Forensic Scientist, Washington State Crime Laboratory, Washington State Patrol, Seattle, WA
(Received 6 June 1995; accepted 6 October 1995)
A number of strong unidentified absorptions were observed in infrared spectra of some U.S. automobile original topcoats (1974–1989) from the Reference Collection of Automotive Paints. Most of these absorptions appeared to arise from specific color-imparting pigments, but with the exception of ferric oxide (Fe2O3), talc, and diatomaceous silica, they could not be attributed to any pigments previously identified in automobile paint using infrared spectroscopy. All of those previously identified, however, were pigments used in undercoats. This paper describes the infrared identification of several inorganic pigments used in automobile topcoats. Lead chromate pigments, which were found in many Reference Collection yellow, orange, and red nonmetallic topcoats, are no longer used in U.S. original finishes. Their presence can thus serve as both a means to differentiate between topcoats and provide some indication of when certain topcoats may have been produced. The far-infrared region below 700 cm−1, which cannot be observed when using a narrowband mercury cadmium telluride (MCT) detector, was found to be important for the analysis of topcoats containing certain pigments.
Paper ID: JFS13925J