trace evidence Flashcards
what is forensic evidence
any and all objects that can establish:
1. A crime has been committed
2. Links between crime scene(s), victim, and offender
define trace evidence
microscopic quantities of material that are of probative value (helps answer questions) in a forensic investigation
what are the two types of trace evidence and give examples
biological (human) - fingerprints, DNA, microscopic pieces of blood
physical (non-human) - paint, glass fragments, fibres, soils, pollen, thorns, etc
give two advantages to trace evidence regarding forensic value
- Mute witness - not biased as it cannot speak
- Microscopic - offender isn’t aware of it
define evidence dynamics
any influence that adds, changes, relocates, obscures, contaminates, or obliterates physical trace evidence, regardless of intent
regarding evidence dynamics what’s the difference between transfer and persistence
transfer = during crime
persistence = post-crime - usually contamination
when is it important to consider evidence dynamics
throughout the entire forensic process
what is Locard’s exchange principle
every contact leaves a trace
what are the two modes of transfer and give an example for each
- One way = only leaves evidence on the offender e.g. GSR
- Two way = leaves evidence on both the offender and on the scene e.g. broken glass
what is the difference between indirect and direct modes of transfer
direct = a footprint
indirect = pollen
what are secondary transfers and give an example
the passing of evidence to another person e.g. a handshake after firing a gun
what are the 3 factors to consider when determining how useful trace evidence is following transfer
variation in transfer
frequency
trace persistence
explain variation in transfer and use examples
the amount and distribution of transferred evidence varies due to different characteristics like; the trace material, its properties of source/ recipient, the environmental conditions and type of contact. For example; clothing - all vary in composition and texture which influences fibre shedding and adhesion of other traces e.g. nylon is very smooth compared to wool
explain the factor of frequency regarding how useful trace evidence is following transfer
this is where we have to look at how common the trace would naturally be encountered in a population as transfer is not a discrete event and someone innocent may have traces on them by accident. For example; Pollen - abundant in the environment, therefore we have to look at individual pollen profiles to discover what type of pollen would be at the scene and who has an excess of this pollen contaminating them
explain the factor of trace persistence in regards to how useful trace evidence is following transfer
this is where we look at how long long evidence is preserved for after a crime event as over time both quality and quantity will be reduced due to a number of factors. For example; Offender activity - burning, washing, cleaning or vacuuming will result in a loss of evidence
name a few techniques used to collect trace evidence
picking/ scraping/ brushing/ combing
tape lifts
vacuum
swabs
what is important to remember when collecting trace evidence
a control sample will also be needed for comparison
how is trace evidence analysed in general
via its class characteristics e.g. colour, shape, refractive index, etc, with an aim to establish points of similarity between samples
how do we classify evidence
morphology, optical, physical, chemical properties
what do we use non-destructive techniques for in trace analysis
to determine the physical and morphological factors of the evidence - often requires a microscope
what do we use destructive techniques for in trace analysis
to gain further information like chemical composition via chromatography or mass spectrometry
what is the most fundamentally important tool to a trace evidence examiner
a microscope
name 3 kinds of microscopes (not light)
stereo microscope
compound microscope
scanning electron microscope
what can a stereo microscope do to trace evidence
segregate evidence from other material - via substance type, size, form, colour, texture and appearance, as well as guiding more in-depth analysis
what can a compound microscope do with trace evidence
high power examinations - important due to trace evidence being so small
why do we often not use scanning electron microscopes
they are more expensive as samples have to be coated in gold
what is an advantage of an SEM
we can combine the microscope with a chemical analysis machine for more information on the evidence
how can paint be used as trace evidence in a crime, how is it classified and what (6) analytical tools do we use
there are 100s of paint types that can be identified due to their different chemical components
they are classified by their number of layers, colour, surface texture and chemical composition
the analytical tools we use include; stereo microscopy, solvent tests, IR spectrophotometry, SEM-EDX, XRD and pyrolysis gas chromatography
how can glass be used as trace evidence of a crim, how is it classified and what (2) analytical tools do we use to analyse it
different glass types have different characteristics and ingredients within them
classified via; pattern matching, fracture shape, density and refractive index
analytical tools = oil immersion method and RI spectroscopy
what can fibres tell us
indicate direct contact between persons and/or scenes
how are fibres classified as trace evidence and what (4) analytical techniques do we use for them
classification; nature of the fibre, colour and pigment distribution, geometry, surface characteristics and additives
analytical tools; microscopy, micro spectrophotometry, Fourier Transform Infrared Spectroscopy and chemical composition using GC-MS
what is the most frequent type of environmental trace evidence
soil
