Lecture 7 - Trace and Contact Evidence Flashcards
What is Locard’s Exchange Principle?
With contact between two items there will be an
exchange.
- every contact leaves a trace
What are the different types of forensic evidence?
Biological, physical, and impression evidence
What are the different lights in visual screening?
Sunlight
White light / torch
Oblique light
What does oblique light help show in visual screening?
Invisible evidence - helps fingerprints
What is crime lite?
Hand-held, high intensity light source
Filters alter wavelength allowing forensic evidence to be targeted.
Blood, semen, saliva, bone, teeth, hair,
fibres, shoeprints, fingerprints, gunshot
residue and accelerants.
What is the standard wavelength for crime lite and why do we alter wavelength?
415-420nm
Different wavelengths for different bodily fluids
What is biological evidence?
Body fluids or cells containing DNA which can be used for identification purposes.
What chemical tests are used for blood?
Combur, KM, Luminol
What chemical test is used for saliva?
Phadebas
What chemical test is used for semen?
ACP
What happens during a COMBUR test?
Catalytic reaction of haemoglobin on a peroxide substrate
Colour change from yellow to green
Sensitive but not specific
What happens during a KASTLE MEYER test?
Phenolphthalein is used to detect possible presence of haemoglobin
Colour change to pink
Less sensitive but more specific - false negative can arise
How is COMBUR test taken?
Add water, apply to evidence, turn green if Hb present - some other things will do
What happens during luminol test?
Reacts with iron in haemoglobin causing luminescence.
What blood test is done last and why?
Luminol
Last test due to mess and disruptive - clean up marks
What substances other than blood may react to luminol test?
Bleach, copper and horseradish.
What happens during PHADEBAS test?
Phadebas Test reacts to α-amylasemby releasing a blue dye.
Presumptive test indicating saliva may be present (amylase is found in other body fluids at lower levels).
What happens during ACP test?
Suspect stain is sampled by swab or damp blotting paper and a solution (sodium alphanapthylphosphate and fast blue B) is applied.
Purple-blue colouration if positive.
What are criteria for collecting biological evidence?
Risk Assessment
PPE – gloves and mask(minimum)
Records – photos and notes
Use sterilised/sterile equipment
How do we collect blood as biological evidence?
One wet swab using sterile water
How do we collect saliva as biological evidence?
Two swabs – one wet then one dry
How do we collect trace/touch as biological evidence?
Two swabs – one wet then one dry
How do we collect cigarette butt as biological evidence?
Entire item into envelope/plastic pottle
How do we collect cigarette butt as biological evidence?
Entire item into paper bag
What is physical evidence?
Physical, microscopic and chemical evidence.
Tangible items which can be collected from a crime scene.
What are examples of physical evidence?
Glass
Paint
Fibres
Soil and Pollen
Hydrocarbons
Gunshot residue
Explosives
What is physical fit?
Pieces left at a scene can be linked back to the source
Physically fitting the pieces back together
Like a jigsaw puzzle
How can glass fragments be linked back to source?
Refractive index
Density
Thickness
Colour
Physical fit
What are collection considerations when collecting glass?
Backscatter ~ 30%
Persistence ~ 24hrs
What are packaging considerations when collecting glass?
Sharp
Can pierce paper - container
What is the glass compared too?
Control samples
Control (or reference) sample from broken window
Refractive Index varies slightly across a window
10 samples taken from around the window
- Not from the ground
When might paint be left?
Hit and run
Ram raid
Assault with painted object
Burglary
How is paint transfer linked back to source?
Layers
Colours
Microscopy
Physical fit
What is the paint compared to?
Control samples
From damaged panel
Away from the area of damage
Multiple samples
All layers, down to the base
What is impression evidence?
Contact between two surfaces leaving a mark which can be used for identification purposes.
What are tool marks in impression evidence?
Created when a tool is used to force open a window or door.
A mark will be left on the softer surface.
Burglary
Why is toolmark compared to tool microscopically?
To identify:
Class characteristics
Individualising characteristics
What are class characteristics for toolmarks?
No damage
Original condition
Type and size of tool used
