Elements of a crime

Scientists wielding a laser and spectrometer analyze chemicals of wood to help shed light on a homicide case.


February 04, 2005|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Police investigating the grisly killing of a young Texas mother have gotten an assist from a laser technology first developed to assay the quality of the enriched uranium made for America's nuclear bombs.

Called laser-induced breakdown spectroscopy, it produces precise, on-the-spot chemical analyses that promise to help forensic detectives quickly match materials linking suspects to crime scenes.

Since 2003, the technique has been used to answer questions in environmental and medical sciences, in astronomy and archaeology. But the Texas homicide investigation is believed to be only the second time it has been applied in a criminal case.

"We are hoping they'll call us more," said Madhavi Martin, the research scientist at the Oak Ridge National Laboratory near Knoxville, Tenn., who assisted in the Texas case.

The story began late last winter in the town of Farmersville, Texas, population 3,200, where 20-year-old Rachelle O'Neil Tolleson disappeared from her small rented house, leaving her 6-month-old daughter alone.

Relatives called police, who joined Tolleson's family and friends in a search. After several days, according to an account in The Dallas Morning News, a relic hunter looking for arrowheads in a streambed outside town came across Tolleson's body. Someone had tried to destroy her remains in a pyre of firewood. But the wood was too green, and it burned poorly.

A tip quickly led police to their suspect, Moises Sandoval Mendoza, 20, a former high-school classmate of Tolleson's. Police said Mendoza confessed, allegedly telling investigators he choked Tolleson, then had sex with her and finally stabbed her in the neck.

But the Collin County sheriff's office continued its detective work, seeking to bolster what would later become a capital case.

Tree-ring expert

On April 20, they contacted Henri Grissino-Mayer, a geographer at the University of Tennessee and an expert in dendrochronology - the study of tree rings as a tool to date old wood and to reconstruct ancient climate patterns.

The sheriff told Grissino he had recovered what looked like partly burned fireplace logs. Most were taken from the crime scene; the rest were logs the suspect had been seen placing in a fireplace at a social gathering.

"At first they wanted to know if I could match the tree ring patterns from the two sets of wood, which makes sense," Grissino said.

Tree rings have been used in criminal cases since the 1932 kidnapping of the 19-month-old son of aviation pioneer Charles Lindbergh. Bruno Hauptmann, who was eventually executed for the crime, was convicted in part with evidence from wood used to make a crude ladder the kidnapper climbed to enter the Lindberghs' Hopewell, N.J., home.

"They matched the tree-ring patterns in the boards on the ladder with the patterns in the boards in his [Hauptmann's] attic, and they matched perfectly," Grissino said.

Grissino has used tree ring analysis to check the authenticity of a $20 million Stradivarius violin (the real thing) and an old Kentucky log cabin that tradition said was Abraham Lincoln's boyhood home (not old enough).

So investigators in the Tolleson case sent him 14 pieces of firewood to examine. At first, he thought it was hopeless. "It's a type of wood that doesn't form nice rings - mesquite," he said. But then he got an idea.

"I thought, I shouldn't concentrate as much on the tree rings as on the chemical properties of the wood," he said.

Wood `fingerprints'

Grissino had heard that colleagues in the University of Tennessee's Forest Products Center were using laser technology to establish chemical "fingerprints" of wood. That led him to Oak Ridge, where Martin was using lasers to determine the geographic origins of wood imported from Canada.

Her chief tool was laser-induced breakdown spectroscopy, or LIBS. It was developed at the Los Alamos National Laboratory in 1980 to analyze the quality of weapons-grade uranium.

Martin said that even trees of the same species, if rooted in different soils or growing in different environments, will absorb different proportions of various chemical elements. LIBS can identify them and map their relative abundance in detail.

Firing a high-energy pulsed laser through a focusing lens, Martin directs a series of 100-millisecond bursts of intense light energy onto a tiny area of the target material.

Each pulse evaporates a tiny volume of the wood into a hot plasma. When each light pulse stops, the chemical bonds in the plasma simmer down and emit light. Those emissions are broken into their various wavelengths by a spectrometer, recorded and graphed on a computer screen as a series of peaks and valleys.

The locations of the peaks and their heights reveal the identities and amounts of each element in the sample. Each has a unique chemical profile.

Martin has also used the technology to identify bones.

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