Flinn Forensic Files—Finding Evidence in Fibers
A crucial element needed to convict a suspect is to find evidence that places the individual at the scene of the crime. One way this can be accomplished is by fiber analysis.
- Physical and chemical properties
- Fiber analysis
- Chemical bonding
Christine Schultz had worked a long day in her job as Accounting Director trying to get the financial statements ready for month's end. She glanced down at her watch and noticed it was already 9:00 p.m. Her associate, Lisa Olszewski, had left about 30 minutes earlier and Christine stayed to wrap up some last minute odds and ends. As she walked out to her car, the parking lot was dark and there was a chill in the air on this fall evening in Lexington, SC. She had quite the walk since her car was located at the far back side of the parking lot. On her way to her car she noticed an old van with temporary plates that she had never seen before. To be cautious she picked up the pace and briskly moved toward her car. As she reached for the door handle an arm came out from behind her, swatting her arm away from the car. She turned around and there was a man about 6 feet tall wearing a white sweatshirt, jeans and a black ski mask. He told Christine to hand over her purse. She refused and tried to get back in her car. At that point the man grabbed and turned her and reached for her purse. Christine put up a struggle but the man overpowered her. He grabbed her purse and pushed her to the ground causing her to hit her head.
The next thing she knew she was being examined by a paramedic and when she opened her eyes she started to remember what had occurred. Officer Brendon Kaiser quickly jogged over when he saw her waking up. He asked what happened and Christine told him. As she was put on a stretcher and into the ambulance, Officer Kaiser said, “Wait a second.” He noticed a section of torn fiber on the button of her coat sleeve that did not match anything she was wearing. He asked, “Is this from him?” Christine glanced at it and immediately knew it was from the man’s sweatshirt. “Yes” she nodded. Officer Kaiser removed an evidence bag from his pocket and placed the piece of fabric in it. “We will take this back to the lab and perform fiber analysis. Take care ma’am; we’ll be in touch,” the officer said before heading back to headquarters.
Later on that evening the police received a call from Jamal Brooks who worked at the Gas Mart across town from where Christine was attacked. He said he saw a white van with temporary license plates dispose of something suspicious in the dumpster at the gas station. He thought it was odd as the van did not fill up with gas and the customer did not come into the store. All he did was throw away what appeared to be tattered clothes. The police collected the evidence from the dumpster. They also reviewed the surveillance video and found the van was registered to man named Kyle Long.
Fibers are the smallest component of a textile material. They are used to form fabric, rope, carpet, etc. Fibers gathered as evidence at a crime scene may arise from numerous scenarios. They may be transferred via personal contact between suspect and victim and they can also be transferred to other items at the crime scene, such as furniture, weapons or flooring, due to a physical struggle.
In most cases, fibers are analyzed based on their physical and chemical properties. The burn test is a primary identification technique used to determine fiber identity. When held near a flame, different fibers will exhibit different behaviors. Some will begin to melt and others will twist or curl up. When ignited they will also react differently. Some fibers burn slowly, others burn quickly. Some will melt and drop off before the flame reaches the rest of the fiber. Odor is also a powerful tool in identifying an unknown burnt fiber. If the smell is similar to that of burning hair, the fiber is most likely silk or wool. If the smell is similar to burning paper or burning wood, the fiber is probably cotton, rayon or linen. If the fiber melts, it is likely a synthetic material such as nylon or polyester.
Fibers may be analyzed by visual inspection, based on their appearance and comparison with known samples. Identifying fibers based on appearance requires the use of a microscope to view miniscule details. Natural fibers are easier to distinguish under a microscope. Synthetic (manmade) fibers are traditionally less descriptive under a microscope because they can consist of practically any strand diameter or color. Synthetic or manmade fibers typically have a more uniform diameter and appearance than natural fibers. Therefore, while microscopic analysis is not the main determining test used to identify fibers, it can be helpful as an ancillary test.
The purpose of this experiment is to examine six different types of fibers against the evidence from the crime scene. This will be done by performing a burn test and using a microscope to perform visual inspection. Each fiber’s unique physical and chemical properties will determine the fabric composition of the obtained evidence.
Part A. Burn Analysis
Multifiber test fabric, 6 cm
Part B. Microscope Analysis
- Based on the information provided in the Background section, name one type of fiber that would likely melt when exposed to flame.
- Why is visual inspection of a fiber sample traditionally done as an ancillary identification means as opposed to a primary means of identification?
Exercise caution when heating fibers over a Bunsen burner. Wear chemical splash goggles and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.
Note: The multifiber test fabric consists of six different fibers in the following order starting at the cream-colored end: wool, acrylic, polyester, nylon, cotton and acetate.
Part A. Burn Analysis
Part B. Microscope Analysis
- Fill a 50-mL beaker with tap water. This will be used to extinguish any stubborn flames resulting from the burn analysis test.
- Obtain the multifiber test fabric strip and pull it apart slightly by grabbing the wool section and the acrylic section. Once the fabric is loose enough, pull out a single wool strand (see Figure 1).
- Light the Bunsen burner. Using forceps, hold the single fiber over the heat but not within the flame. Record observations on the Finding Evidence in Fibers Worksheet.
- Place the fiber into the flame. Does it ignite quickly or slowly? Note: hold each fiber in the same location within the flame to achieve comparable results.
- Remove the fiber from the flame of the Bunsen burner. Does it extinguish quickly? Does it melt or drip? Record observations on the worksheet.
- Repeat steps 2–5 to single test strands of each of the different fibers in the multifiber fabric. Note: With some fibers the reactions in steps 2–5 occur very quickly. Repeat tests as necessary in order to make accurate observations.
Part C. Evidence Analysis
- Obtain the test fabric strip and pull it apart slightly by grabbing the wool section and the acrylic section. Once loose enough, pull out a wool strand.
- Place the strand on a microscope slide. Add a few drops of immersion oil to the strand and place a cover slip over the specimen.
- Examine the specimen underneath a compound microscope. Record observations at 100X on the Finding Evidence in Fibers Worksheet.
- Repeat steps 7–9 with the five remaining fibers.
- Obtain a sample fiber obtained at the crime scene from your instructor.
- Perform the tests from Parts A and B on the evidence fiber. Record observations on the Finding Evidence in Fibers Worksheet.
- Obtain a sample fiber from the possible evidence collected at the gas station.
- Repeat step 12 with the fibers collected at the gas station.
- Consult your instructor for appropriate disposal procedures.