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Science Chemistry
- Science of the everyday
- Study of NON-BIOLOGICAL trace evidence
- Locard’s Exchange Principle
- E.g. paint, glass, fibres, drywall, glue, cleaner fluid, adhesive tape etc.
- Crime scene? – ordinary people, ordinary place
- E.g. criminal in woolly sweater, victim’s makeup, duct tape, bomb, materials
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Forensic Chemist’s analyze
- Anything that is not a body fluid
- Used to be part of hair and fibre (still is in US).
- Now: fibre – chemistry
- hair - biology
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Qualifications
- B.Sc. Honours (minimum)
- Chemistry – lots!
- All basic analytical techniques
- Civilians
- Then basic training in lab under a mentor ~ 15-18 months
- Mock trial
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Types of Crimes
- B & E
- broken glass,
- fragments of drywall
- Hit and run
- Car light glass
- Car lights
- Paint
- Safe cracking
- Packing material from safe
- Most crimes – many specialists
- E.G. Home invasion
- Ident (the REAL CSI!) - scene
- Pathologist - body
- Biology – DNA, semen
- Firearms – spent cartridges
- Chemist – all aspects!
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Chemists involvement
- Fibres left behind
- Soil from assailant’s shoes
- Glass on suspects clothes
- Rope used to tie homeowners
- Strange stain on floor (gasoline?)
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Analysis (Chemist)
- Ø Substance could be anything!Ø Broad range of tests to determine constituentsØ First tests – gasolineØ Further tests – type of gasoline – Esso
- Further tests – batch of Esso gas
- Qualitative analysis – what substances are present
- Quantitative analysis - % of each
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Techniques of analysis (chemistry)
- Basic chemistry techniques
- Chromatography
- Electrophoresis
- Spectrophotometry
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Chromatography
- Separates substance into components (so purifies them)
- Gas Chromatography – separates based on distribution between liquid and gas phase
- Gas chromatograph (GC) - inject sample
- Pyrolysis – adds heat, e.g. paint chip-gas
- High-Performance Liquid Chromatography (HPLC)
- Thin-layer Chromatography (TLC)
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Electrophoresis
Separates substances on basis of electrical potential
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Spectrophotometry
- Different substances absorb light in different ways
- Used to identify
- Mass Spectrometry (MS)
- Gas chromatography – tentative ID
- GC +MS (GC/MS) – positive ID
- Very basic techniques, first year chemistry
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Significance
- Identifying substance - basic chemistry
- Significance of substance – FORENSiC chemistry
- Class evidence
- In order to have value - must determine significance
- May have 0 significance
- May be very significant
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Significance
- Class evidence – may be high or low significance
- Exact match? But may be very common – so exact match to many different items
- If rare – more significant
- How common is evidence?
- Need databases for comparison
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Determining Significance 1.
- How common is this evidence?
- Develop database – parent population
- E.g. B & E, glass at scene
- Suspect has glass on him
- Physical properties – do they match?
- How common is that sort of glass?
- If common, does it mean anything? Determining Signif. 1 contd.
- Glass on person?
- Develop a database
- How common is it for regular people to have glass on them?
- Get a sample of people
- See how much glass on them
- Shoes - ~30%
- Clothes ~<2%
- So: -
- Glass itself is common
- But rare to have glass on clothing
- So, significant that suspect had matching glass on clothing
- <2% population have any glass
- Even more significant that it is that type of glass
- Less significant if just on shoes (position on shoes?)
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Determining Significance 2
- How valid is your database?
- Does it fit this person – or how likely is it that this person could innocently have this evidence?
- Glass on clothing rare – but what if guy is a glazer?
- Suspect must fit in research population
- Validity:
- Drywall?
- Rare in average populationØ
- Not in person renovating
- Gun residue?
- Rare in average population
- Not in person at gun range
- Therefore, need database into which suspect fits
- How common is it to find glass on glazier’s clothes?
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Determining Significance 3
- What is population/situation?
- Some things may be very common in some situations and very rare in others
- Livingston in Africa
- Scenario?
- Manure from bomb?
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Determining Significance 4
- More than one type of class evidence
- Cumulative - the more there is the more significant – multiplies probabilities
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Fibres
- Natural fibres – animal or plant sourcesØ Manmade fibers
- Polymers –
- e.g. nylon (large number of atoms)also include paint, adhesives, plastics
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Fibre examination
- Colour
- Striations on fibre
- Cross-sectional shape
- Refractive index
- Infrared spectrophotometry
- Etc.
- How rare is the fibre? Circumstances?
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Significance/circumstances (fibre)
- Hit and Run
- Fuzzy sweater fibres
- Very common
- BUT – not common stuck in broken head light glass of a car with a smashed fender
- Glass from light on victim?
- Exchange of evidence – very significant
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Paint
- Everywhere!
- Cars – all have at least 4 layers normally
- Match to database of car colours
- Chemically examine paint – e.g. chromatography
- If repaint job – more significant but harder to find
- Car brake lights – on or off?
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Arson
- Chemist may attend AND may be special fire investigators
- May be direct arson for the sake of the fire
- May be insurance
- May be to disguise a crime
- Very difficult to destroy a body!
- Usually lots of evidence left behind!
- Remember – body in dumpster?
- Arson?
- Was it arson?
- Accelerant?
- Gas?
- Chemicals?
- Kindling?
- “bounce”?
- Significance?
- Where is it found?
- Samples
- Collect Sample
- Must take a control
- If an accelerant is found, is it also on control?
- Then maybe not used as an accelerant
- Many accelerants can be specifically identified
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Explosions, bombs
- Usually homemade
- How was it constructed?
- What was it made of?
- What explosive was used?
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Explosion
- Combustion with release of gases, which expand with the heat
- Very fast
- If confined – pipe bomb – shrapnel
- Large blast – gases escape - >7000mph
- Gale force – knocks over buildings, walls, kills
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Classification of explosives
- Based on speed at which the explosive decomposes
- Low explosives – speed of deflagration (burning) – rapid oxidation, heat, light, subsonic pressure wave.
- High explosives – speed of detonation – creation of supersonic shock wave – instantaneous build up of heat and gases
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Low explosives
- Throwing or propelling motion
- Normally just burn
- Black powder, smokeless powder
- e.g. fuse to gunpowder
- Lethal when confined - pipe bomb
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High Explosives
- Primary explosives – ultra-sensitive to heat, shock or friction. Detonate violently instead of burn. Primers, blasting caps
- Secondary explosives – insensitive to heat, shock or friction. Burn rather than detonate if in open air e.g. dynamite, TNT etc.
- Bombs must be detonated by an initiating explosion
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Bombs
- Explosive
- Timers
- Detonators
- Fuses
- Batteries?
- Duct tape?
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