fires and explosions

Question 1

fires and explosions are often linked

Answer 1

explosions are almost always accompanied by fires
fires often result in explosions when energetic materials at the scene become involved

Question 2

fires and explosions are a result of combustion

Answer 2

chemical rxn of a fuel with oxygen
products are carbon dioxide, water and energy

Question 3

activation energy

Answer 3

all chem reactions require a small amount of energy to get started

Question 4

exothermic reactions

Answer 4

chem reactions that releases energy as one of its products
produced energy > activation energy
all combustions are exothermic

Question 5

endothermic reactions

Answer 5

require the input of energy to take place

Question 6

fires vs explosion

Answer 6

whether a fuel burns or explodes depends on the nature of the fuel
how close the oxygen and fuel are to each other during the reaction

Question 7

fires

Answer 7

important to know cause
arson is deliberately set with criminal intent
take place by a slow and ordinary combusion
oxygen must be broken up into atoms so they can get close to fuel
takes time and activation energy
must determine characteristics and damage caused by fire
point of origin and cause

Question 8

conditions for a fire

Answer 8

four elements
heat/energy
fuel
source of pxygen
chain reaction between fuel and oxygen
self sustaining until one element is lost

Question 9

ignition temperature

Answer 9

temperature of heat or energy necessary

Question 10

fuel contions

Answer 10

fuel must be a vapor in order to combust

Question 11

flash point

Answer 11

lowest temp that will allow a liquid to produce a flammable vapor
not hot enough to ignite fuel

Question 12

flame point

Answer 12

temperature that will ignite fuel and sustain the combustion

Question 13

accelerants

Answer 13

sometimes used to start a fire more quickly
fuels that are easily vaportized
support combustion
highly exothermic
poured around the area to be burnt and ignited
burn easily
provide activation energy to start other things on fire
leave behind residue even if burnt
good for forensic examination
common: gasoline, kerosene, charcoal lighters, paint thinners

Question 14

types of fires

Answer 14

natural accidental deliberate (incendiary, forensic interest)

Question 15

natural

Answer 15

lightning strikes

Question 16

accidental

Answer 16

may be difficult to distinguish from deliberate fires
furnace fire (accidental)
furnace tampered with to cause a malfunction that causes. a fire (deliberate, arson)

Question 17

deliberate fires

Answer 17

all possible causes of natural and accidental causes must be eliminated
need to determine if it is arson
accelerant residues
multiple points of origin
fire trails

Question 18

source of ignition/cause of fire

Answer 18

direct ignition
most basic
direct application of flame source to fuel
may be a time delay mechnaism
usually easy to determine intent
electrical
may be accidental or deliberate
weather related
mechanical
parts over heat due to misuse, overuse, incorrect placement
car fires, lint fires

Question 19

recognition and collection of evidence

Answer 19

critical to determine point of origin
most likely place to find physical evidence
accelerant
timers
trace
can be very difficult to investigate
severe damage done by burning and collapse
attempts to suppress fire may obscure evidence
dark due to cutting off electricity
dangerous
weakened structure and smoldering embers

Question 20

point of origin

Answer 20

location where initial ignition took place
most intense damage and burning
characteristics
low burning fire burns in an upward direction
V patterns
near a wall, smoke damage usually occurs in a V
wood charring
depth depends on time and heat
spalling of plaster or concrete
destruction due to heat
material distorition
metal or glass melt or distort
soot and smoke staining
large amount of sooting

Question 21

indications of arson

Answer 21

presence of accelerant
necessary elimination of natural or accidental causes
fire trails
pouring accelerant, uneven burning in a line
multiple points of origin

Question 22

evidence

Answer 22

finding can be difficult
focus on trace at points of entry exit and origin
evidence may be changed or destroyed due to fire and accelerants

Question 23

methods to detect potential accelerant

Answer 23

arson dogs
can sniff out hydrocarbons used in accelerants
anaytical devices (GC with gas sensors)
investigator’s experience

Question 24

evidence packaging

Answer 24

package trace as you would any other science, post it notes and bindle
if packaging evidence to be tested for accelerant, use airtight containers
accelerants are volatile and evaporate easily
metal unused paint cans are best
airtight durable can puncture a hole in top to analyze and can withstand heat

Question 25

analysis of evidence

Answer 25

two steps
isolate accelerant
determine the nature of accelerant residue

Question 26

isolation of accelerant residues

Answer 26

headspace method
adoption methods
passive adosption
active adsorption
solid phase micro extraction
solvent extraction
steam distillation

Question 27

headspace method

Answer 27

liquid accelerant is placed into airtight metal can
some will vaporize (headspace) and some will stay liquid until an equilibrium is reached
some of the headspace vapor can be sampled and injected into gas chromatograph

Question 28

adsorption methods

Answer 28

using charcoal to absorb large quantities of hydrocarbon and eluting the sample off the charcoal

Question 29

passive adsorption

Answer 29

analyst puts a charcoal strip into the metal can either at the scene or the lab

Question 30

active adsorption

Answer 30

air is actively pumped into and or through two charcoal tubes in the evidence container
the exit charcoal tube absorbs the accelerant

Question 31

solid phase microextraction

Answer 31

fibers made of fused silica and coated with charcoal are placed into the evidence container
fiber can be directly inserted into gas chromatograph

Question 32

solvent extraction

Answer 32

solvent added to evidence container
solvent is poured off, filtered, and evaporated
leaving accelerant residue

Question 33

steam distillation

Answer 33

some accelerant residue is put into distillation apparatus with water
boiled and distilled
accelerant residues either sit as a layer on distilled water and can be removed
or if soluble in water, the first aliquot of water and collected and analyzed

Question 34

gas chromatography

Answer 34

almost universally used as a separation technique
mass spectrometry usually detector
produces chromatogram of the diffeerent components of the accelerant
compares to the knowns in the spectral library

Question 35

explosion

Answer 35

difference is the speed of the combustion reaction
explosions happen very rapidly
fuel combines with oxygen to create gaseous products
gases expand rapidly away from the origin of the explosion, bomb seat

Question 36

three primary effects

Answer 36

blast pressure
fragmentation effects
thermal effects

Question 37

blast pressure

Answer 37

rapid expansion of gases create a wave that moves away from bomb seat
positive pressure phase
vacuum is created and the gases pulled back toward the bomb seat
negative pressure phase

Question 38

fragmentation effects

Answer 38

bomb casing can shatter and be propelled
bomb may be wrapped in potential shrapnel (nails, etc.)
blast can break up objects in its path and propel them

Question 39

thermal effects

Answer 39

least damaging effects
large ball of flash produced at bomb seat
high explosives, very hot and brief
low explosives, less hot and longer

Question 40

velocity categorizations

Answer 40

low explosives
below 3280 fps
oxygen is physically mixed with the fuel
slower controlled rate
push objects rather than shatter them
used in blasting operations, gunpowder
high explosives
above 3280 fps
oxygen and fuel are chemically combined
designed to chatter and destroy objects
detonation
usually require a shock
TNT, nitroglycerine, C4

Question 41

types of explosions

Answer 41

nothing to do with type of explosive
describes efficiency
high order explosion
occurs at or near the maximum detonation velocity
low order
occurs at less than optimum efficiency
due to old explosives, explosive subject to humidity or moisture, improperly constructed device, improper placement of divide
usually more forensic evidence some things still intact

Question 42

explosions analysis

Answer 42

two major types of explosive residues
undetonated explosive
exploded residues that are products or side products of the explosion
bomb seat may yield parts of the devide
may be able to tell type of device how it was detonated and what part of the world it came from
who detonated it
get trace DNA fingerprints of devices or parts
may need visual examination to remove explosive materials
after explosive residues have been isolated
several analysis techniques
Thin layer chromatography TLC
high performance liquid chromatography, HPLC
capillary electrophoresis
infrared spectrophotometry