Just a trace is all FBI needs
The FBI's X-ray spectral database is still in its infancy, but two recent cases hint at its crime-solving potential.
The FBI's X-ray spectral database is still in its infancy, but two recent cases hint at its crime-solving potential.
They prove that spectroscopy has moved from simple identification of chemical elements to providing investigative leads, FBI Laboratory chemist Dennis C. Ward said.
In one upstate New York case, microscopic powder samples were lifted from a potential murder suspect's jacket. Most firearm cartridges contain a primer of lead, antimony, barium and other elements. Tiny primer particles lodge on clothing or other nearby surfaces during firing.
Local investigators sent the jacket to the FBI Lab in Washington, where Ward removed trace amounts of gunshot residue for study with a scanning electron microscope. The X-ray spectra showed traces of cobalt in addition to the usual primer ingredients.
A database search revealed that one ammunition manufacturer uses cobalt in its cartridge primer, and investigators confirmed that brand of handgun cartridge was used in the shooting.
Investigators often don't have a clue about the compositional uniqueness of gunshot residue, paint, adhesives and so on until they have compared hundreds or thousands of materials in a given category, Ward said. The more comprehensive the database becomes, the more analysts will know about the similarities and differences.
Also, using spectroscopy to trace gunshot residue highlights the need to involve other nations in populating the spectral database, Ward said. European researchers have access to ammunition from Eastern bloc countries that isn't sold legally in the United States.
In another closed criminal case, medical examiners recovered white granules from beneath the fingernails of a woman who was slain in her apartment. Because they could not identify the white stuff, they sent it to the FBI Lab.
X-ray spectroscopy showed the particles contained calcium, sulfur and oxygen, and the database suggested an ordinary drywall composition, Ward said. Detectives then sought and found scratch marks on a wall, which helped them piece together the death scenario.
Investigators also found a few unusual-looking smudges on the victim's clothing. Ward and his colleagues retrieved enough material from the smudges to take an X-ray spectrum and learn from the database that the substance might be a metal-polishing compound.
This clue led detectives to a suspect who worked in a machine shop near the victim's apartment. They found the smudges on the suspect's clothing, on rags in his truck and throughout his work environment.
Although the SEM/spectrometer combination 'is already a workhorse in the laboratory,' the database will revolutionize its use in solving crimes, Ward said.
'Patricia Daukantas
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