An image showing the use of LIBS on a tire

A tire’s unique chemical signature based on its elemental composition could help forensic scientists link a car to the crime scene. The discovery could become a new tool for examining tires and their debris, which has often been overlooked in car crash and crime scene investigations.

“This would provide incredible additional information to law enforcement agencies and forensic laboratories that rely on traditional skid marks to determine primarily the speed and energy that provoked the accident,” explains Matthieu Baudelet, a chemistry professor at the University of Central Florida, whose team does the Study carried out. He says some scuff marks on tires can help forensic scientists, but knowing their chemical makeup could link a tire to skid marks on the road. This is particularly useful in hit-and-run accidents or accidents involving multiple cars.

Baudelet’s team used laser-induced breakdown spectroscopy (Libs) to analyze 34 samples of tire treads. The technique focuses a high-intensity laser pulse on a surface, creating a luminous plasma that then emits light according to its chemical composition.

Because degradation and ionization aren’t specific to an element or material, Libs can potentially reveal any element on the periodic table that could be in any sample, be it a gas, liquid, or solid, says Baudelet.

All tires have their own chemical signature and unique, appropriate skid marks. Tires are mainly made of a rubber compound that is hardened by vulcanization, in which the polymer chains are cross-linked along with some additional chemicals such as sulfur or zinc oxides. Oxides of other elements such as silicon, calcium and iron are also added to the tires for stabilization and performance. This means that every tire model is chemically different from every other.

Baudelet admits, however, that there is still a major challenge in determining more clearly how elements on the road such as oil, rainwater and other vehicles affect and change a tire’s chemical signature. “Elemental analysis is powerful, but it is very sensitive to interference unless we properly prepare the sample for analysis,” he explains. To understand this better, Baudelet and his team built a machine to recreate skid marks on road materials. Together with a colleague from civil engineering and his students, they will soon be able to test the effects of oil, pollutants and road materials on the chemical signature of a tire.

Vincent Motto-Ros, professor at the Light Matter Institute in France who has Libs expertise, describes the study as “very original and innovative”. He says it demonstrates the ability of this technique to identify and classify tire rubber. “This is one of those applications that really brings technology to the fore and shows that it has great potential for the various analytical needs of the forensics community,” says Motto-Ros Chemistry world.

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