A murder occurs in Collin County, Texas. The suspect tries to dispose of the body by burning it on a pyre made of fire wood.

When detectives learn during the course of the investigation that the suspect had brought logs to a party at about the same time the victim came up missing, they collected ten logs from the murder scene fire and four logs from the party fireplace for comparison purposes. If the logs could be somehow shown to match, it might help place the suspect at the crime scene.

“Our initial plan was to compare the tree-rings from the samples,” said Lt. Larry R. Smart, the lead detective on the case for the Collin County Sheriff’s Department. Tree-ring analysis is frequently achieved simply by counting, dating (to the nearest exact year), and measuring (to the nearest 0.001 mm) the tree rings found in specimens.

Smart contacted the Laboratory of Tree-Ring Science at the University of Tennessee directed by Prof. Henri D. Grissino-Mayer to see if the lab could use tree-ring dating to determine whether the two sets of fire wood came from the same tree. Tree-ring analysis, or dendrochronology, has been a reliable forensic tool since playing a major role in solving the 1932 murder of Charles Lindbergh's infant son.

Now You Tree It, Now You Don’t
“ Unfortunately, when I saw the pieces of mesquite (Prosopis glanduloa) I realized that tree-ring dating would not be practical with this type of wood,” Grissino-Mayer said. Mesquite, the most common shrub or small tree of the Desert Southwest, does not form familiar tree-rings patterns due to the erratic nature of tree growth from year to year, which produces annual rings that are not clearly distinguishable. Grissino-Mayer told Smart that tree-ring analysis in this case would not only be challenging, but time-consuming, costly, with no guarantee of results.

While Smart considered the implications of these obstacles, Grissino-Mayer wondered whether some kind of chemical identification technique could identify the wood samples as having come from the same location. He contacted Suzanne Fisher at the Tennessee Valley Authority in Knoxville, who once had conducted dendrochemical analysis for her master’s thesis. Fisher informed Grissino-Mayer of research scientist Madhavi Z. Martin at the Oak Ridge National Laboratory who had already begun initial studies on chemical properties of wood using a technique known as laser-induced breakdown spectroscopy.

“Maybe we could use this technology to obtain enough information from the chemical composition of the wood to perhaps tie the two sets of wood together,” Grissino-Mayer said.

LIBS Service
Laser-induced breakdown spectroscopy (LIBS) itself is not new. For the past 15 years or so, for instance, Los Alamos National Laboratory has conducted research and development on elemental analysis using LIBS technology.

The LIBS technique has many advantages compared to conventional analysis methods that make it particularly suited for field based forensic measurements under harsh conditions. For one, LIBS measurements are generally carried out in ambient air at atmospheric pressure, whereas some analytical methodologies require laboratory vacuum chamber processing. The limits of LIBS detection, however, can be improved when analysis is performed with the aide of vacuum processing, Martin indicated.