Raman analysis of multilayer automotive paints in forensic science: measurement variability and depth profile

Abstract

The aim of this work is to study the influence of several analytical parameters on the variability of Raman spectra of paint samples. In the present study, microtome thin section and direct (no preparation) analysis are considered as sample preparation. In order to evaluate their influence on the measures, an experimental design such as ‘fractional full factorial’ with seven factors (including the sampling process) is applied, for a total of 32 experiments representing 160 measures. Once the influence of sample preparation highlighted, a depth profile of a paint sample is carried out by changing the focusing plane in order to measure the colored layer under a clearcoat. This is undertaken in order to avoid sample preparation such a microtome sectioning. Finally, chemometric treatments such as principal component analysis are applied to the resulting spectra. The findings of this study indicate the importance of sample preparation, or more specifically, the surface roughness, on the variability of the measurements on a same sample. Moreover, the depth profile experiment highlights the influence of the refractive index of the upper layer (clearcoat) when measuring through a transparent layer. Copyright © 2014 John Wiley & Sons, Ltd.

Applications of Raman spectroscopy in art and archaeology

“…The analytical parameters influencing the variability of Raman spectra of car paint samples were studied by Lambert et al [46] Through 32 experiments and 160 measurements, including chemometrics, it was found that the only factor that significantly influences the Raman spectra variability is the sample preparation. No sample preparation gives less variability than the analysis on cross sections, even in the case of clearcoat presence.…”

A model for interpreting depth profiles of confocal Raman measurements in reflective and transmitting materials

“…Grauw et al adapted this formalism and considered a Gaussian beam, instead of a spherical beam, for the image‐formation in the back‐focal plane, and accordingly obtained a Lorenzian profile of Raman intensity from a thin polystyrene film. For thick transparent samples, the effect of refractive index mismatch has been studied in several articles . Everall formulated the axial spreading of focus inside the sample between paraxial and oblique rays using a geometrical optic approach and provided an analysis of the Gaussian intensity distribution within the axially broadened depth‐of‐focus as a function of the starting position of the ray on the objective, that is, the pupil parameter.…”

“…As a result, the otherwise near-perfect converging beam is broadened in the axial direction inside the sample. This results in dramatic deterioration of the depth-of-focus and makes it difficult to accurately reconstruct the true depth profile and to extract effective parameters, especially axially inhomogeneous parameters such as thickness [7][8][9][10][11][12] and material properties. [13][14][15] This particular problem is prevalent in high numerical aperture (NA) dry objective systems.…”

“…For thick transparent samples, the effect of refractive index mismatch has been studied in several articles. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] Everall [8,20] formulated the axial spreading of focus inside the sample between paraxial and oblique rays using a geometrical optic approach and provided an analysis of the Gaussian intensity distribution within the axially broadened depth-of-focus as a function of the starting position of the ray on the objective, that is, the pupil parameter. Baldwin and Batchelder [21] analysed the mean focal position and depth-of-focus based on the axial intensity distribution within the broadened depth-of-focus.…”

Chemical discrimination of multilayered paint cross sections for potential forensic applications using time‐of‐flight secondary ion mass spectrometry

“…Forensic evidence involving automobiles frequently comes in the form of small paint chips and smears, which are valuable forms of trace contact evidence, obtained from vehicles involved in hit‐and‐run accidents, road accidents, or from victims of vehicular homicide. The chemical analysis of such trace evidence is used to link suspected vehicles with traces collected at an accident scene and for the potential discrimination of vehicles by their make, model, and year using reference samples. Examinations of the layer structure, color, and chemical composition of the paint are some of the features that are routinely investigated.…”