The FBI has a library of paint samples and the National Automotive Paint File contains more than 40,000 original paint finish samples. This tremendous stock of material enables examiners to identify the year, make and model of any car by comparing the physical characteristics and chemical composition of paint found at a crime scene with that of known specimens. The chemical constituents of paint can be analyzed via releasing gases and using gas chromatography. This technique creates characteristics for each layer and establishes points of comparison. Also the shape of a chip can be matched to an area where the chip is missing.
With x-ray fluorometry, by bombarding the metal elements in the paint with high-energy electrons, an inner orbital electron of the atom hit by the beam is displaced or knocked out. In order to stabilize, an outer orbital electron falls to fill the void. The atom emits an x-ray and the x-ray emitted is specific to the atom (and therefore the element) that it came from. This technique permits examination of the composition of the paint on an atomic level. The elemental composition of the fragment should match that of its place of origin—allowing the fragment to be linked back to the crime scene.
Although a few flakes of paint may not seem like much, in cases with hit-and-run injuries they can make all the difference.
A ten-year-old Los Angeles boy was killed by a hit-and-run driver. Examiners took the boy’s clothes into an airtight scraping room and carefully rubbed paint off with a metal spatula. Tiny paint chips fell off the boy’s pants, which had probably been pressed into the clothes by his blood and fluids. The gold metallic paint identified was used on three model cars by the same manufacturer. The lab quickly notified the Los Angeles police department that the killer would be driving one of these models.
Soon after, police discovered one of the boy’s neighbors drove a gold sports car with a recently replaced hood. The suspect claimed the body work was the result of a different traffic accident. To his dismay, the hood was recovered, and the examiners were not only able to match the paint chips but they also found fibers caught in the metal that matched the boy’s jacket. Faced with this evidence, the owner of the car confessed to the crime.
In addition to the collections of paint samples, the FBI has standards against which to test typewriting samples, watermarks, plastics and polymers, duct tape, gemstones, cosmetics, and even lubricants like eighty-five types of petroleum jellies. Anything that can be used to build a bomb or aid in a crime is compared microscopically, microchemically, and instrumentally. All specimens have to be identified using two different scientific methods and following established protocols, before it can be used in court.
As forensic science advances with computers and increasingly more accurate means of detecting the component parts of small samples, trace evidence may soon play even more significant roles. As it is, the more trace evidence an investigator can collect at a crime scene, the better the chances of making a case.