Raman Uncovered – What Tiny Traces Are Hiding

Article 1: Forensic analysis of white automotive paint of same manufacturer with Raman spectroscopy and chemometrics _^[2]

L. Lei & G. Massonnet
School of Criminal Justice (ESC), University of Lausanne, Lausanne, Switzerland

Differentiating automotive paints of the same color has long posed a challenge in forensic and quality control contexts — particularly when the paints originate from the same manufacturer and share identical color codes. Raman spectroscopy, a non-destructive and highly sensitive technique, is commonly employed to distinguish between paints by analyzing pigments and extenders in the topcoat. However, its application to other paint layers remains less explored.

In this study, 54 white automotive paint samples from a single manufacturer (Volkswagen) were analyzed using Raman spectroscopy. Each individual layer—clearcoat, basecoat, and primer—was measured to assess the technique’s discrimination power. Statistical methods were applied to classify the samples and evaluate correlations with factors such as vehicle model, production year, topcoat color code, and assembly plant.

Raman spectroscopy demonstrated high discriminatory capability, successfully differentiating 92.8% of the 1,431 sample pairs. Only 103 pairs remained undistinguished. Despite the limited range of pigments in white paints, Raman spectroscopy proved effective in detecting variations in binders and extenders across different layers, with clearcoats showing the highest discrimination power.

To ensure accurate identification of Raman bands, IR spectroscopy was used to cross-reference and confirm spectral assignments. The analysis revealed that unique chemical characteristics are present even among paints from the same manufacturer, model, and year - highlighting the influence of the assembly plant as a key variable in paint formulation.

Interestingly, while variations in model, production year, and assembly plant contributed to differentiation, no correlation was found between the topcoat color code and the chemical composition of the basecoat, despite the presence of three different white color codes. This suggests the need for further investigation and the integration of complementary analytical techniques to gain a deeper understanding of multilayer automotive paint compositions.
 

Article 2: Raman Microspectroscopic Mapping: A Tool for Identification of Fused Materials in Fire Debris ^_[3]

Post-Fire Material Identification Using Raman Spectroscopy

T.J. Kerr, L. Myers &  K.L. Duncan
Department of Physics, University of the West Indies, Kingston, Jamaica

The examination of fire debris can provide valuable insights into the types of materials present at the time of the fire, aiding both fire scene reconstruction and understanding of compartment fire dynamics. This study demonstrates the effectiveness of Raman spectroscopy in identifying materials after a fire, even in complex cases where different substances fuse into visually indistinguishable masses due to extreme heat.

By combining a validated Raman spectral library with Raman mapping, three real-world fire case studies were investigated. These involved fused masses formed during fire dropdown, composed of various common polymers. Raman mapping was conducted over a 10 mm × 10 mm area on each sample. The technique achieved material identification matches ranging from 85% to as low as 40%, depending on the extent of material degradation.

The results show that Raman spectroscopy can successfully resolve complex fire debris into individual material components, providing both identification and spatial distribution. Importantly, the method is not limited to identifying single, isolated materials, but can also distinguish multiple materials fused together during high-temperature exposure.

This capability ensures that forensic investigators are not disadvantaged when analyzing heavily altered or fused debris. However, the study also highlights the need for expanding and refining Raman spectral libraries to improve identification accuracy—especially for materials that are highly decomposed or only partially degraded.

Overall, the findings underscore the strong potential of Raman spectroscopy as a powerful, non-destructive tool for post-fire material analysis, capable of handling the challenges posed by complex and degraded fire debris.

The Raman spectra were collected using the HORIBA Aramis dispersive Raman spectrometer, LabRAM IR2, equipped with a 785-nm laser. The confocal microscope provides a very good reduction in fluorescence and out-of-focus beams, making it optimal for the testing.

Article 3: New Raman spectroscopic methods’ application in forensic science _^[4]

E-R. Mojica & Z. Dai
Forensic Science Program, Department of Chemistry and Physical Sciences, Pace University, New York, NY 10038

This article reviews recent developments and applications of advanced Raman spectroscopic techniques in forensic science. Surface-enhanced Raman spectroscopy (SERS) has been utilized for the highly sensitive detection of trace amounts of controlled substances. Shifted-excitation Raman difference spectroscopy (SERDS) enables the acquisition of fluorescence-free Raman spectra, which is especially useful for certain types of forensic evidence. Spatially offset Raman spectroscopy (SORS) has been applied to examine materials such as drugs and explosives directly through packaging or container surfaces. These techniques are increasingly being combined to tackle complex forensic scenarios, providing powerful, non-invasive tools for evidence analysis. Their potential to support justice and security by enhancing forensic investigations is significant.

Beyond these established methods, several new Raman-based approaches show great promise for future forensic applications. For instance, gel-based techniques have been introduced for in situ SERS analysis. Optimized gel substrates enhance Raman signals while minimizing sample damage, making them ideal for on-site, non-destructive investigations where preserving the integrity of the evidence is critical.

Micro-SORS, coupling SORS and microscopy, has been successfully employed in art conservation to analyze thin, turbid paint layers. This technique can be adapted for forensic analysis, such as identifying the composition and thickness of vehicular paint chips at crime scenes.

Time-gated Raman spectroscopy (or time-resolved Raman spectroscopy, TRRS) uses short laser pulses and time-gated detection to suppress fluorescence. Additionally, TRRS can selectively probe different layers within stratified samples by adjusting the detector's gating delay. When combined with SORS (TR-SORS), it enables the non-invasive analysis of concealed substances, such as explosives hidden in opaque plastic.

Another powerful combination involves time-gated Raman and defocused micro-SORS, which allows simultaneous analysis and imaging of both fluorescent and non-fluorescent materials beneath turbid layers. This approach employs spectral multiplexing to suppress interfering surface signals and holds strong potential for forensic applications.

More recently, Frequency Offset Raman Spectroscopy (FORS) has emerged as a method for probing diffusely scattering samples at various depths. By leveraging the dependence of optical properties—like absorption and scattering—on excitation wavelength, FORS selectively targets different layers within a sample. Unlike SERDS, which uses excitation wavelength differences of less than 2 nm, FORS employs much larger wavelength offsets (tens of nanometers), achieving high spatial resolution and signal-to-noise ratios. Hybrid approaches combining FORS and SORS promise even greater analytical performance and are expected to become valuable tools in forensic science.

To conclude, Raman spectroscopy continues to prove itself as a vital technique in forensic analysis. The advent of advanced methods such as SERS, SERDS, SORS, TRRS, FORS, and their hybrid implementations has significantly enhanced the sensitivity, specificity, and practicality of Raman-based forensic investigations. As these techniques are increasingly integrated with chemometric analysis, their adoption is expected to grow, supporting more accurate and timely forensic assessments and strengthening the administration of justice.