Criminal investigations have been used for centuries in an effort to fight crime. The techniques used have evolved over the years, and the field of forensic science can now solve more crimes than ever before. Modern forensics labs utilize cutting edge techniques such as DNA fingerprinting, forensic entomology, and trace chemical analysis. While these procedures use state-of-the-art technology, many crimes are still solved by using classical techniques to examine the physical evidence at a crime scene.
One of the frequently used methods to identify a suspect is called fiber analysis. When clothing from a suspect and victim come in physical contact at a crime scene there is a possibility for fibers to be exchanged. Fiber transfer depends on the type of clothing worn by the suspect and victim. Loosely woven fabrics tend to shed more that tightly woven ones, and damaged fabrics tend to shed more than undamaged ones.
Although fibers are a useful piece of physical evidence, fiber transfer during a crime is never exact. For example, the length of time that physical contact occurs between the suspect and victim can influence fiber transfer. Additionally, the mobility of the victim or suspect will affect how long fibers can stick to a piece of clothing or other evidence. Finally, weather like rain and snow can completely destroy fiber evidence at a crime scene. Consequently, investigators and medical personnel must carefully preserve any evidence as soon as possible to avoid losing any fibers.
Forensic scientists have multiple methods available for analyzing fibers recovered at a crime scene. The most common method uses microscopes to examine longitudinal and cross-sectional samples of the fibers. Forensic scientists prefer Scanning Electron Microscopy and Atomic Force Microscopy, which use a beam of electrons or the movement of a microscopic probe, over light microscopy due to the higher resolution. In addition to microscopy, scientists will can also identify fibers by measuring the rate at which they burn or solubilize in a solvent. Upon matching the fibers from a suspect to those recovered from a victim or crime scene, scientists can conclude that the two individuals most likely had contact during the commission of a crime.
Below is a simple fiber and hair analysis activity teachers can perform in their classroom. Students can gather fiber or hair evidence found from a crime scene and can perform a microscopic analysis.
- Compound Microscope
- Microscope slides
- Cover slips
- Paper for evidence (4 x 4 cm)
- Fiber evidence (natural or man-made)
Fiber Analysis Procedure
For Teachers: Gather several different types of fibers obtained from a “crime scene” as evidence for students to analyze. You should also select known fiber types for the students to use during their analysis.
For Students: Identify the unknown fiber evidence found on the victim by comparing it to known fiber types.
- USE a pair of forceps and COLLECT an “unknown” piece of fiber evidence found on the “victim”.
- PLACE the piece of evidence on a clean piece of paper. FOLD the paper twice to avoid contaminating the evidence.
- PREPARE a wet-mount slide of the evidence by placing a drop of water on the slide.
- PLACE the fiber evidence on it very carefully and COVER the evidence with a cover slip.
- USE a microscope to make observations of the fiber evidence under low, medium, and high magnification.
What does the fiber look like? Are there striations on it? Sketch your observations in your lab notebook or use the data table below:
|Fiber Type||Low Magnification||Medium Magnification||High Magnification|
After sketching what you see, repeat the procedure with the known fiber types (cotton, polyester, wool, etc). Make observations using low, medium, and high magnification. Based on your observations, determine the fiber type found on the victim.
Burn Test (Optional): Students can do a similar experiment using a burn test to observe the rate at which each fiber type burns. Be sure to wear appropriate safety equipment and check school policies first!