trace evidence

Question 1

what is forensic evidence

Answer 1

any and all objects that can establish:
1. A crime has been committed
2. Links between crime scene(s), victim, and offender

Question 2

define trace evidence

Answer 2

microscopic quantities of material that are of probative value (helps answer questions) in a forensic investigation

Question 3

what are the two types of trace evidence and give examples

Answer 3

biological (human) - fingerprints, DNA, microscopic pieces of blood
physical (non-human) - paint, glass fragments, fibres, soils, pollen, thorns, etc

Question 4

give two advantages to trace evidence regarding forensic value

Answer 4

1. Mute witness - not biased as it cannot speak
2. Microscopic - offender isn’t aware of it

Question 5

define evidence dynamics

Answer 5

any influence that adds, changes, relocates, obscures, contaminates, or obliterates physical trace evidence, regardless of intent

Question 6

regarding evidence dynamics what’s the difference between transfer and persistence

Answer 6

transfer = during crime
persistence = post-crime - usually contamination

Question 7

when is it important to consider evidence dynamics

Answer 7

throughout the entire forensic process

Question 8

what is Locard’s exchange principle

Answer 8

every contact leaves a trace

Question 9

what are the two modes of transfer and give an example for each

Answer 9

1. One way = only leaves evidence on the offender e.g. GSR
2. Two way = leaves evidence on both the offender and on the scene e.g. broken glass

Question 10

what is the difference between indirect and direct modes of transfer

Answer 10

direct = a footprint
indirect = pollen

Question 11

what are secondary transfers and give an example

Answer 11

the passing of evidence to another person e.g. a handshake after firing a gun

Question 12

what are the 3 factors to consider when determining how useful trace evidence is following transfer

Answer 12

variation in transfer
frequency
trace persistence

Question 13

explain variation in transfer and use examples

Answer 13

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

Question 14

explain the factor of frequency regarding how useful trace evidence is following transfer

Answer 14

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

Question 15

explain the factor of trace persistence in regards to how useful trace evidence is following transfer

Answer 15

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

Question 16

name a few techniques used to collect trace evidence

Answer 16

picking/ scraping/ brushing/ combing
tape lifts
vacuum
swabs

Question 17

what is important to remember when collecting trace evidence

Answer 17

a control sample will also be needed for comparison

Question 18

how is trace evidence analysed in general

Answer 18

via its class characteristics e.g. colour, shape, refractive index, etc, with an aim to establish points of similarity between samples

Question 19

how do we classify evidence

Answer 19

morphology, optical, physical, chemical properties

Question 20

what do we use non-destructive techniques for in trace analysis

Answer 20

to determine the physical and morphological factors of the evidence - often requires a microscope

Question 21

what do we use destructive techniques for in trace analysis

Answer 21

to gain further information like chemical composition via chromatography or mass spectrometry

Question 22

what is the most fundamentally important tool to a trace evidence examiner

Answer 22

a microscope

Question 23

name 3 kinds of microscopes (not light)

Answer 23

stereo microscope
compound microscope
scanning electron microscope

Question 24

what can a stereo microscope do to trace evidence

Answer 24

segregate evidence from other material - via substance type, size, form, colour, texture and appearance, as well as guiding more in-depth analysis

Question 25

what can a compound microscope do with trace evidence

Answer 25

high power examinations - important due to trace evidence being so small

Question 26

why do we often not use scanning electron microscopes

Answer 26

they are more expensive as samples have to be coated in gold

Question 27

what is an advantage of an SEM

Answer 27

we can combine the microscope with a chemical analysis machine for more information on the evidence

Question 28

how can paint be used as trace evidence in a crime, how is it classified and what (6) analytical tools do we use

Answer 28

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

Question 29

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

Answer 29

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

Question 30

what can fibres tell us

Answer 30

indicate direct contact between persons and/or scenes

Question 31

how are fibres classified as trace evidence and what (4) analytical techniques do we use for them

Answer 31

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

Question 32

what is the most frequent type of environmental trace evidence

Answer 32

soil