As if in a Hollywood script, an ORNL scientist helps solve a murder mystery in Texas.
In March 2004, a woman who had been missing for a week was found strangled and stabbed to death in a field in Collin County, Texas. Around her body lay pieces of partially burned logs. Her murderer apparently had tried to ignite the wood pieces in an attempt to burn her body. The attempt failed because the wood was too green.
The sheriff's office obtained warrants to search the homes of people who had known the murdered woman. A criminal investigator found similar, partially burnt wood pieces at the site of a gathering attended by a male suspect. The investigator theorized that the woman's murderer had cut more wood from the same tree, and perhaps took the wood elsewhere.
The sheriff's office collected and numbered 14 logs—some from the murder site and the others from the place of gathering the suspect attended. The wood pieces were sent to Henri Grissino-Mayer, an expert in tree-ring analysis who had relocated from the University of Arizona to the Geography Department at the University of Tennessee at Knoxville.
Grissino-Mayer determined that the logs came from the mesquite tree, which grows too erratically to be useful for studies of annual tree rings. Thinking that a chemical technique might show the wood pieces came from a particular location, Grissino-Mayer contacted a Tennessee Valley Authority chemist. She suggested that he contact Madhavi Martin, a physicist in ORNL's Environmental Sciences Division. Martin had begun studies of the chemical composition of wood, using laser-induced breakdown spectroscopy.
Grissino-Mayer called Martin and asked if she would apply her LIBS technique to the analysis of wood pieces to help solve a murder mystery. Martin agreed. She received three boxes of wood pieces, each piece about 2 feet long and 6 inches in diameter.
Martin was not told that, of the 14 logs she was asked to analyze, 10 were retrieved from the crime scene and 4 from another place the suspect had been. "I analyzed all 14 pieces and found they had an identical spectrum," she says. "They all came from the same tree or from trees found in the same geographical area. I examined the burned parts of the logs and the unburned parts. The data revealed that the wood pieces had the same elemental content."
Equipped with this key piece of evidence, authorities connected the suspect to the crime scene. In June 2005 a jury found the suspect guilty of murder.
LIBS is a technique that produces a chemical "fingerprint" of wood or any other material, based on heavy metals and other trace elements. A tree draws metals from the soil into its wood structure. Trees in one location may have high concentrations of titanium, represented in the spectrum as a tall peak. Trees in another location may have little, if any, titanium.
In LIBS a high-intensity, pulsed laser is focused through a lens onto a solid, liquid, or gaseous sample. The hot laser pulse breaks down the sample, creating a plasma. As the plasma cools, excited atoms of different elements emit light of distinct wavelengths. The light is collected by optical fibers, delivered to a spectrometer, and detected by an intensified charge-coupled detector. The computer-controlled spectrometer is triggered to acquire and read out the data simultaneously on the display screen, to provide rapid elemental analysis of materials.
Through various collaborations, Martin has used LIBS to detect and measure concentrations of heavy metals in emissions from coal-fired power plants, sulfur and carbon in coal gasification plant emissions, and silver and palladium dispersed in cellulose membranes developed for fuel cells. She also has examined silicon and carbon in layers enveloping nuclear fuel cores to determine if layer thicknesses are uniform.
Martin laughs at the notion her research may be equally valuable to the writers of CSI, noting that LIBS also enables her to distinguish between real money and counterfeit bills.
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