Fire and ignitable liquid residues Flashcards
What is in the fire tetrad?
Oxygen(if falls below 15% then combustion stops because flames can no longer be supported and will go out. Also include chemical oxidisers such as ammonium nitrate and H2O2 with readily released o2) , fuel (organic material containing H+C), ignition(gases released from heated objects mixed with oxidising agent that gives heat opportunity to ignite gas) and heat (more energy released and temp increases)
What are the 3 types of combustion?
Flaming combustion, Glowing combustion and explosive combustion
What is flaming combustion?
Ignition temp of fuel is reached and combustion begins
What is glowing combustion?
Burning without flame, oxygen limited, aka smouldering, occurs on surface of solid fuel in absence of high heat enough to pyrolyse
When solid object burns it will either do what?
Pyrolysis or Pulverisation
What is pyrolysis?
Under heat flow, decomposition solid producing gas products
What is pulverisation?
Transformation of a solid into small particles with form a cloud mixed with air
What is flash point?
Lowest temperature at which fuel can form an ignitable mixture with air
What is fire point?
Temperature at which the vapour continues to burn after being ignited
What is the Auto-ignition temperature (AIT)?
Temperature at which fuel will ignite on its own without additional ignition source
What is the aim of fire investigation?
To find origin, cause and spread
What are the 3 modes of heat transfer?
Conduction, radiation and convection
What is conduction?
Movement of heat through solid object via electrons and atoms within heated object which collide with each other. Objects such as metals with loosely held electrons are good conductors of heat. Objects at scene could conduct heat
What is radiation?
Transfer of heat energy from heated surface via electromagnetic radiation. All surfaces facing heated surfaces impacted by radiant heats
What is convection?
Transfer of heat as result of movement of molecules within a liquid or gas. As heat up, it moves upwards and by convection radiates heat to surface below. Convection air can cause pyrolysis
Define arson
Reckless or malicious and intentional act of starting a fire or causing an explosion
Define accelerant
Introduced to scene to help initiate fire, spread fire, increase intensity of fire. Could be ignitable liquid or combustible material (paper) or improves mixtures (sugar and chlorate). May not burn completely and instead may soak into carpet and materials leaving residues after fire
What are detection techniques for accelerants?
Point of origin, multiple seats, burn patterns, instruments, canines
What are collection methods for fire scenes?
For sample use air tight containers and always get control. Or package in fresh nylon bag or arson tin
What are the 6 laboratory preparation techniques for fire scenes?
Steam distillation, Solvent extraction, direct headspace, SPME, Dynamic headspace and Passive headspace
What is steam distillation?
Accelerant separated from liquid or solid by vaporisation and subsequently condensed to liquid. Old technique, high risk of contamination, lots of time required, lack of sensitivity and limited range of products successful
What is solvent extraction?
Solvent placed within debris that extracts accelerant residues. Liquid is then decanted, filtered and concentrated through evaporation. Matrix products also extracted. Bad is labour intensive, destructive and lots of solvent
What is direct headspace?
Sample headspace drawn through gas syringe which is directly injected in GC. Used as first screening. Advantages are interferences reduced. Disadvantage is less concentrated samples will give poor resulting chromatograms and heavy products
What is Dynamic headspace?
Adsorbent tube or purge. Vapour of sample forced over or through adsorbent material at given temp. Accelerant vapours trapped in adsorbing material which later extracted usually by solvent extraction. Temp 60-90 degrees
