lecture 4 - trace evidence

Question 1

what is trace evidence?

Answer 1

• can literally be anything
• Locard’s exchange principle tells us that every contact leaves a trace

Question 2

different types of transfers

Answer 2

primary - direct contact
secondary - indirect contact

Question 3

microanalysis

Answer 3

• application of a microscope and microscopical techniques to the observation, collection and analysis of micro-evidence that cannot be clearly observed or analyzed without such devices

Question 4

microanalysis often operates using different forms of energy:

Answer 4

• visible light
• UV light
• infrared light
• electrons

Question 5

microanalysis can get useful information such as

Answer 5

• morphology - size, shape, etc
• analytical data - optical properties, molecular spectra, etc.

Question 6

4 types of microscopes

Answer 6

• stereo
• compound binocular
• comparison
• basic scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDS)

Question 7

spectroscopy

Answer 7

• science of how radiation/energy interacts with matter

Question 8

if you probe matter with energy…

Answer 8

you can obtain useful information used to tell you the chemical composition of the sample

Question 9

stereo binocular microscope

Answer 9

• most likely to be employed first in examinations of evidence
• used in preliminary evaluations of submissions (recovery of glass particles)
• compound microscope
• total magnification is multiply the power of objective lens by eyepiece lens
• constructed with two similar but separate microscopes
• results in 3D image
• works with reflected light

Question 10

compound binocular microscope

Answer 10

• second most common, commonly used in science labs
• has two eyepieces and only one objective
• mainly used to collect morphological information (size, shape, etc.)
• works with transmitted light

Question 11

magnification of compound microscope

Answer 11

• 25-1200 magnification possible
• 40-400x commonly employed in forensics

Question 12

micrometry

Answer 12

• calibrated micrometer placed in the EP of microscope enables accurate measurements to be take

Question 13

comparison microscope

Answer 13

• very important for forensics
• can simultaneously view 2 independent images in one field
• suggest this is the only valid way to compare two pieces of trace evidence
• two microscopes joined by optical bridge

Question 14

scanning electron microscope

Answer 14

• enables far greater magnification (10-100,000x)

Question 15

scanning electron operation

Answer 15

• beam of electrons sweep over small spot
• beam of electrons interact with surface, and area slightly below surface of sample
• initial beam of electrons removes additional electrons from sample, which are converted to an image

Question 16

glass fragments

Answer 16

• common type of microscopic evidence
• encountered as transfer evidence

Question 17

what is glass

Answer 17

• sideline glass - drinking glass - not very strong
• glass is like sand

Question 18

comparing glass consists of

Answer 18

• finding and measuring properties that will associate one glass fragment with another
• minimizing or eliminating with possible existence of other sources

Question 19

individualization of glass

Answer 19

• glass posses its greatest evidential value when it can two pieces can be physically matched together - individualized

Question 20

individualization comparisons require:

Answer 20

• piecing together irregular edges of broken glass
• matching all irregularities and striations on broken surfaces
• often dealing with smaller fragments

Question 21

comparing glass fragments

Answer 21

• can vary greatly in terms of composition
• small pieces of glass are considered to be class evidence

Question 22

common characteristics of glass that can be measured

Answer 22

• colour
• thickness
• surface features
  -density
• refractive index
• thermal history

Question 23

density

Answer 23

• defined as the mass per unit volume
• remains the same regardless of sample size
• used as an aid in identification

Question 24

refractive index

Answer 24

• light waves travel in air at a constant velocity until they penetrate another medium
• when light reaches another medium, such as water, light waves are slowed causing rays to bend

Question 25

refractive index is the ratio...

Answer 25

- of velocity of light in a vacuum to that in the medium under examination

Question 26

bending of light waves because of change in velocity is called...

Answer 26

- refraction

Question 27

flotation method!!!

Answer 27

- glass particle immersed in liquid - density of liquid carefully adjusted by addition of small amounts of appropriate liquid until the glass chip remains suspended in liquid medium - at this point, glass and liquid will have same density - relevant pieces of glass will then be placed inside same liquid for comparison

Question 28

glass immersion method

Answer 28

- best used to determine a glass fragment's refractive index - makes use of Becke line

Question 29

what is the Becke line

Answer 29

- bright halo near the border of a particle that is immersed in a liquid of a different refractive index

Question 30

steps of the glass immersion method

Answer 30

- glass particle immersed in liquid - liquid refractive index is varied until it is equal of glass particle - when refractive index of particle and liquid match, becke line disappears - minimum contrast is observed between liquid and particle

Question 31

order of impact - 3 cracks

Answer 31

- radial crack -> originate from impact point and propagate outward - concentric crack -> circle around impact point - terminated crack -> T junctions

Question 32

direction of impact - radial crack

Answer 32

- when glass breaks, edge characteristics known as conchoidal lines often develop - they will meet one edge of glass 90 degree - for radial crack, this is the opposite surface from which the force originated