Trace Evidence Flashcards
What is trace evidence?
Microscopic quantities of material that are of probative value in a forensic investigation
Types of trace evidence
-Biological (human)
-Physical (Inc non human)
-Particles, substances, marks and impressions
-Natural vs manufactured materials
Whats the forensic value?
-Mute witness- not impacted by bias
-Microscopic- offender doesn’t know they left it
-Determines circumstances
-associates a person with a crime
-Establish spatial and temporal links between a person and scene
What’s the forensic process?
-Transfer
-Persistence
-Collection
-Analysis
-Interpretation
-Presentation
What’s transfer (Forensic process)
When the crime happens
What’s Persistence (forensic process)
How long the evidence persists for, people may make an effort to remove the evidence afterwards
What’s collection (forensic process)
Police collecting evidence to try and get trace evidence
What’s interpretation (forensic process)
Exert witnesses look at evidence, questioning why they had that evidence on them
What’s presentation (forensic process)
In court
How is trace evidence transferred
Locards principle ‘every contact leaves a trace’ and its often undetected
What’s one way transfer
Transferred from one place to another and not the other way around
What’s two way transfer
The exchange of materials between two objects
Direct vs indirect modes of transfer
Direct involves touching while indirect is through an intermediary (pollen onto clothes)
Secondary modes of transfer
Transfer of a substance through an intermeditary source (like from shaking hands with someone)
How to analyse trace evidence
-identify the evidence via class characteristic
-establish points of similarity between samples
-If the properties differ then its not from the same source
-Absolute identification often isn’t possible
How do you classify evidence
-Morphology, optical, physical and chemical properties
-melting and boiling point
-Refractive index
-absorption and emission spectra
-Density
-Colour
-species characteristics
Techniques
-non destructive techniques are a priority
-Destructive analysis gives you more useful and further information. there often chemical like chromotography and mass spec
-depends on the amount of evidence, type of crime, aims of enquiry and resources
Stereo microscope
-preliminary examination (30X mag)
-used to segregate evidence that is from another material
-Guides for more in depth analysis/ isolation of individual particles
Compound microscope
-High power examination (100-1000x mag)
-transmitted light
-light from the base, through the condenser and specimen
-the light passes through the objective lense and ocular lense
SEM (Scanning electron microscopy)
-High power mag (10x to 50,000x mag)
-The sample in gold coated
-Scans the surface with focused high energy electron bam
-Secondary selections defected= creates an image
-Combine with chemical analysis
Paint
-1,000 paint types- all with different components
-fragments, chips particles
-can classify by the number of layers, colour, texture and chemical composition
What analytical techniques do you use to classify paint
Stereo microscopy, solvent tests, IR spec, SEM-EDX, pyrolysis gas chromotography
Glass
-sand+ sodium carbonate+ calcium chloride +impurities
-Different types of glass have different characteristics
-can microscope entire shards
-can classify based on pattern matching, fracture shape, density and refraction index
What analytical tools do you use for glass
IR- how light bends through as it passes through so its very specific
Fibres
-Smallest unit of textile material- clothing carpets, furniture
-Natural and synthetic
-Can indicate direct contact between a person and the scene
What analytical techniques do you use for fibres
Mircroscopy, MSP, FTIR and GC/MS
