Attribution of tire marks via their elemental profile

Tire marks left by tires during road incidents, including from braking, can be found on a routine basis, especially in hit-and-run and multiple car incidents, due to the friction of rubber with the road surface. Even with the Antilock Braking System, tire rubber is always transferred to the road, even as trace evidence in small amount. Its chemical composition could be used for identification or classification. For many years, the focus of tire rubber chemical analysis has been on the molecular signature of the rubber. Nonetheless, the road material (asphalt in particular) can be a critical molecular interference, in addition to the heat induced by braking degrading the polymers. But tire rubber also contains different inorganic elements besides C, H, O, N, S, introduced as oxides and traces in the manufacturing process, present either voluntarily for adjusting their performance, or involuntarily as impurities along the fabrication. This inorganic elemental profile is the support for the attribution of a skid mark to a tire, in the same way two pieces of glass can be matched by their elemental profile.

The goals of this proposal are to develop a protocol to obtain the quantitative elemental profile of rubber (from skid mark and tire), to initiate a database of tire rubbers and to identify the source of a mark via its elemental composition. This project will be led by the National Center for Forensic Science at UCF, with field experiments with the Florida Department of Highway Safety and Motor Vehicles.

 These goals will be fulfilled in three parts, starting with (i) a protocol to obtain a quantitative elemental profile of tire samples and skid marks (both lab-made and field-made) using ICP-MS, leading to (ii) the initiation of a preliminary database of 80 tires, based on their elemental composition, with their skid mark on concrete and asphalt. Finally, (iii) a statistical protocol to link a skid mark to the tire that left it on several road material based on the likelihood of their elemental composition.

This project will provide the forensic community with its first quantitative elemental analysis of tire rubber. A protocol will be developed that can be used for skid mark analysis and its attribution to a tire. The outcomes of this study will be a database of elemental profiles of tires, the protocol to obtain this profile, and its use for comparison to a questioned tire or a database entry. CA/NCF