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Chapter 23 Fires and Explosions
Forensic Science (CHEM1118)
Fairleigh Dickinson University
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**● What is a Fire? ** ○ Fire is the evolution of energy in the form of light and heat and smoke as the result of combustion. ○ Combustion is a type of chemical reaction whereby a fuel reacts with oxygen to release energy. ○ Reactions that give off energy are termed ƈexothermic.Ɖ ○ The atomic oxygen now reacts with the methane in the combustion. ○ The amount of activation energy needed to get the reaction started is very little compared to the energy that is emitted by the reaction. ○ The activation energy can be in the for of a spark or small flame. ○ Exothermic reactions like methane and oxygen produce energy because the energy stored in C-H bonds is greater than that in C-O or O-H bonds. ○ The excess energy is given off in the form of heat and flame and smoke. ○ Gasoline is a petroleum distillate that contains more than 300 substances. ○ Most of them are hydrocarbons, which are substances made up of carbon and hydrogen. ○ Methane is also a hydrocarbon. ○ The gasoline combusts in the air present in the engine cylinders and the energy produced causes the pistons to move and propel the car. ○ One of the compounds in gasoline is octane. ○ It combusts with oxygen; the activation energy needed for this reaction serves two purposes. ○ It breaks the oxygen molecule up into oxygen atoms and it vaporizes the octane, which is a liquid at room temperature. ○ There are many other substances that can act as fuels in combustion reactions/ ○ Wood, plastic, natural, and synthetic fibers and fabrics, carpeting, tile, drywall, and most building materials will undergo combustion as long as there is sufficient activation energy available to vaporize the fuel. **● Extinguishing a Fire ** ○ Fire experts use the concept of the fire tetrahedron when describing the materials and conditions needed to sustain a fire. ○ All four of these elements must be present to have a fire. ○ If any one of them is removed, the fire goes out. ○ Fire extinguishers are based on this principle; they remove one or more of the elements of a fire. **● Incendiary Fires ** ○ Fire experts and the legal system use two closely related terms to describe fires that are deliberately set. ○ The term ƈincendiary fireƉ means a fire that is willfully and intentionally set. ○ The National Fire Protection Association recently has changed from using the term ƈincendiaryƉ to ƈintentionallyƉ but incendiary is still widely used. ○ In 2001, the Federal Emergency Management Agency published the results of a research study on arson.
○ A summary of their finding included the following: ■ Arson is the leading cause of fires in the US and the second leading cause of death. ■ Fifty percent of arson fires occur outdoors, while 30% occur in buildings and 20% occur in vehicles. ■ Half of all arson arrests are juveniles. ■ Poorer neighborhoods experience 14 times the number of arson fires as do more well-off neighborhoods. ■ Church arsons have increased since 1996. **● Investigation of a Fire ** ○ Trained fire scene investigators normally carry out investigation of fires. ○ Most of these people are trained first as firefighters and come to fire scene investigation from a career as a firefighter. ○ Some have college degrees; a few universities and community colleges offer classes and even degrees in fire science where students learn about the chemistry of fire and how fires start and spread, how they are suppressed, and how to investigate a fire properly collect and preserve evidence. ○ This is done by a physical inspection of the fire scene, discussion with the firefighters and witnesses, support of the forensic science laboratory in the analysis of fire residues, and perhaps even the use of dogs that are specially trained to detect common accelerants. ○ When a fire is discovered, the fire department will respond as quickly as possible. ○ They have two major duties at a fire scene. ○ The first is to remove everyone who may be trapped in the fire and the second is to extinguish the fire. ○ The out, the electricity and the gas are turned off if the fire didn’t already knock them out. ○ After the fire is out and the firefighters have left, the fire scene investigator takes over. ○ Fire scene investigation can be very difficult. ○ As aforementioned, there may be no light or heat or air conditioning. ○ Everything will be very wet; the fire may have weakened the structure of the building and walking through it can be hazardous. _○ Causes of Fire _ ■ From the standpoint of the fire scene investigator, there are three causes of fires: ● Natural: this could be a fire that is started by a lightning strike. ● Accidental: someone who accidentally drops a match in bed or an electrical circuit becomes overloaded. ● Deliberate: arson. ■ There are two ways that a fire scene investigator determines that a fire is arson. ■ The first is to have compelling evidence at the scene.
○ Accelerants can also greatly increase the damage of a fire because they give off so much heat. _○ Detection and Collection of Accelerants _ ■ Fire scene investigators are well trained to spot signs that accelerants were used in a fire. ■ Evidence includes extreme heat and damage, sooty V-pattern burning, and fire trails. ■ There are hydrocarbon sniffer instruments that can detect the presence of small quantities of common accelerants. ■ Some material are better than others for trapping and holding accelerants. ■ The best materials are those that can easily absorb liquids. ■ These include bedding, furniture with cushions, carpeting, clothing, and soil. ■ Substances like tile, wood, wall board, and other building materials do not absorb and trap liquids very well and are less suitable candidates for containing accelerant residues. **● Analysis of Fire Scene Evidence ** ○ The biggest challenge facing forensic chemists in the analysis of fire scene evidence is separating and concentrating the accelerant residues from the fire debris. ○ This can be a real challenge because of the amount of debris present the way it is packaged, and the nature and concentration of the accelerant. ○ The concentration methods that are commonly used today are described here: ■ Passive Headspace: this is the most popular method of concentration of accelerant residues. ● The container is airtight; it is gently heated so that some of the accelerant will evaporate into the air space above the debris. ● Eventually, the amount of accelerant in the headspace will be in equilibrium with the amount in the debris. ■ Absorption/elution: this is a modification of the passive headspace method. ● Two small holes are punched in the top of the can and a tube containing activated charcoal is put in each hole. ● A vacuum pump is then connected to one of the tubes. ● When the vacuum is turned on, it pulls the air out of the headspace of the can. ● The air is pulled through one of the tubes containing the charcoal. ● Instead of a vacuum, the accelerant can be removed by pumping an inert gas such as nitrogen through one of the tubes. ■ Solid-phase Microextraction: SPME is the newest technique in accelerant concentration.
● It takes advantage of the sensitivity of today’s modern GC/MS instruments that require only a few micrograms of analyte. ● The SPME apparatus consists of a syringe whose needle is coated with charcoal or another polymer that is efficient at absorbing ● accelerant molecules. ■ Solvent Extraction: solvent extraction is used to be a popular method of accelerant concentration. ● It is performed by opening the can of fire residue and adding a suitable solvent, usually pentane or a similar solvent and mixing well. ● Then, the mixture is filtered and the solvent evaporated to a small volume and injected into a gas chromatograph. **● Analysis of Accelerants ** ○ The universal method for the analysis of accelerant residues is by GC, usually coupled to a mass spectrometer. ○ The most common accelerants are gasoline and other consumer products such as charcoal lighter, paint thinners, and lamp oils. ○ Fuels used in camping lanterns and stoves are also popular accelerants. ○ Each of these products contains many components. ○ Gasoline has more than 300 substances; the purpose of GC is not to identify each component, but to display the pattern of peaks obtained from a sample of fire debris. ○ Interpretation of gas chromatograms of accelerants can be difficult. ○ The chromatograms of gasoline and kerosene are obtained from pristine materials, right out of the can. ○ Accelerants that have been subjected to the heat from a fire undergo physical and chemical changes that will affect their chromatograms. ○ On the basis of the laboratory analysis of fire residues, a forensic chemist can reasonably conclude that accelerant residues are present in fire debris. ○ If there is enough present, the chemist may be able to determine the type of product it is. ○ One must be careful here; there are a number of commercial products made from the same material. **● Explosions ** ○ If a fuel such as gasoline is confined to a closed space and then set on fire, gaseous products are produced along with energy. ○ This will cause the pressure to build up in the container until it ruptures. ○ Most people would refer to this as an explosion, but it is actually just a fire that has been confined. ○ To someone standing nearby, this is a distinction without a difference. ○ It sure looks and sounds like an explosion. _○ Deflagration _
Chapter 23 Fires and Explosions
Course: Forensic Science (CHEM1118)
University: Fairleigh Dickinson University
