ac detective
When trees talk Analysis of logs used to burn a victim’s body could help solve a Texas murder mystery.
ast spring, a 20-year-old Texas woman was abducted from her home. Days later, police found her raped, strangled body in a dry creek bed. Now, the wood logs her killer tried to burn her body with could help put him behind bars. “The wood was too green to burn, and so it remained on top of her body,” explains Henri Grissino-Mayer, a dendrochronologist at the University of Tennessee (UT) in Knoxville. The suspect later attended a gathering during which he placed some other logs in a fireplace. These logs were also too green to burn. Because of Grissino-Mayer’s work with tree-ring analysis—which includes authenticating the Messiah violin made by Antonius Stradivarius in 1716— Collin County, Texas, detectives sent him 14 logs they had retrieved from both the crime scene and the fireplace to see if he could determine whether the logs came from the same tree. “I couldn’t,” Grissino-Mayer concedes, because they were from a common tree species found in the Southwest, namely mesquite, that doesn’t form welldefined rings. He thought, however, that there had to be some type of chemical method for determining whether one piece of wood is related to another, so he contacted researchers at UT’s Forest Products Center. As it turned out, Madhavi Martin, a physicist at Oak Ridge National Laboratory (ORNL), was using a technique called laser-induced breakdown spectroscopy (LIBS) to help them analyze the chemical properties of wood. LIBS is a surface analytical technique that essentially produces a chemical “fingerprint” of wood on the basis of heavy metals and other trace elements, according to Martin. Because such elements aren’t found in abundance in the natural © 2005 AMERICAN CHEMICAL SOCIETY
COURTESY OF ORNL, PHOTO BY CURTIS BOLES
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Answers in the wood. Madhavi Martin of ORNL analyzes a log from a crime scene in search of clues to a murder mystery.
environment, their presence in wood is highly dependent on the type of soil found at the site where the tree grew. LIBS is a simple method in which “you just shoot an object with a laser beam, and it creates gas plasma as the object right at the surface vaporizes,” Grissino-Mayer explains. A spectrometer records the elements in the gas plasma, which can be identified by their spectral peaks. “Then, all you have to do is match up the spectral peaks with the elements, and you have a fingerprint,” he notes. Martin sampled different areas on both the charred and unburned sections of the confiscated logs, essentially scanning their entire surface, to assess the presence and amounts of certain elements —in this case, carbon, nitrogen, calcium, aluminum, iron, titanium, silicon, magnesium, manganese, and sodium. Little difference was seen in the chemical spectra between the charred and unburned portions, which was to be expected, because the temperatures that caused the charred surfaces were too low to volatilize and remove the elements, she says. A further statistical correlation analysis of the elemental data helped the scientists
to establish with 99.999% confidence that the chemical spectra from the logs were identical. “It was staggering,” GrissinoMayer says. “The only way that can happen is if the logs came from the same tree or the same stand of trees.” The finding puts the person who placed the logs on the fireplace at the crime scene. Martin has used LIBS for various environmental applications, such as monitoring the amount of carbon that can be sequestered in soil, as well as in other forensics applications for identifying chemical fingerprints in bones and detecting counterfeit currency. Now, researchers at the UT Forest Products Center are using LIBS to look at wood treated with preservatives, particularly chromated copper arsenate, which the U.S. Environmental Protection Agency recently phased out for use in residential wood products and children’s play areas. The challenge with chemically treated wood is that it doesn’t decay, notes Tim Rials, the center’s director. So when it’s taken out of service, there’s a disposal issue. Consequently, “it’s really important to be able to quickly and conveniently determine whether it’s in fact preservative-treated wood or not,” and if it is, what type of treatment was used, Rials says. “LIBS is a very sensitive analytical tool for that, and the real attraction from our perspective is not only is it fast, but it’s affordable, robust, and has the potential for miniaturization.” As for the murder-mystery case, jury selection for the trial is expected to begin in June, according to Leigh Hornsby, spokesperson for the Collin County district attorney. Grissino-Mayer and Martin could be called to testify. a —Kris Christen
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