l4 trace evidence Flashcards
what is Locard’s Exchange Principle
every contact leaves a trace
what is primary transfer? secondary transfer?
primary- direct contact
secondary- indirect, usually intermediate source
what is trace evidence?
Evidence that is transferred is small or even microscopic
what is the most important tool in forensic trace analysis and what is the proper term of this study?
microscope, microanalysis
two types of info you can gain from microanalysis ?
morphology, analytical data
probing matter with energy can tell you the what of a sample? what is this study called?
chemical composition, spectroscopy
which microscope is most likely to be employed first in the examination of evidence?
stereo binocular microscope
use of stereo binocular microscope
preliminary evaluation, location of microscopic particles
stereo binocular microscope is what kind? How does it work?
compound type (more than 1 stacked lens), works using reflected light (like eyes)
second most common type of microscope
compound binocular microscope
what is the use of a compound binocular microscope
collect morphological information
calibrated micrometer placed on EP of microscope that enables accurate measurements to be taken
micrometry
how does a compound binocular microscope work?
transmitted light, uses slides
most important microscope in forensics
comparison microscope
how are we able to see two independent images at the same time using a comparison microscope?
two microscopes joined by an optical bridge
use of comparison microscope
Simultaneously view two independent images in one field
Microscope that enables far greater magnification than any others
Scanning electron microscope (SEM)
briefly explain how a scanning electron microscope works
- beam of electrons swept over area
- beam interacts with surface and slightly below surface of sample
- initial beam removes extra electrons from sample which is converted to image
- x-rays are also created, to obtain chemical info
what is the use of a scanning electron microscope?
generates image and chemical compound of sample at the same time
what is glass? (characteristics)
amorphous solid, hard, brittle, transparent, lacks ordered arrangement of atoms in most solids
common, mass-produced glass usually found in glassware, contains sodium carbonate and calcium carbonate
soda lime glass
similar to lime-glass but with boron added, heat resistant
pyrex glass
what is tempered glass? what is another name for it
when it breaks, it shatters into small pieces so there are no big pieces that will cut you, safety glass
comparing glass consists of..?
- finding and measuring the properties of a piece
- minimizing or eliminating possible sources based on the properties
glass possesses its greatest evidential value when it can..?
be individualized to one source by being physically matched together
small pieces of glass are considered as what type of evidence?
class evidence
what does “density is an intensive property” mean? why is this important?
remains the same regardless of sample size, useful in identification of glass
what is refractive index?
ratio of velocity of light in a vacuum to velocity of light in another medium
t/f refractive index is temperature dependant
true
explain the flotation method for comparing glass densities
- sample glass immersed in liquid
- density of liquid is adjusted by adding small amts of more liquid until the glass sample is suspended in liquid
- now, the glass and liquid have same density
- comparative pieces are placed in the same liquid, they will sink, float or remain suspended based on their density
- the pieces that remain suspended with the sample are relevant
what method is used to compare glass densities?
flotation method
what method is used to determine the refractive index of glass?
glass immersion method
explain the glass immersion method, what does it make use of?
makes use of Becke line- bright halo on boarder of particle immersed in liquid of different refractive index
- glass particle immersed in liquid
- liquid’s refractive index is varied by temp until equal to glass
- when refractive indexes match, Becke line disappears
- minimum contrast observed b/w liquid ad particle
what are radial cracks in glass?
originate from impact point outwards
what are concentric cracks?
circle around the point of impact
what are terminated cracks?
caused by later impact, cracks perpendicular off of pre-existing crack
what are conchoidal lines?
edge characteristics that occur when glass breaks, they meet one edge of the glass at 90 degrees
for radial cracks, how will conchoidal lines form?
opposite the surface from which impact originated
hair grows out of an organ called..?
the hair follicle
how can order of impact be determined by cracks in glass?
radial cracks are sign of first impact, then concentric, then terminated
hair shaft is composed of what three layers?
cuticle, cortex, medulla
scale structure covering the exterior of the hair
cuticle
the scales of a cuticle always point to which end of the hair?
tip
what is scale pattern useful for? What scale pattern is characteristic of human hair?
species identification, imbricate
main body of hair shaft embedded with pigment granules that impart color
cortex
important points of comparison among hair of different individuals when looking at cortex include..?
colour, shape and distribution of pigment granules
center coloumn running through center of hair
medulla
for humans, how much diameter of the shaft does the medulla occupy?
less than one third
three stages of hair growth?
anagen, catogen, telogen
medullas can be..?
continuous, interrupted, fragmented or absent
translucent tissue surrounding shaft when hair is pulled from head
follicular tag
why are follicular tags important?
DNA can be used to individualize hairs
important factors to distinguish human from animal hair
scale structure, medullary index, medullary shape
when comparing human hair we look to match..?
(3pts)
colour, length, diameter
a hair comparison report cannot rely on morphology alone, it must include..?
DNA info
most common request of hair comparison is to determine..?
if hair from suspect matches hair found at crime scene
what are natural vs man-made fibers? examples of natural?
natural derived in whole from plant or animal sources (cotton, wool, cashmere, hemp)
man-made are manufactured
what are the two types of man-made fibers?
regenerated fibers and synthetic fibers
what are regenerated fibers? examples?
manufactured from raw materials, ex rayon, acetate
what are synthetic fibers? examples?
produced solely from synthetic chemicals ex. nylon, polyester, acrylic
the quality of fiber evidence depends on..?
- ability to identify origin
- ability to narrow possibilities to limited number of sources
what is the common goal of fiber analysis? How are they analyzed?
determining common orgin, done with comparison microscope b/w sample and ref
three morphological features that are important when comparing fibers
- cross-sectional shape
- lengthwise striations on surface
- presence of de-lustering particles
t/f the more complex and unique a fiber is the better evidence it is
true
what is the use of infrared spectrophotometry?
used to identify generic fiber class
explain infrared spectrophotometry
Molecules that compose manufactured fibers selectively absorb light to form a characteristic pattern
colours of fibers compared through spectral patterns
