trace evidence analysis Flashcards

1
Q

trace evidence

A

microscopic quantities of material that are of probative value in a forensic investigation.

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2
Q

types of trace evidence

A

biological- (human)
physical- biological (non human)
particles, substances, marks/impressions
natural vs manufactured materials

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3
Q

forensic value

A

establish spatial and temporal links between persons and scenes of interest.

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4
Q

forensic process- trace evidence

A

transfer-> persistence-> collection-> analysis-> interpretation-> presentation

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5
Q

evidence dynamics

A

any influence that adds, changes, relocates, obscures, contaminates, or obliterates physical trace evidence , regardless of intent. transfer- during crime. persistence- after crime.

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6
Q

modes of transfer

A

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.

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7
Q

modes of transfer

A

direct- person touches the evidence
secondary transfer- person not at the crime scene but evidence has been transferred to them

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8
Q

how useful is trace evidence following transfer- 1. variation in transfer

A

amount and distribution of transferred evidence varies.

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9
Q
  1. variation in transfer- clothing
A

varies in composition and texture.
influences fibre shedding (creation of evidence). influences adhesion of other traces. eg nylon vs wool.

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10
Q
  1. variation in transfer- weather
A

local conditions affect transfer of environmental trace evidence (source). influences deposition of other types of evidence at the scene.

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11
Q
  1. variation in transfer- trace characteristics
A

particle size/ shape. glass- fragments (mm) or particles (um). pollen- different sizes and surface textures.

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12
Q

how useful is trace evidence following transfer 2. frequency

A

is the trace evidence easily transferred? how commonly is the trade naturally encountered. transfer is not a discrete event- pre and post crime transfers.

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13
Q

3 trace persistence- wear

A

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.

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13
Q

2 frequency- pollen

A

abundant in the environment. transport mechanisms- direct/ indirect transfer. present in most peoples hair, shoes, clothing. individual pollen profile.

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13
Q

2 frequency- glass

A

population studies- frequency in general populations. rarity amongst general public= forensic significance.

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13
Q

how useful is trace evidence following transfer 3. trace persistence

A

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.

14
Q

3 trace persistence- offender activity

A

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.

15
Q

trace analysis

A

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.

16
Q

classifying evidence

A

morphology, optical, physical, chemical properties. general physical characteristics- mpt, bpt, RI, absorption and emission spectra (IR, vis, UV), density, molecular mass, colour, species characteristics.

17
Q

classifying evidence techniques

A

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.

18
Q

microscopy

A

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.

19
Q

stereo microscope

A

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.

20
Q

compound microscope

A

high power examinations (100-1000x mg) transmitted light= sample preparation. light from the base, through condenser & specimen. light passes through objective lens & ocular lens

21
Q

scanning electron microscope

A

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).

22
Q

summary

A

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.