Application of infrared microscopy and alternating least squares to the forensic analysis of automotive paint chips (original) (raw)

2020, Journal of Chemometrics

To collect infrared (IR) absorbance spectra from an automotive paint chip with an IR imaging microscope, it is a common practice to cast the paint chip in epoxy and then cross section it using a microtome to reveal the individual layers of paint. Ideally, the epoxy should present little or no spectral interference. However, the epoxy can infiltrate individual layers of the paint chip as it cures contaminating the IR spectra of the layers and impairing the accuracy of a search of each of these layers against an automotive paint library. In this study, we have demonstrated that automotive paint chips can be successfully cross sectioned without the use of embedding media. Sample preparation is easier, and more importantly, interfering peaks in the spectra due to the epoxy are eliminated. To demonstrate the advantages of this approach for sample preparation, IR image maps of four automotive paint chips that were not cast in epoxy prior to cross sectioning were collected. After each IR image was unfolded using an oblique transit to traverse the image, the spectra of the individual paint layers comprising the line map were reconstructed by alternating least squares. Comparing each recovered IR spectrum against a spectral library, we show that high quality spectral matches were obtained for spectra from the same line/model of the vehicle from which the paint sample originated. When the same paint chips were cast in epoxy prior to cross sectioning, high quality spectral matches could not always be obtained. K E Y W O R D S chemometrics, cross sectioning, forensic automotive paint analysis, microtome, multivariate curve resolution, spectral library matching, transmission infrared imaging microscopy 1 | INTRODUCTION Automotive paint is an important category of trace evidence as it can provide crucial links in the early phase of an investigation or evidence of association in a criminal proceeding. Automotive paint in the form of a chip is often the only evidence recovered at the crime-scene of a vehicle related fatality such as a hit-and-run where injury or death to a pedestrian has occurred. Studies 1,2 conducted by the Royal Canadian Mounted Police (RCMP) almost 45 years ago showed that automotive vehicles could be discriminated by directly comparing the color, layer sequence, and chemical composition of each individual layer in an original equipment manufacturer (OEM) paint system. To make these