mass among compounds of the same type
- this doesn’t necessarily apply if low molecular mass oxygenate additives are being compared with higher molecular mass hydrocarbons for example
What are the other substances present in fire residues?
- need to identify these for comparative control samples
- combustion and pyrolysis products of other materials
- e.g. carpets (wool, nylon), carpet underlay, polymers, adhesives, wood products, clothing
- fire retardants
- e.g. brominated hydrocarbons (remove free radicals and effectively interrupt the chemical mechanism of the combustion process)
- environmental and toxicity concerns about some of these chemicals
What is the rule that professional arsonists follow?
to use only fuels available at the scene
- accumulations of rubbish etc.
- less likely that witnesses will see the arsonist carrying suspicious objects
- less likely that an investigator will conclude that the fire was arson
