Application of Infrared Imaging and Chemometrics to Facilitate the Forensic Examination of Automotive Paints

As submitted by the proposer: In the forensic examination of automotive paint, each layer of paint is typically analyzed by FTIR. The paint sample examined often consists of multiple and unique layers of paint. The more unique the paint layers are, the more information is contained in the sample and the stronger are the forensic conclusions that can be drawn. However, for each additional layer of paint in a sample, more time must be spent analyzing the sample because, currently, each layer must be analyzed individually. Some laboratories will embed the entire paint fragment, cross-section it, and then analyze each layer. Other laboratories are more likely to hand-section each layer and present each separated layer to the spectrometer for analysis. With either technique, sampling too close to the boundary between adjacent layers may produce an IR spectrum that is a mixture of two layers. Not having a "pure" spectrum of each layer will prevent a meaningful comparison between each paint layer or, in the situation of searching an automotive paint database, will prevent the scientist from developing an accurate hit list of potential suspects. One way to minimize the time necessary for data collection is to collect concatenated IR data from all paint layers in a single analysis. This would be achieved by scanning across the cross-sectioned layers of a paint sample using an FTIR imaging microscope equipped with a linear array detector, which can perform a complete scan of all layers in less than one minute. Once the data has been collected, it could then undergo decatenation using chemometrics to obtain a "pure" IR spectrum of each paint layer. This approach, not only eliminates the need to analyze each layer separately, but would also ensure that the final spectrum for each layer is "pure" and not a mixture. Minimizing the probability of collecting a mixed spectrum would result in a time savings as well as objectively ensuring that only "pure" spectra from each layer have been collected and are used in subsequent searches. IR spectra of automotive paints are well-suited to the exploration and application of concatenation-decatenation methods because a large database of pure IR spectra from automotive paint systems already exists. By integrating the proposed imaging experiment with a prototype pattern recognition IR library searching system developed by the PI in a previously funded NIJ project, the forensic examination of automotive paint will be facilitated in terms of both speed and accuracy. ca/ncf