Trace Evidence

• Proper handling and collection techniques must be employed to avoid contamination or loss of trace evidence.
• • Exculpatory evidence: points to someone other than the suspect, indicating innocence
  • Conflicting evidence: irreconcilable evidence from different sources that confuses the case
  • Tainted evidence: discovered as a result of an illegal search, inadmissible in court
  • Hearsay testimony: adds nothing to the strength of a case, given by a witness who tells not what he or she knows personally, but what others have said
• Different kinds of evidence:

  • Real evidence: generated as part of the crime and recovered at the scene or at a place where the suspect or victim had been before or after the crime
  • Demonstrative evidence: items created to augment or explain real evidence, not generated directly from the incident
  • Circumstantial evidence: based on inference and not on personal knowledge or observation
  • Conclusive evidence: strengthens a case, overshadowing any other evidence to the contrary
  • Corroborating evidence: differs from but strengthens or confirms other evidence
• Transfer and Persistence:

  • Edmond Locard’s Exchange Principle: "Every contact leaves a trace"
  • Transfer evidence occurs when two objects come into contact
  • Conditions affecting material transfer: pressure applied during contact, number of contacts, ease of material transfer, form of evidence, amount of item involved
  • Types of transfer: direct transfer (A → B) and indirect transfer (A → B → C)
  • Persistence: evidence remains in a location until further transfer, degradation, or collection
• • Factors affecting how long evidence can persist: type of evidence, location, environment, time from transfer to collection, activity around the evidence location
• • Relationship and context of evidence within a crime scene are critical for interpretation and analysis
• Uniqueness of Identity, Class, and Individualization:

  • All objects are unique in space and time
  • Identification categorizes objects based on chemical and physical properties
  • Class characteristics place objects into groups with similar traits
  • Individualization occurs when an object can be classified into a group with only one member
  • Traits allowing for individualization depend on raw materials, manufacturing methods, and history of use
  • Questioned items vs. known/standard items: questioned evidence has an unknown source, known/standard evidence has a known source
• Analysis of Evidence:

  • Identification: discovering physical and chemical characteristics of evidence to categorize it
  • Comparison: attempting to discover the source of evidence and its relatedness to questioned material
  • Association: relationship between evidence and a putative source, strength depends on various factors
  • Individualization: occurs when unique characteristics link evidence to a single source
• Individualization occurs when at least one unique characteristic is found to exist in both the known and the questioned samples
• Identification can significantly reduce the number of items in the class that evidence belongs to, but individualization cannot be accomplished by identification alone
• Finding similarities is not enough, significant differences should not exist between the questioned and known items
• A significant difference is a class characteristic that is not shared between the questioned and known items, such as tread design on shoes or shade differences in fiber color
• Examples of significant differences include a few millimeters’ difference in fiber diameter, or distinct cross-sectional shape of fibers or hair color
• The scientific method on trace evidence involves:

  • Identifying a problem with investigative questions
  • Creating or developing a hypothesis
  • Gathering necessary data including known hairs from suspect and victim, questioned hairs from the crime scene, and other relevant data
  • Analyzing and interpreting the gathered data through careful examination
  • Testing the hypothesis to prove or disprove/accept or reject
  • Creating a conclusion based on the results
• In cases where more than one kind of analysis must be done on the same item of evidence, tests such as firearms testing, serology, developing of latent fingerprints, and DNA analysis should be conducted in a specific order to avoid precluding or spoiling the next test
• Forensic scientists conduct evidence analysis by forming many hypotheses and perhaps rejecting some as the investigation progresses
• Preliminary questions must be answered before formulating hypotheses, including sampling of large amounts of materials, sufficiency of samples, and what happens when more than one kind of analysis must be done on the same item of evidence
• Visible light represents a small range of the electromagnetic spectrum, and technology can be used to "see" beyond the visible range
• The electromagnetic spectrum is the full range of electromagnetic radiation, described in terms of mass-less particles called photons traveling in a wave-like pattern at the speed of light
• Electromagnetic waves transmit energy and include radio waves, microwaves, infrared waves, visible light, ultraviolet radiation, X-rays, and gamma rays
• Light waves behave in similar ways across the electromagnetic spectrum, being transmitted, reflected, absorbed, refracted, polarized, diffracted, or scattered depending on the composition of the object and the wavelength of the light
• When light moves from one medium to another, it can be absorbed, reflected, or transmitted, with different wavelengths interacting in different ways with the medium
• Diffraction is the bending and spreading of waves around an obstacle or through an opening, scattering occurs when light bounces off an object in various directions, and refraction is the change in direction of light waves passing from one medium to another
• The speed of light in a vacuum is a constant value, but it slows down when traveling through matter, including minerals
• Electromagnetic waves contain electric and magnetic fields that change or oscillate, with the electric field vector always perpendicular to the magnetic field vector
• Refractive index is the measurement of the bending of light when passing from one medium to another, and it is an intensive property
• Snell’s law states that the sine of the angle of incidence is the same as the sine of the angle of refraction, and birefringence occurs when the sample polarizes the light and refracts it into two directions
• Dichroic materials are examples of materials that have the property of birefringence