Trace evidence

What is trace evidence

Microscopic quantities of material that are probative value in a forensic investigation

Types of trace evidence

• Biological

• Physical

• Particles, substances, marks/impressions

• Natural v manufactured materials

Forensic value of trace evidence

• Mute witnesses- cant speak, but presence can tell us alot

• Microscopic

• Determine circumdtances of what happended

• Associate a person with a crime

• Establish spatial and temporal links between persons and scenes of interest

Forensic process- trace evidence- transfer

Someone commiting an assaut which transfers trace evudence

Persistance- second stage

May run away/gunshot residue is transferred ont clothes

Third stage- collection

Police have been and collected evidence, maybe have a suspect in mind and make an arrest

Fourth stage- analysis

look at the evidence and analyse the gunhot residue

Fifth stage- interpretation

has the residue came from same gun

Sixth stage- presentation

Presentation of evidence in court

Transfer of evidence

• Evidence deposited/removed

• Often undetected as evidence is microscopic

The modes of transfer

One way- e.g shooting a gun

Two way- e.g burgarly can look at fibres in crime scene and glass on suspect- reduces chances that the trace evidence has been picked up innocently somewhere else

When was the soham murders

2002

How many hairs and fibres were examined inthe soham murders

40,000 from victims clothing, suspects clothing and house

In the soham murders how many fibres were two way transfer

154

What other trace evidence types were found with soham murders

pollen, soil, concrete, petrol

What are the different modes of transfer

• Direct vs indirect

• Secondary transfers

Variation in transfer- clothing

• variation in composition and texture

• influences fibre shredding (creation of evidence)

• influences adhesion of other traces

• e.g nylon v wool

Variation in transfer- weather

• Local conditions affect transfer of environmental trace evidence

• Influences deposition of other evidence types at the scene

Variation of transfer- trace characteristics

• Particle size/shape

• Glass:fragments (mm) or particles (hm)

• Pollen: different sizes and surface textures

Frequency - example- pollen

• It is abundant in the environment

• Transport mechanisms- direct/indirect transfer

• Present oon most peoples hair, shoes

• Individual pollen profile

Frequency- example,glass

• Population studies- frequency in general population

• E.g Jackson et al (2013)- glass on hair/headwear- 6 fragments found on 232 members f public and 138 fragments found on 15 people in industry

Trace persistence- example- wear

• Loss of evidence over time- ‘J curve@

• Fibre persistence on different clothing items

• Similar trends with nost traces- other types of trace evifence more complex

Trace persistence- example- offender activity

• Post crime activity may contribute to loss of trace evidence- offender, bystanders and police/csi

• E.g washing/burning ecidence

Case example of offender activity- when ?

1998 terrorism trial

car set on fire- physical evidence lost

soil recovered from tires- compared to scene

defence- evidence not reliable

research found soil evidence was valid- (persistence)

Trace collection

• Scene, lab, person

• Control samplesare important

• Range of techniques:

  -picking,scraping, brushing and combing

  -tape lifts

  -vacuum

  -swabs

Trace analysis

• identified via class characteristics

• Aim to establish poits of similarity between samples

• If properties differ- not from same source

• Absolute identification often not possible- exclusion

Classifying evidence

• Morphology, optical, physical and chemical properties

What are the general physical characteristics of evidence

• Melting point and boiling point

• Refractive index

• Density

• Molecular mass

• Colour

Different techniques to classify evidence- non destructive

Physical and morphological e.g microscopy

Classifying evidence- destructive

Chemical e.g chromatpgraphy and mass spectrometry

Microscopy for trace evidence

Is the most important tool

Surface analysis and look through object

what are the different microscopes

stereo

compound

scanning electron

What is a stereo microscope

• Preliminary examination (30x magnification)

• Segregate evidence from other material- substance type, size, form, colour, texture and appearence

• Guides more in dpeth analysis

• Isolation of individual particles

What is a compound microscope

• High power examinations (100-1000x mag.)

• Transmitted light- sample preparation

• Light from the base, through condenser and specimen

• Light passes through objective lens and ocular lens

What is a scanning electron microscope

• Higher magnification (10x to 500,000x mag) often needed

• Samples gold coated

• Scans surface with focused high energy electron beam

• Secondary electrons detected- creates image

• Combine with chemical (XRD, EDX, EDS)

Examples of trace evidence analysis in practice- paint

• Various crime scenes- burglary, vandalism, assault, vehicular

• 1,000s paint types- different chemical components

• Often applied in sequence layers

• Fragments, chips, particles

Classification of paint

Number of layers

colour

surface texture

chemical composition

Analytical tools for paint

• Stereomicroscopy

• Solvent tests

• IR spectrophotometry

• SEM-EDX, XRD

• Pyrolysis gas chromatography

Case example of paint

• Green river killer (1982-1995), seattle

• Suspected of murdering up to 104 women

• DNA identification in 2001

• Physical evidence- paint spheres (spray paint)

• Suspect confessed

Glass

• Various crime scenes- burgalrly, vandalism, assault

• Sand and sodium carbonates + calcium oxides + impurities

• Different glass types- different characteristics

• Whole shards- microscopic patricles of glass

Classification of glass

• Pattern matching (physial fit)

• Fracture shape

• Density (flotation method)

• refractive index

What is refractive index of glass

How light bends as it passes through

• Oil immersion method (becke-line)

• Temperature varied util disappears

Case example of glass

• Hit and run (2010) w australia

• Onevictim died, one critically injured

• Glass recovered from victims clothign and suspects house

• Two way transfer of glass and black foam

• Strong circumstantial evidence

Fibres

• Smallest unit of textile material

• Natural and synthetic fibres

• Indicate direct contact between persosn and or scenes

• BUT prevelant within the environment

Classification of fibres

Nature of the fibre

Colour and pigment distrubution

Geometry

Surface characteristics

Additives

Techniques for fibres

• Microscopy

• Microspectrophotometry MSP

• FTIR

• Chemical composition

Case example 1- fibres

• Murder and kidnapping- 2000

• Suspect identified, van seized

• Fibres from suspect present on victims hair and shoes

• Single hair on suspects jumper- the victim

• Suspect guilty

Environmental evidence

Environmental transfer from crime scenes to persons

Soil most frequently transferred- physical, chemical and bilogical analyses

Plant traces also transferred in abudnace

Case example- environment

Pollen- leanne tiernan murder (2001)

Diatoms- natalie william murder (2002)