Trace Evidence

What is Trace Evidence?

Trace evidence is created when objects make contact.

Locard’s exchange principle tells us that every contact leaves a trace (hair, glass shards, soil, fibres). The transfer can be either:

• Primary (direct contact; ex: soil from a grave embedded into shoe treads of killer)
• Secondary (indirect contact; ex: you sit in my car, get dog hair on you, but no dog was present)

Quite often the evidence that is transferred is small or even microscopic in size, making analysis quite difficult. Should be of no surprise that our microscope is one of the most important tools of the forensic trace analyst. Analysis is typically called “microanalysis”.

Microanalysis

The 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.

• Generally deals with samples in the milligram or microgram
• Often operate using different forms of energy (visible, uv light, infrared, elecrons and more)
• Can get useful information like morphology (size, shape, etc.) and analytical data (optical properties, molecular information, elemental information)

Instruments of Microanalysis

There are many types of microscopes available for use in a forensic laboratory for a wide variety of materials:

• Stereo microscope
• Compound binocular microscope
• Comparison microscope
• Basic scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDS)

Light and Matter

If you probe matter with energy, you can obtain useful information used to tell you the chemical composition of the sample

• Heat, Chemical Reaction, Photons, Electrons go into Sample which gives information
• Spectroscopy: the science of how radiation/energy interacts with matter

Stereo Binocular Microscope

• The most likely to be employed first in the examination of evidence
• Used in preliminary evaluations of submissions, and for the location of microscopic particles and materials
  • Recovery of fragments of red wool fibres from a victim’s sweater found on the denim jacket of a suspect in an assault case
  • Recovery of glass particles from the jeans of a burglary suspect
• This microscope is of a compound
• Total magnification is computed by multiplier the power of the objective lens (OBJ) by that of the eyepiece lenses (EP)
  • OBJ power * EP power = total magnification
• Microscope is constructed from two similar, but separate microscopes
• The views are offset by 15 degrees so that each eye sees a slightly different perspective
  • Results in a 3D image
• Many significant preliminary and analytically important observations are made with this microscope
• This microscope works with reflected light, just like our eyes

Compound Binocular Microscope

• Second most common microscope
  • Commonly used in science laboratories
• Has two eyepieces and only one objective
  • 25-1200x is possible
  • 40-400x commonly employed in forensics
• Mainly used to collect morphological information: info relating to structure (size, shape, etc.)
• Micrometry: a calibrated micrometer placed in the EP of a microscope enables accurate measurements to be taken
• This microscope works with transmitted light

Comparison Microscopy

• At least one type will be present in every major crime laboratory
• Can simultaneously view two independent images in one field
  • Also allows superposition of the two images
• Classical, transmission illumination, and even PLMs can be oriented similarly
  • Critical for side-by-side comparisons of evidence
• Two microscopes joined by an optical bridge
• Many experts suggest this is: the only valid way to compare two pieces of trace evidence

Scanning Electron Microscope

• Enables far greater magnification (10 - 100,000x)
• Principle of operation:
  1. Beam of electrons is swept over a small spot
  2. Beam of electrons interacts with surface, and area slightly below the surface of the sample
  3. Initial beam of electrons removes additional electrons from the sample, which are converted to an image
  4. X-rays are also created, which can be measured with an energy-dispersive x-ray spectrometer (EDS) to obtain chemical information

Glass

Glass Fragments

• Common type of microscopic evidence, frequently encountered as transfer evidence
• Broken and shattered glass fragments can place a suspect at a crime scene
• Chips of broken glass from a window may lodge in a suspect’s shoes or garments
• Particles of headlight glass found at the scene of a hit-and-run can lead to clues about the identity of a suspect vehicle

What is glass?

Glass is an amorphous soild. A hard, brittle material that is usually transparent and lacks ordered arrangement of atoms found in most solids

Forensics Analysis of Glass

• Comparing glass consists of:
  1. Finding and measuring the properties that will associate one glass fragment with another
  2. Minimizing or eliminating the possible existence of other sources

Individualization of Glass

• Due to the ubiquity of glass in our society, analysis can be quite a challenge
• Glass possesses its greatest evidential value when it can be individualized to one source
  • This can only truly be done when two pieces of glass can be physically matched together
• Individualization comparisons require:
  • Piecing together irregular edges of broken glass
  • Matching all irregularities and striations on broken surfaces
• Often dealing with much smaller fragments

Comparing Glass Fragments

• Can vary greatly in terms of composition
  • Allows it to be discriminated by physical, optical, or elemental characteristics
• Due to mass production, many types of glass do not possess any characteristics that can be used for individualization
  • Small pieces of glass are considered to be class evidence
• Common characteristics of glass that be measured are:
  • Colour
  • Thickness
  • Density
  • Surface features
  • Refractive index
  • Thermal history (strength)

Density

• Density is defined as the mass per unit volume
  • Density is an intensive property of matter, meaning it remains the same regardless of sample size
  • It is considered a characteristic property of a substance and can be used as an aid in identification

The Floatation Method

• The flotation method is a rather precise and rapid method for comparing glass densities
• In the flotation method:
  1. A glass particles is immersed in a liquid
  2. The density of the liquid is carefully adjusted by the addition of small amounts of an appropriate liquid until the glass chip remains suspended in the liquid medium
  3. At this point the glass and liquid will have the same density
  4. Relevant pieces of glass will then be placed inside the same liquid for comparison. They will remain suspended, sink, or float depending on density

Refractive Index

• Light waves travel in air at a constant velocity until they penetrate another medium
  • When light reaches another medium, such as glass or water, light waves are slowed, causing the rays to bend
• Refractive index is the ratio of the velocity of light in a vacuum to that in the medium under examination
• @25 C, RI of water is 1.3333
• The bending of light waves because of a change in velocity is called refraction

Glass Immersion Method

• The immersion method is best used to determine a glass fragment’s refractive index
• Makes use of the Becke Line
  • A bright halo near the border of a particle that is immersed in a liquid of a different refractive index
• In the immersion method:
  1. A glass particle is immersed in a liquid
  2. The liquid’s refractive index is varied until it is equal to that of the glass particle
  3. When the refractive index of the particle and liquid match, known as the match point, the Becke Line disappears
  4. Minimum contrast is observed between liquid and particle

Order of impact

• Radial cracks - originate from impact point and propagate outward
• Concentric cracks - circle around impact point
• Terminated cracks - “T” junction

Direction of impact - Radial crack

• When glass breaks, edge characteristics known as conchoidal lines often develop
• They will meet one edge of the glass 90 degrees
• For radial cracks, this is the opposite surface from which the force origination
• Was it an “inside job”?
• Where did the impact take place?

Hair and Fibres

• Hair is encountered as physical evidence in a wide variety of crimes
• Individualization of a human hair is not possible through its morphology alone
  • Isolation and characterization of DNA is possible
• Hair can provide strong corroborative evidence for placing an individual at a crime scene
• Requires:
  1. Requires properly collected and submitted samples
  2. Adequate number of standard/reference samples

Morphology

• Hair is an appendage of the skin that grows out of an organ known as the hair follicle
• The length of a hair extends from its root or bulb embedded in the follicle, continues into a shaft, and terminates at a tip end
• The shaft is composed of three layers the cuticle, the cortex, and the medulla
  • These layers are subjected to the most intense examination by the forensic scientist

Cuticle

• The cuticle is the scale structure covering the exterior of the hair
  • The scales always point towards the tip of the hair
  • The scale pattern is useful in species identifications

Cortex

• The cortex is the main body of the hair shaft
  • Its major forensic importance is the fact that it is embedded with the pigment granules that impart hair with color
  • Important points of comparison among the hairs of different individuals include the: colour shape and distribution of these granules

Medulla

• The medulla is a cellular column running through the center of the hair
  • Medullary index = Diameter of Medulla/ Diameter of Hair Shaft
  • For humans, the medulla generally occupies less than one-third the diameter of the shaft, while for animals it is generally one-half or greater
• The medulla may be continuous, interrupted, fragmented, or absent
  • The presence of the medulla varies from individual to individual and even among hairs of a given individual
  • Medullae also have different shapes, depending the species

Root

• The root and other surrounding cells in the hair follicle provide the tools necessary to produce hair and continue its growth
• The three phases of hair growth are the anagen, catagen, and telogen phases
• When pulled from the head, some translucent tissue surrounding the hair’s shaft near the root may be found
  • Follicular tag
• By using DNA analysis on the follicular tag, the hair may be individualized

Comparison of Hairs

• The comparison microscope is an indispensable tool for comparing the morphological characteristics of hair
• Particularly important in distinguishing animal from human hair are:
  • Scale structure, medullary index, medullary shape
• When comparing strands of human hair, we are particularly interested in matching:
  • Colour, length, diameter
• Other important features for comparing human hair are:
  • The presence or absence of a medulla
  • The distribution, shape and color intensity of the pigment granules present in the cortex
• The most common request is to determine whether or not hair recovered at the crime scene compares to hair removed from the suspect
  • Microscopic hair examinations are highly subjective
  • Depend on skill & integrity of analyst
  • A report of hair comparison must contain DNA information

Types of Fibers

• Natural fibers are derived in whole from animal or plant sources
  • Wool, mohair, cashmere, furs, cotton
• Man-made fibers are manufactured
  • regenerated are manufactured from natural raw materials and include: rayon, acetate, triacetate
  • synthetic are produced solely from synthetic chemicals and include: nylons, polyesters, acrylics
• Synthetic fibers are composed of polymers, or macromolecules, which are a large number of atoms arranged in repeating units known as monomers

Fiber Evidence

• The quality of the fiber evidence depends on:
  1. The ability to identify origin of fibre
  2. To be able to narrow the possibilities to a limited number of sources
• Determining a common origin is the goal:
  • The strongest evidence that two different fabrics were of common origin: two torn edges fit together
• Microscopic comparisons are performed initially
  • Between questioned and standard/reference fibers
  • Evaluated for colour and diameter
  • Using a comparison microscope
• Morphological features that could be important in comparing fibers are:
  1. The cross-sectional shape of the fiber
  2. Lengthwise striations on the surface of the fiber
  3. The presence of delustering particles that reduce shine
• Compositional differences may exist in the dyes that were applied to the fibers during the manufacturing process

Methods for Fiber Comparison

• Identification of generic fiber class can be accomplished using infrared spectrophotometry
  • The molecules that compose a manufactured fiber selectively absorb infrared light to form a characteristic pattern
• The microspectrophotometer allows a forensic analyst to view an object under a microscope while simultaneously obtaining its visible or infrared absorption pattern
  • Input of visible light or infrared light to compare the colors of fibers through spectral patterns