trace evidence analysis Flashcards
trace evidence
microscopic quantities of material that are of probative value in a forensic investigation.
types of trace evidence
biological- (human)
physical- biological (non human)
particles, substances, marks/impressions
natural vs manufactured materials
forensic value
establish spatial and temporal links between persons and scenes of interest.
forensic process- trace evidence
transfer-> persistence-> collection-> analysis-> interpretation-> presentation
evidence dynamics
any influence that adds, changes, relocates, obscures, contaminates, or obliterates physical trace evidence , regardless of intent. transfer- during crime. persistence- after crime.
modes of transfer
one way- evidence is passed from one thing to another.
two way- evidence is dispersed onto multiple objects. reduces the chance that the trace evidence has been picked up innocently.
modes of transfer
direct- person touches the evidence
secondary transfer- person not at the crime scene but evidence has been transferred to them
how useful is trace evidence following transfer- 1. variation in transfer
amount and distribution of transferred evidence varies.
- variation in transfer- clothing
varies in composition and texture.
influences fibre shedding (creation of evidence). influences adhesion of other traces. eg nylon vs wool.
- variation in transfer- weather
local conditions affect transfer of environmental trace evidence (source). influences deposition of other types of evidence at the scene.
- variation in transfer- trace characteristics
particle size/ shape. glass- fragments (mm) or particles (um). pollen- different sizes and surface textures.
how useful is trace evidence following transfer 2. frequency
is the trace evidence easily transferred? how commonly is the trade naturally encountered. transfer is not a discrete event- pre and post crime transfers.
3 trace persistence- wear
loss of evidence over time. fibre persistence on different clothing items. similar trends with most traces (GSR, paint, glass). other types of trace evidence is more complex.
2 frequency- pollen
abundant in the environment. transport mechanisms- direct/ indirect transfer. present in most peoples hair, shoes, clothing. individual pollen profile.
2 frequency- glass
population studies- frequency in general populations. rarity amongst general public= forensic significance.
how useful is trace evidence following transfer 3. trace persistence
temporal dynamics of transferred evidence. how long is the evidence preserved for after crime events? impacted by source/ recipient, environment, evidence characteristics, initial transfer.
absence of evidence is not evidence of absence.
3 trace persistence- offender activity
post crime activity may also contribute to loss of trace evidence- offender, bystander, police/CSI. eg washing burning, cleaning, vacuuming. packaging exhibits- loss or redistribution of evidence.
trace analysis
identified via class characteristics (colour, shape, RI) aim to establish points of similarity between samples- how rare. if properties differ= not from the same source. absolute identification often not possible- exclusion.
classifying evidence
morphology, optical, physical, chemical properties. general physical characteristics- mpt, bpt, RI, absorption and emission spectra (IR, vis, UV), density, molecular mass, colour, species characteristics.
classifying evidence techniques
non destructive- priority. physical eg microscopy.
destructive- further information. chemical eg chromatography, mass spectrometry.
depends on the amount/ value of evidence, type of crime, aims of enquiry, resources.
microscopy
the microscope is the most fundamentally important tool to trace evidence.
it analyses many types of evidence- 3D, microscope slide. surface analysis & look through objects. many times of microscopy.
stereo microscope
preliminary examination (30x magnification). segregate evidence from other material- substance type, size, form, colour, texture, appearance.
guides more in depth analysis. isolation of individual particles.
compound microscope
high power examinations (100-1000x mg) transmitted light= sample preparation. light from the base, through condenser & specimen. light passes through objective lens & ocular lens
scanning electron microscope
higher magnification (10x to 500,000x mg) often needed. samples gold coated. scans surface with focused high energy electron beam. secondary electrons detected= creates image. combine with chemical analysis (XRD, EDX, EDS).
summary
trace evidence is valuable when recognised, detected and collected.
important to use non destructive analytical techniques first.
understanding trace evidence dynamics assists with crime reconstruction.
anything can be trace evidence= diverse analysis approach.
importance to exclude rather than match.