Lecture 20

Question 1

what type of burn pattern suggests an accelerant was used (4)

Answer 1

severe and localised damage

not uniform across a floor

coming out from the source and expanding due to how liquids absorb through materials

a ‘sloshing’ pattern or spots just outside of the main part are likely to suggest a liquid was used

Question 2

what is meant by a streamer or a trailer spot of accelerant

Answer 2

the little spot seen just away from the main area

suggesting liquid has splashed on a hard floor

Question 2

what is meant by a smoke record and what is it used for

Answer 2

the smoke deposits left on all open surfaces

a good indicator of what may have happened at a scene

regions without smoke deposits are likely to have been covered

glass can show if broken before or after fire
objects on desks leave ‘shadows’
door frames show door position

Question 3

what three things can be used to help reconstruct a fire scene

Answer 3

witness statements

pre-fire photos - use of social media and digital records make this easier

matching objects up to burn patterns

Question 3

what three things are done/used by firefighters that can destroy a fire scene

Answer 3

high pressure hoses

windows smashed for ventilation

items cleared out to take away fuel that could increase fire

Question 3

what do arcing wires develop

what does this suggest

Answer 3

beads at the ends

this suggests the electrical fault was prior to the fire

Question 4

why is the excavation of the debris at a fire scene needed

Answer 4

debris may have fallen and could cover the seat of the fire or remnants of ignition sources

Question 5

what type of electrical good may be the cause of a fire

Answer 5

counterfeit ones e.g chargers

due to design faults or incorrect use

Question 6

what can excess current in a cable do

Answer 6

melt the insulation and this means wires can come into contact

Question 7

fires cause ……. but ……. also cause fires

Answer 7

electrical faults

Question 8

what are two examples of timing devices that may be used in an arson

Answer 8

a candle burning until it reaches a fuel source

soldering connections into an automated socket timer

Question 9

what information is lab testing of samples used for

Answer 9

information regarding the ignition of a fire

in the search of liquid residues

Question 10

what is the presence of accelerants at a scene evidence but not proof of

how are these identified

Answer 10

arson

using chemical analysis

Question 11

where is unburnt accelerant likely to be at a scene

Answer 11

within porous materials

carpets, floorboards, upholstery, plaster, rags, floor cracks, concrete

these places are where we want to swab

Question 12

where is unburnt accelerant less likely to be present

Answer 12

when flashover has occurred

Question 13

what can be sprinkled over concrete surfaces to absorb traces of accelerant

Answer 13

absorbent mineral powders e.g diatomaceous earth or celite

disposable nappies have also seen to be used in an emergency

Question 14

what three methods can be used for the detection of accelerants for on site analysis

but what does this do and what else is needed for evidence to go to court

Answer 14

portable GC-MS or in lab

sniffer dogs = less reliable but high sensitivity

portable hydrogen sniffer - organic vapours affect the electrical conductivity of a semiconductor detector

only indicates possible accelerants, lab samples and analysis is still needed

Question 15

what three samples are needed from a fire scene

Answer 15

fire sample = near as possible to the seat of the fire

comparative control sample = same material as fire sample but from elsewhere in the room and uncontaminated with accelerant

negative control sample = tools and containers of the same type used in the sampling process (ensures no contamination)

Question 15

why is time of the essence to collect unburnt accelerants

Answer 15

they are volatile liquids so need to collect before they evaporate

Question 15

is it illegal to possess accelerants such as petrol, diesel or butane

Answer 15

no

Question 16

what samples should be taken from a suspect of a fire scene if there is one

Answer 16

clothing, materials from thei premises, hand swabs

Question 17

how could samples from a fire scene be packaged (5)

Answer 17

in airtight containers so they can retain long chain hydrocarbons and aromatic molecules e.g toluene and xylene

ordinary evidence bags are too porous

bags must be free of plasticisers - seal by knotting the neck and sealing with a cable tie, often see double bagging of an inner nylon and then outer polyethylene or polypropylene

glass jars and metal lids - lid needs to withstand vapour pressure of liquids

metals cans - not lined with plastic as this is a hydrocarbon, resistant to puncture but not good for corrosive samples

Question 18

give 4 types of samples that may be taken from a fire scene

Answer 18

flooring where accelerant was
(include burnt and unburnt and underlay, at least 1m^2)

swab-able surfaces
(tiles and lightbulbs)

floorboards

soil
(under completely burnt floorboards)

ALWAYS REMEMBER CONTROLS

Question 19

what is essential for the tracking of fire scene evidence (3)

Answer 19

accurate labelling of samples - record original location in contemporaneous notes

good record keeping ensures the chain of custody - ideally electronic records

tamper evident seals on sample bags

Question 19

what type of tools should not be used to recover evidence from a fire scene

Answer 19

petrol powered tools as petrol is a type of accelerant

Question 20

what is the name of the seal used to bag fire scene evidence

Answer 20

swan neck seal

Question 21

in lab analysis what are the two types of sample analysed as well as the fire scene sample

Answer 21

a positive control = the volatile liquid, packaged and treated the same as the scene sample

negative control from within the analysis lab, to monitor accelerant traces in lab equipment

Question 22

when is the positive control sample normally analysed in the lab

Answer 22

at the end of the analytical sequence to reduce the risk of contamination

Question 23

what is the name of the two methods used for sampling possible accelerant traces collected from a scene

Answer 23

passive headspace sampling

dynamic headspace sampling

Question 24

what is passive headspace sampling

Answer 24

the vapour in a sealed container is exposed to activated charcoal which is a chemical adsorbent where it is trapped for analysis later

stored at 60-80degrees for 2 hours

can also use tenax

Question 25

what is dynamic headspace sampling

Answer 25

vapour is drawn from the sample through an adsorbent using a flow of carrier gas e.g dry nitrogen

the adsorbent is activated charcoal or tenax

can be used with alcohols or ketones so applicable and low detection limit

Question 26

how are accelerant traces removed from the adsorbent in the two sampling methods discussed

how are these samples then analysed

Answer 26

they are eluted using a solvent such as pentane, diethyl ether or carbon disulfide

then analysed by GC
- sample is injected where is it vaporused
- carried into a separating column by a inert carrier gas e.g helium, nitrogen
- column separates solutes by adsorption and each has its own retention time
- lower molecular weight tend to be eluted earlier

Question 26

what is TENAX

Answer 26

a porous polymer based on diphenylene oxide which traps organic volatiles but has low affinity for water

Question 27

what is meant by a ramping cycle

when is this used

Answer 27

a programmed change in tend used in the GC analysis

used when a sample has a broad boiling point range

Question 28

can aromaticity affect retention time

Answer 28

yes

Question 29

what does retention time primarily rely on

secondarily

Answer 29

molecular mass

the molecule structure = linear, branched, cyclic, aromatic

Question 30

what type of distribution from GC is a characteristic pattern of distillate fuels (fuels fractions by distillation)

what does this allow

Answer 30

a Gaussian distribution shown by petrol

allows these fuels to be distinguished from mixtures of unrelated single substances (e.g pyrolysis products)

Question 31

in GC analysis why may we not see any data for the first portion of the chromatogram and why a sample may not directly compare to a reference

Answer 31

samples at scene may become weathered due to aging or heating at the scene = evaporation of some components

could also be incorrect storage of samples

the most volatile and lower molecular weight components are likely to evaporate first here so wont be seen in the chromatogram

Question 32

what three things do we need to be aware of when analysing GC data from scene samples that could suggest the presence of certain molecules so we don’t misinterpret the data

Answer 32

1. other substances in fire residues
2. combustion and pyrolysis products of other materials than the fuel e.g carpets, carpet underlay, wood, clothing
3. fire retardants that may have been used at the scene e.g brominated hydrocarbons

Question 32

what is something an arsonist may do to prevent an investigator tracing the accelerant/fuel they used back to them

Answer 32

use fuels available at the scene

so no accelerant doesn’t mean no arson

Question 32

what does the NCFS ignitable liquids reference collection include

when should this be used

Answer 32

common accelerants with mass spec data which is often coupled with GC

this should not replace reference comparisons within the lab they should be used aswell

Question 33

what can surfactants and fire suppressants used by firefighters be confused with

how can we prevent this

why is this a difficult kind of database

Answer 33

pyrolysis products of fuels and accelerants

use a online database of GC retention times with things commonly used at fire scenes so the results can be compared

GC retention times are not absolute they depend on the parameters of the GC analysis but these can also be defined on the database

pattern matching is important of chromatograms and most fuels are complex mixtures not pure substances