Forensic Science: Trace Evidence and Microscopy Comprehensive Study Guide (copy)

Overview of Forensic Science and Trace Evidence

  • Definition of Trace Evidence: Any small, minute physical material transferred between people, objects, or a crime scene during the commission of a crime.

  • Visibility: Trace evidence is not typically seen with the unaided eye.

  • Distinction Between Macroscopic and Microscopic Evidence:

    • Macroscopic Evidence:

    • Refers to larger evidence contexts that contain numerous microscopic pieces of evidence or are composed of many smaller crime scenes.

    • Examples: Bodies, fields, buildings, vehicles.

    • Microscopic Evidence:

    • Refers to specific types of trace evidence requiring magnification to analyze.

    • Examples: Hair, paint, soil, fibers, glass.

    • Forensic crime laboratories maintain specialized databases containing characteristics and manufacturing details of materials found at crime scenes for systematic comparison.

  • Core Characteristics of Trace Evidence:

    • Small in physical size.

    • Transferred easily upon contact or movement.

    • Occurs in various physical forms (e.g., hair, paint, glass, soil).

  • Common Forms and Specific Applications:

    • Forms: Hair, fiber, glass, soil, paint, explosives, pollen, drugs, feathers, plants, and additional minute materials.

    • Pollen: Can assist in regionalizing geographical locations where an individual or object may have been.

    • Duct Tape Fibers: Can be physically and chemically matched to specific rolls of tape recovered from crime scenes.

Principles of Transfer and Persistence

  • Locard's Exchange Principle:

    • Fundamental Axiom: "Every contact leaves a trace."

    • Formulated by Edmond Locard, who established the first police crime lab.

    • Serves as the foundational backbone of trace evidence theory and practice.

    • Defines the principle of cross-transfer between interacting physical entities.

  • Types of Evidence Transfer:

    • Direct / Primary Transfer: Direct transfer of material from one source to another without an intermediary (e.g., hugging someone results in direct fiber transfer).

    • Indirect / Secondary Transfer: Transfer occurring via an intermediate surface or entity (e.g., leaving hair on a seat, which is subsequently picked up when someone else sits down).

    • Non-Direct / Contact Transfer: Material transfer occurring without direct physical contact between surfaces (e.g., airborne particle settling).

  • Factors Influencing Evidence Transfer:

    • Types of material involved.

    • Characteristics of the surface to which material is being applied.

    • Degree to which the material was shed.

    • Specific nature of contact (e.g., rubbing, tapping, applied force).

    • Number of discrete contacts made.

    • Total size of the surface area where contact occurs.

    • Environmental conditions (e.g., presence or absence of wind, moisture).

  • Persistence and Degradation:

    • Persistence: The second phase of the transfer process, defined as how long transferred evidence lasts on a surface.

    • Rule of Persistence: Transferred evidence remains on a surface until it undergoes further transfer, degrades, decomposes, or is collected as evidence.

    • Factors Affecting Persistence:

    • Nature of the accepting surface.

    • Physical activity occurring after transfer.

    • Addition of other materials onto the receiving surface.

    • Depth to which the material becomes embedded in the surface.

    • Nature of the material transferred.

    • Amount of force with which the material was applied.

    • Specific physical location of the evidence.

  • Interpretation and Negative Findings:

    • Negative Findings Axiom: A negative finding does not mean that no contact took place.

    • Contextual Significance: Evaluating the physical relationships between people, places, and things involved in crimes helps determine the legal and forensic significance of evidence and how to interpret it correctly.

Contamination Prevention Protocols

  • Definition of Contamination: Any transfer of material that takes place once activity surrounding the crime has ceased is considered contamination.

  • Standard Steps to Prevent Contamination:

    • Wear protective gear including gloves, Tyvek suit, disposable booties, and a protective mask.

    • Strictly limit access to the crime scene.

    • Minimize the number of investigators present at the scene.

    • Establish and maintain a single designated pathway into and out of the scene.

    • Package every piece of evidence separately.

    • Secure and protect the physical integrity of the scene.

    • Thoroughly clean and sanitize tools between uses.

    • Collect proper reference samples for control and comparison.

Fundamentals of Microscopy in Forensic Science

  • Historical Evolution of Microscopes:

    • Microscopy origins date back to the Middle Ages (11th11\text{th} century).

    • Galileo (16291629) mastered the art of grinding optical lenses.

  • Classification of Microscopes:

    • Low Power Microscopes:

    • Includes stereo microscopes (typically fixed magnification with zoom capabilities up to 100×100\times).

    • Used specifically to examine LARGE objects.

    • High Power Microscopes:

    • Compound microscopes using more than one lens arranged in sequence.

    • Feature ocular lenses that are binocular (two eyepieces) or monocular (single eyepiece) with ocular magnification values typically ranging around 10×10\times.

    • Used specifically to examine SMALL objects.

    • Major Types: Simple, stereo, compound, comparison, and electron microscopes.

Optical Components, Lighting Systems, and Image Properties

  • Microscope Lighting Systems:

    • Transmitted Light: Light shines from underneath the specimen stage; used primarily for transparent specimens.

    • Incident Light: Light shines over the top of the specimen; used primarily for opaque specimens.

  • Specialized Optical and Lighting Components:

    • Condenser Light / Lens: Located directly below the stage to focus light onto the specimen.

    • Field Diaphragm: Controls the total amount of light passing through the specimen.

    • Köhler Illumination: A standard scientific method used in transmitted microscopy to achieve a bright, evenly illuminated field of view.

  • Magnification Equations and Relationships:

    • Objective Lens: Lens magnifications typically range from 2×2\times to 100×100\times.

    • Total Magnification Formula:     Total Magnification=Ocular Lens Magnification×Objective Lens Magnification\text{Total Magnification} = \text{Ocular Lens Magnification} \times \text{Objective Lens Magnification}

  • Field of View (F.O.V.):

    • Defined as the total physical area visible when looking through the eyepiece.

    • Relationship: A lower power magnification provides the greatest field of view.

    • Low Power = Great (Large) F.O.V.

    • High Power = Low (Small) F.O.V.

  • Depth of Field (Aperture / Photographic Properties):

    • High magnification results in a reduced field of view and a reduced depth of field.

    • Increasing the depth of field increases the overall sharpness of the image.

Advanced Microscopy Techniques and Calibration

  • Resolution:

    • Definition: The minimum distance required between two separate objects such that they can still be distinguished as two distinct objects.

    • Directly influences effective magnification.

  • Comparison Microscope:

    • Consists of two compound microscopes joined by an optical bridge.

    • Allows two distinct specimens to be viewed side-by-side simultaneously in a split field.

  • Electron Microscope:

    • Utilizes electromagnetic lenses instead of optical glass lenses.

    • Capable of zoom magnifications up to 10,000,000×10,000,000\times.

  • Microscope Calibration and Micrometry:

    • Purpose: Calibration is performed to improve measurement accuracy and precision.

    • Primary Unit of Measurement: Microns (μm\mu\text{m}).

    • Measurement Tools:

    1. Ocular Micrometer: A ruler scale built directly into the optical ocular lens.

    2. Stage Scale Micrometer: A specialized microscope slide containing a precise known scale placed on the stage where a specimen slide is normally placed.

    • Ocular Scale Properties:

    • Ocular scale divisions serve as arbitrary measurement units.

    • The scale appears superimposed over the specimen being observed.

    • Because the scale is arbitrary, it MUST be calibrated independently for EACH objective lens.

    • Standard Unit Metric Conversions:

    • 1 mm=1000 μm1\,\text{mm} = 1000\,\mu\text{m}

    • 0.1 mm=100 μm0.1\,\text{mm} = 100\,\mu\text{m}

    • 0.01 mm=10 μm0.01\,\text{mm} = 10\,\mu\text{m}

    • 0.001 mm=1 μm0.001\,\text{mm} = 1\,\mu\text{m}

    • Calibration Procedure:

    • Place the stage scale micrometer on the stage.

    • Align the stage micrometer scale and ocular micrometer scale until the division lines match up.

    • Ocular Calibration Factor (OCF) Formula:     OCF=SSD×KASD\text{OCF} = \frac{\text{SSD} \times K}{\text{ASD}}     Where:

    • OCF\text{OCF} = Ocular Calibration Factor

    • SSD\text{SSD} = Stage Scale Divisions (number of divisions on the stage micrometer)

    • KK = Conversion factor (known micrometer length per stage division)

    • ASD\text{ASD} = Arbitrary Scale Divisions (number of matching ocular scale divisions)