1 / 31
Chapter 4. The Microscope and Forensic Identification of Hair and Fibers. Objectives (1 of 2). Students should gain an understanding of: The parts of a compound microscope and how it works The use of a comparison microscope to compare two objects
Presentation posted in :
An Image/Link below is provided (as is) to download presentation
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other sites. SlideServe reserves the right to change this policy at anytime.
While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
The Microscope and Forensic Identification of Hair and Fibers
Objectives (1 of 2) Students should gain an understanding of: The parts of a compound microscope and how it works The use of a comparison microscope to compare two objects The large working distance and the larger depth of field afforded by the stereomicroscope Differentiation of amorphous and crystalline materials by use of a polarized light microscope Objectives (2 of 2) The structure of hair and the microscopy techniques used to identify human hair The characteristics of natural fibers, human-made fibers, and the fabrics made with both types of fibers The use of microspectrophotometers and scanning electron microscopes in the forensic lab Introduction Trace evidence: small, often microscopic, objects that are readily transferred between people and places Microscopic comparison of fibers and hairs: started at the FBI laboratory in the early 1930s Capabilities of forensic laboratories: greatly expanded with the development of modern analytical instruments Magnifying Small Details Forensic scientists need to analyze many different types of materials Early labs relied on the light microscope This microscope offered less than 10 times magnification Refraction Refraction: magnifying glass bends (refracts) light rays as they pass through air and back through the lens Focal length: depends on the change in refractive index Refractive index: ratio of the velocity of light in a vacuum to the velocity of light in any other medium Types of Microscopes (1 of 9) A microscope has at least two lenses: Objective (lower) lens: produces a magnified and inverted version of the object Ocular (smaller) lens: produces a virtual image in the viewer’s brain Magnifying power = power of the objective lens × power of the ocular lens The ability to distinguish extremely small objects depends on the wavelength of light used to illuminate the object Types of Microscopes (2 of 9) Compound microscopes have six parts: Base: stand on which it sits Arm: support for the tube body Body tube: hollow tube that holds the objective and eyepiece lenses Stage: platform that supports the specimen Coarse adjustment: knob that focuses the microscope by raising and lowering the body tube Fine adjustment: knob that adjusts the height of the body tube in smaller increments Types of Microscopes (3 of 9) The optical system of a compound microscope has four parts: Illuminator: electric lighting (e.g., tungsten, fluorescent, halogen) Condenser: part that focuses light rays through a lens at the center of the stage Eyepiece: part you look through Objective: second lens of the microscope A higher numerical aperture (NA) allows for more detail Anything beyond 1000× is considered “empty magnification” Types of Microscopes (4 of 9) Comparison microscopes Are used to compare two specimens Consist of two compound microscopes connected by an optical bridge Provide a single eyepiece through which the examiner sees both images side by side Can be lighted from below the stage or via a vertical or reflected illumination system Types of Microscopes (5 of 9) Stereoscopic microscopes Are the most commonly used microscope in crime labs Include two eyepieces Produce a three-dimensional image with a right-side-up, frontward orientation Offer a large working distance Can be lighted from below or vertically from above Types of Microscopes (6 of 9) Polarizing microscopes Can provide information on the shape, color, and size of minerals Can distinguish between isotropic and anisotropic materials Include two polarizing filters, a polarizer lens (fixed below the specimen), and an analyzer lens (fixed above the specimen) Through analysis of plane-polarized light, can determine whether the sample exhibits pleochroism Are used to identify human-made fibers and paint Types of Microscopes (7 of 9) Microspectrophotometers Optical microscopes have been attached to spectrophotometers. The lamp emits radiation that passed through the sample. Light is separated according to its wavelength and the spectrum formed is observed with a detector. These devices can determine the composition of unknown materials. Types of Microscopes (8 of 9) Microspectrophotometers Can measure the intensity of light reflected from a sample, the intensity of light emitted when a sample fluoresces, or the intensity of polarized light after it has interacted with a sample Allow for more precise measurements of a sample while eliminating interference from surrounding material Are useful for analysis of synthetic fibers Types of Microscopes (9 of 9) Scanning electron microscopes Can magnify 100,000× Have a depth of focus more than 300× that of an optical microscope Use electrons rather than light Offer much greater resolution than with a light microscope Forensic Applications of Microscopy: Hair (1 of 8) An individual hair cannot result in definitive identification of a person unless it has a DNA tag attached. Hair samples can exclude suspects. Hair is often contributing evidence that connects a suspect to a crime scene or connects multiple crime scene areas to each other. Forensic Applications of Microscopy: Hair (2 of 8) Hair is composed primarily of keratin, which makes hair resistant to physical change. Each strand grows out of a follicle. Forensic Applications of Microscopy: Hair (3 of 8) Three parts of a hair: Cuticle: scales of hardened, flattened, keratinized tissue that are unique to animal species Cortex: orderly array of cortical cells that allows for comparison of hair samples Medulla: rows of dark-colored cells organized in a pattern specific to the animal species Forensic Applications of Microscopy: Hair (4 of 8) Hair growth stages: Anagenic: hair follicle is actively producing the hair; follicle is attached to the root Catagenic: transition stage in which the root is pushed out of the follicle Telogenic: hair naturally becomes loose and falls out Forensic Applications of Microscopy: Hair (5 of 8) Ask two questions when hair evidence is found at a crime scene: Is the hair human? Does it match the hair of the suspect? Forensic Applications of Microscopy: Hair (6 of 8) When analyzing hair, the investigator must: Distinguish between animal and human hair Assess the hair color, length, and diameter Compare features of the hair samples, including their distribution, color, and shape of pigment granules Forensic Applications of Microscopy: Hair (7 of 8) Collect hair evidence by hand Wide, transparent sticky tape Lint roller Evidence vacuum cleaner Forensic Applications of Microscopy: Hair (8 of 8) Microscope examination might reveal two pieces of information: Area of body from which the hair originated Race of the hair’s owner Microscopy cannot determine the age or sex of the hair’s owner. Forensic Applications of Microscopy: Fibers (1 of 8) Most fibers do not degrade at a crime scene. Fibers are easily transferred from one object or person to another. Fibers provide evidence of association between a suspect and a crime scene. Fiber evidence must be carefully secured to avoid its loss or cross-contamination. Most fiber evidence can only be placed within a class. Forensic Applications of Microscopy: Fibers (2 of 8) Natural fibers are derived from plant or animal sources. Cotton is the most widely used natural fiber. Forensic Applications of Microscopy: Fibers (3 of 8) Yarn is classified into two types: Filament: continuous length of human-made fiber Spun: short lengths of fibers that are twisted or spun together Physical properties of yarn include its texture, number of twists per inch, number of fibers per strand, blend of fibers, color, and pilling characteristics. Forensic Applications of Microscopy: Fibers (4 of 8) Woven fabrics consist of intertwining of two sets of yarns. They are woven on a loom. Basic weaves are plain, twill, and satin. Forensic Applications of Microscopy: Fibers (5 of 8) A wide variety of synthetic fibers have replaced natural fibers in fabrics, garments, and rugs. There are two types of synthetic fibers: Cellulosic: produced from cellulose-containing raw materials such as trees and plants Synthetic: produced from chemicals made from refined petroleum or natural gas Forensic Applications of Microscopy: Fibers (6 of 8) Plastics: malleable materials easily formed into different products Polymers: huge molecules formed by chemically linking together smaller molecules Production of synthetic fibers: Produced by melt spinning process Shapes of holes in spinneret determine cross-sectional shape of the polymer Forensic Applications of Microscopy: Fibers (7 of 8) Step 1 in comparison of synthetic fibers: examination with a comparison microscope Pay special attention to the fibers’ color, diameter, cross-section shape, pitting or striations, and presence of dulling agents Advantages of comparison microscopy: Fiber is not destroyed Technique is not limited by the sample size Microscopes are readily available Forensic Applications of Microscopy: Fibers (8 of 8) Step 2 in comparison of synthetic fibers: analysis of chemical composition Try to place fiber in a specific polymer subclass Use refractive index to identify synthetic fibers