Trace evidence analysis Flashcards
What can forensic evidence establish?
- A crime has been committed
- Links between scenes, victim, offender
How can forensic evidence help reconstruct an event?
Can establish the who, what , where, when and why
When does forensic evidence vary?
Depending on the circumstances of the crime
What is trace evidence?
Microscopic quantities of material that are of probative value in a forensic investigation
What are the 4 types of trace evidence?
-Biological
-Physical
-Particles, marks/impressions
-Natural
What is the forensic value of trace evidence?
-Answers key questions
-Mute witness
-Can associate a person with a crime
-Can determine chronology
-Links between person and scenes of interest
Why can trace evidence be imprecise?
Gives a circumstantial link
What is the transfer stage in the forensic process?
When a crime occurs. The transfer of trace evidence occurs onto the perpetrator.
E.g. Person breaks through a window, fibres transfer to window, glass transfers to perpetrator
What is the persistence stage in the forensic process?
How long will the evidence persist and how long will it be useful.
Have efforts been made to remove the evidence?
What is the evidence collection stage in the forensic process?
Police have collected the evidence and arrest the suspect and make efforts to collect trace evidence from their clothing
What is the analysis stage of the forensic process?
Lab analysis of trace analysis retrieved from the suspect
What is the interpretation stage of the forensic process?
Interpretation of the trace evidence and the results following analysis in the lab
What is the presentation stage in the forensic process?
Presentation of evidence in court
Why are the transfer and persistence stage important?
Any influence that adds, changes, relocates, obscures, contaminates, or obliterates physical trace evidence, regardless of intent
When does the transfer stage occur?
During the crime
What can affect the transfer of evidence during the crime?
-Weather: Affects the spread
-Time: How long event occurs
When does the persistence stage occur?
Post-crime
What can affect the spread of trace evidence post crime?
-Clothes can be left in dry cleaners
-Shoes worn
Why is the transfer of evidence often undetected?
Microscopic
Where does the transfer of trace evidence occur?
-Victim & Crime scene
-Perpetrator & scene
-Perpetrator & victim
What are the modes of evidence transfer?
-One way
-Two way
-Direct vs indirect
-Secondary transfers
What is one way evidence transfer?
Gunshot residue fired onto a victim
What is two way evidence transfer?
Burglary, burst through window, glass found on suspect, fibres picked up on glass at scene
What is direct evidence transfer?
Contact between someones shoe and the ground
What is indirect evidence transfer?
Pollen onto hair and clothing
What are secondary evidence transfers?
Person shoots gun, meets friend and shakes hand, gunshot residue is transferred to second person
How does clothing affect trace evidence?
-Clothing fibres varies in composition and texture
-Influences fibre shedding
-Surface characteristics influences the adhesion of other traces
How does weather affect trace evidence?
-Local conditions affect transfer of environmental trace evidence
-Influences deposition of other evidence types at the scene
How does trace characteristics vary in trace evidence?
-Particle size/shape
-Glass: Fragments or particles
-Pollen: Different size and surface textures
How is the frequency of trace evidence affected?
-Is the trace easily transferred e.g. pollen, hair, soil
-How commonly is the trace ‘naturally encountered’ significant to the case?
How frequent is pollen as trace evidence?
-Abundant in the environment
-Direct transport mechanisms (walk over it)
-Indirect transport mechanisms
How frequent is glass as trace evidence?
- Uncommon to general population
-Less common as trace evidence, more significant to the case
What is the trace persistence of forensic evidence?
-How long is evidence preserved for after the crime scene
What is the trace persistence of forensic evidence impacted by?
-Source/recipient
-Environment
-Evidence characteristics
-Initial transfer
-Activity
How does wear impact trace evidence?
-Loss of evidence over time
-Fibre persistence on clothing items
-Similar trends with most traces
How does offender activity affect trace evidence?
-Post crime activity can contribute to loss of evidence
-Washing, burning, cleaning, vacuuming
-Packaging exhibits
When must trace evidence dynamics be considered?
During collection, analysis and interpretation
What are the techniques for evidence collection?
-Picking, scraping, brushing, combing
-Tape lifts
-Vacuum
-Swabs
What can trace evidence analysis identify?
Class characteristics: Colour, shape, RI etc.
What does trace analysis establish?
- If properties differ significantly = evidence not from the same source and can be excluded
How can evidence be classified?
-Morphology
-Optical
-Physical
-Chemical properties
What are the general physical characteristics of trace evidence?
-Melting point
-Boiling point
-RI
-Absorption and emission spectra
-Density
-Molecular mass
-Colour
What are trace evidence analysis techniques?
-Non-destructive techniques e.g. microscopy
-Destructive analysis e.g. chromatography
What information does non-destructive analysis techniques produce?
-Physical and morphological
What information does destructive analysis techniques provide?
chemical
What does analysis techniques depend on?
-Amount of evidence
-Value of evidence
-Type of crime
-Resources
-Aims of enquiry
What is a stereo microscope used for in analysis?
Preliminary examination
What magnification does a stereo microscope use?
30X
What information can a stereo microscope provide?
-Substance type
-Size
-Form
-Colour
-Texture
-Appearance
-Isolation of individual particles
What is a compound microscope used for in analysis?
Higher power exam
What magnification does a compound microscope use?
100-1000X
How does a compound microscope work?
-Transmitted light = sample prep
-Light from the base, through condenser and specimen
-Light passes through objective and occular lens
What magnification does a scanning electron microscope use?
10-500,0000X
What are samples coated in using a SEM?
Gold coated
How does an SEM work?
-Scans surface with high focused high energy electron beam
-Secondary electrons are detected
-Creates images
What can be referred to when interpreting results?
-Databases
-Scientific reports
-Scientific surveys
-Examiner experience
How can paint be classified?
-Number of layers
-Colour
-Surface texture
-Chemical composition
What analytical tools are used to analyse paint?
-Stereo microscopy
-Solvent tests
-IR spectrophotometry
-SEM-EDX
-GCMS
How can paint be found as trace evidence?
As paint chips, fragments and particles
What does glass consist of?
-Sand
-Sodium carbonates
-Calcium oxides
-Impurities
How can glass be classified?
-Pattern matching
-Fracture shape
-Density
-Refractive index
What type of fibres can be found as trace evidence?
Natural and synthetic
How can fibres be classified?
-Nature of fibre
-Colour
-Pigment distribution
-Geometry
-Surface characteristics
What techniques are used to analyse fibres?
-Microscopy
-FTIR
-GCMS
