Lecture 7 - Trace and Contact Evidence Flashcards
What are the 3 categories of forensic evidence?
- Biological
- Physical
- Impression
What are some visual screening techniques for searching evidence?
- Sunlight
- White light/torch
- Oblique light
- Crime lite
What is a crime lite?
Hand-held, high intensity light source
How does a crime light work?
Filters wavelengths allowing forensic evidence to be targeted
What are 3 examples of things crime lites can detect?
- Blood
- Semen
- Saliva
- Bone
- Teeth
- Hair
- FIbres
- Shoe print
- FIngerprints
- Gunshot residue
- Accelerants
What is biological evidence?
Body fluids or cells containing DNA which can be used for identification purposes
What are 3 bodily fluids that can be chemically tested?
- Blood
- Saliva
- Semen
What chemical tests are used for blood?
- Combur
- Kastle Meyer
- Luminol (often used last)
How does the Combur test work?
- Catalytic reaction of haemoglobin on a peroxide substrate
- Colour change from yellow to green
- Sensitive but not specific
How does the Kastle Meyer test work?
- Phenolphthalein is used to detect possible presence of haemoglobin
- Colour change to pink
- Less sensitive but more specific
How does Luminol test work?
- Reacts with iron in haemoglobin causing luminescence
- Non-specific, other substances may react including bleach, copper and horseradish
What chemical test is used for saliva?
Phadebas
How does the Phadebas test work?
- Saliva is rich in the enzyme alpha-amylase (a-amylase)
- Phadebas test reacts to a-amylase by releasing blue dye
- Presumption test indicating saliva may be present (amylase is found in other body fluids at lower levels)
What chemical test is used for Semen?
Acid phosphatase (ACP)
How does the Acid Phosphatase test work?
- ACP is found in high concentrations in semen
- Suspect stain is sampled by swab or damp blotting paper and a solution (sodium alphanapthylphosphate and fats blue B) is applied
- Purple blue colouration if positive
What to do when collecting biological evidence?
- Risk assessment (safety always before evidence)
- PPE (gloves and mask minimum)
- Records (photos and notes)
- Use sterilised/sterile equipment
How would you collect blood sample?
One wet swab using sterile water
How would you collect saliva sample?
Two swabs - one wet then one dry
How would you collect trace/touch sample?
Two swabs - one wet then one dry
How would you collect cigarette butt?
Entire item into envelope/plastic pottle
How would you collect clothing?
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 some examples of physical evidence?
- Glass
- Paint
- FIbres
- Soil and pollen
- Hydrocarbons
- Gunshot residue
- Explosives
What is physical fit?
- Pieces left at a scene that can be linked back to the source
- Physically fitting the pieces back together
- Like a jigsaw puzzle
In what 5 situations could glass be left behind?
- Hit and run
- Ram raid
- Assault with bottle
- Broken windows
- Smashed windows
How is glass linked back to its source?
- Refractive index
- Density
- Thickness
- Colour
- Physical fit
What percentage of glass backscatter is there likely to be?
30%
What should glass be packaged in?
Plastic bottle/container
What is the glass compared to? (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
In what situations could paint often be left behind?
- Hit and run
- Ram raid
- Assault with painted object
- Burglary
How is paint linked back to its 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
When can tool marks occur?
- When used to force open a window or door
- Softer surfaces
- Burglary
How are tool marks collected?
A cast is created by putting a paste in the tool mark, letting it dry and peeling it off
What are the class characteristics when linking tool marks to tools?
- No damage
- Original condition
- Type and size of tool used
What are the individualizing characteristics when linking tool marks to tools?
- Wear and damage
- Microscopy used
- Type, size and specific tool used
What are the class characteristic when linking shoeprints to shoes?
- No damage
- Size and make of shoe
What are the individualising characteristic when linking shoeprints to shoes?
- Wear and damage
- Size, make and specific shoe
