Automotive paint smears are often recovered from a crime scene where damage to a vehicle and/or injury or death to a pedestrian has occurred. Extracting investigative lead information from a paint smear is currently problematic as there are no reference materials available in a paint database to generate hit-lists of potential suspect vehicles.  Automotive paint smears are difficult to create in a laboratory and have also proven challenging to analyze because of the mixing of the various automotive paint layers. A procedure based on a Charpy impact tester has been developed to generate smears due to paint transfer from an automotive substrate to the surface of interest under conditions simulating those of a real collision. The paint smears can be separated from the inert metal substrate using a sharp knife and transferred to an infrared (IR) microscope for analysis.  As revealed by multivariate curve resolution (MCR) analysis of the spectral line maps of a paint smear obtained using an iN10MX IR microscope equipped with an ATR imaging accessory,  separation of the automotive paint layers may occur in some regions of the paint smear.  In contrast, the paint smear appears intact in all regions of the line map when using transmission IR microscopy.  The unmixing of the layers is evident only with ATR-IR microscopy because of its superior spatial resolution.  Because the smeared paint in all likelihood may undergo melting due to the heat generated by the impact, a separation phenomenon may occur during fluid flow due to differences in viscosities and/or affinities for the substrate.  This result indicates that paint smear standards can be developed for proficiency testing using the approach described here.  The Charpy impact tester can simulate a vehicle collision in terms of collision speed and direction as well as momentum/energy transfer.  The long term goal of this proposed paint transfer effort is the development of forensic standards for automotive paint transfer and paint smear analysis.

Infrared microscopy applied to the forensic analysis of automotive paint smears

255th National Meeting (2018)

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