‘Ewww. Let’s try it.’

From forensics to environmental science, elemental fingerprints uncover the clues

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  Madhavi Martin subjects a chunk of wood to laser-induced breakdown spectroscopy.

Madhavi Martin subjects a chunk of wood to laser-induced breakdown spectroscopy.

The Environmental Sciences Division's Madhavi Martin has reinvented an old analytical method, finding some very new applications for the technology in the process. She uses laser-induced breakdown spectroscopy-a technique invented in the 1960s-to collect elemental fingerprints.

The LIBS method uses a focused laser to evaporate sample materials, breaking down all of the bonds and producing optical emissions of very specific wavelengths depending on the elements present within the sample.

"The wavelength emissions are the fingerprints of the elements in the sample," Madhavi says. "One advantage of the method is that I can analyze any kind of sample: liquid, solid, gas or aerosol."

One of her collaborators at the Lab has continued to test the claim that her method can work with any kind of sample. Arpad Vass approached her about five years ago, asking if the method she had been using to analyze wood chemistry could be applied to bone.

"My first reaction was, 'Ewww, bone?' My second reaction was, 'Let's try it,'" says Madhavi.

After looking at the first few bone samples, she realized that she could see clear differences between them. "Well, you are what you eat," Madhavi says.

Her method can even determine whether you took your multivitamins or not.

"We need more bones in the experimental data set before the method is ready to be used for biological profiling, but the results are promising," Madhavi says, pointing out the peaks in the spectrum that tell her the bone sample is probably from a woman who took her daily supplements.

The same method can be used to differentiate between human and animal bones.

"Sometimes people find a fragment of bone in the backyard, and this method can quickly determine whether it is human or not," says Madhavi, explaining that the wait for DNA sequencing may be as long as a month and could be inconclusive for old bones. The method could also be used to analyze bones that have been exposed to conditions that degrade DNA, like high heat or the passage of time.

Her work has even helped to solve a murder, connecting a suspect to the victim by showing that firewood at the scene of the crime was from the same tree that the suspect later took to a bonfire and burned at his own home.

Early one morning, she got a call from Henri Grissino-Mayer, an expert in tree-ring analysis in the University of Tennessee's geography department, who had been called in to consult on the case. Unfortunately for Grissino-Mayer, the firewood turned out to be from a mesquite tree-one of the worst for analyzing tree rings because of its erratic growth patterns.

"He called and asked if I could test some logs for him, but he didn't mention anything about a murder," says Madhavi. "When I finished testing all 14 of the logs, I was very disappointed because they all looked the same except for one." She called Grissino-Mayer back with the results and he immediately asked which one was different-it was the control, a piece of firewood that wasn't from the case in question.

"He was very excited; I was very surprised when I found out," Madhavi says. "The data that I thought was so uninteresting turned out to be very important-the elemental fingerprint of the logs was the same, tying the suspect to the scene."

LIBS can shed light on not only where bones come from, but also whether their owners took their vitamins.

Madhavi has gotten a lot of press for her contributions to forensics, but she hastens to add that environmental threats like water and air pollution can also be characterized using the LIBS technique.

She can analyze the environmental conditions experienced by a tree during its growth, pulling samples from each ring. Global climate change can be traced by looking at the elements present throughout the lifetime of a tree. Material from 100 years of tree growth takes her about 20 minutes to analyze and requires only a few thousand shots of the laser. Conventional wet-chemistry techniques take much longer and involve extracting and chopping up the wood samples from every ring to be analyzed.

Madhavi is also using the technique to improve phytoremediation of large-scale polluted areas.

The use of plants to clean up environments that have high levels of pollutants such as mercury or TNT shows much promise. Some plants take up and sequester the pollutants better than others. Using her method, Madhavi says that she can monitor the amount of pollutants present in prospective plants, helping to choose the best candidate for phytoremediation efforts.

"It's been exciting because the tools that we have now-better lasers, better instruments for detection-have revived the LIBS technology," she says.

People are using LIBS to look at all kinds of materials, but Madhavi is one of the few using it for environmental analysis. "I get calls and get to collaborate with all kinds of people because I am in this wonderful niche," Madhavi says.—Sarah Wright