Forensic Applications of Infrared Chemical Imaging: Multi-Layered Paint Chips

Abstract

This paper examines the potential of infrared chemical (hyperspectral) imaging as a technique for the forensic analysis of automotive paint chips in particular, and multicomponent (e.g., layered) samples in general. Improved sample preparation procedures for the infrared analysis of paint chips are detailed, with the recommendation that where mounting resins are chemically incompatible with the sample, it is better to mount and section the sample in a soft wax from which the sections can be removed and pressed into a KBr disk for transmission analysis. Infrared chemical images of multilayered paint chips have been successfully obtained, with the chief advantage over conventional infrared analysis being that thousands of infrared spectra are collected in a few minutes across the whole sample, at a spatial resolution of around 5 µm. As with conventional infrared spectroscopy, chemical species can be identified from their spectra, but the wealth of information available can be also extracted in a number of different ways that make multicomponent spectral (and hence chemical) comparisons between two samples easy to visualize and understand. In one approach, the infrared chemical images of two paint chips being compared side-by-side can be viewed as a “movie,” in which each frame is an intensity map of the two samples at a given wavenumber (frequency) value. In another approach, the spectra (pixels) in the image files are classified into chemically similar groups, resulting in a “cluster” image that makes it possible to simultaneously compare all of the layers in two paint chips. These methods are applicable to other multicomponent samples, and also to other chemical imaging techniques

Hyperspectral imaging for non-contact analysis of forensic traces

“…Because both spectral and spatial data were gathered, differences in paint layers could easily be highlighted visually, as an alternative to a spectral comparison. This was also shown by Flynn et al who analyzed more multi-layered paint specimens using IR HSI [50]. They presented several ways to display hyperspectral data, which make chemical differences and similarities between heterogeneous specimens easy to visualize and understand for the layperson (such as a juror).…”

Forensic Analysis of Bicomponent Fibers Using Infrared Chemical Imaging

“…It should be noted that only integrated peak intensities were used here to attempt imaging of fibers. Multivariate statistical techniques such as hierarchical cluster analysis (HCA) can be used to highlight small but consistent differences between spectra (10). Alternatively, composite images based on spectral intensities at more than one wavelength can be constructed.…”

Forensic Analysis of Fibres by Vibrational Spectroscopy

“…Each detector element measures a spectrum which relates to a small part of the specimen, and by analysing the spectra using band intensities, areas or ratios, or more sophisticated techniques such as principal component analysis (PCA), false colour chemical images of the specimen can be constructed. Mid-IR chemical imaging has been applied to the forensic characterisation of thin sections of multi-layer paint chips [27] and has the advantage that many spectra are collected in a short time (around 5 min) at a lateral resolution of about 5 mm. Each layer could be identified from individual spectra, and comparison between samples was facilitated by the construction of chemical images so that all layers in any two samples could be simultaneously compared.…”