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Unravelling tool paint using FTIR spectroscopy: Chemical identification, discrimination study and creation of a
Florian Vanneste1, Kyra Lunstroot1
1National Institute of Criminalistics and Criminology (NICC/INCC), Brussels, Belgium.
Abstract:
Tool paint, although frequently encountered at crime scenes, has not received as much attention as other types of paint in the forensic literature. The current study fills this gap with a unique database of 400 paint samples from 388 tools, developed over 15 years. After examination, the paint on each tool was sampled and analysed using Fourier Transform Infrared (FTIR) spectroscopy in transmission mode. Based on these results, the chemical composition of each paint was determined. Additionally, these spectra were compared followed by a calculation of the discriminating power (DP) and an evaluation of FTIR's effectiveness in distinguishing between different tool paint samples. Red, black, blue and orange paints dominate the tool collection with red being the most prevalent colour in the database. FTIR analysis revealed 22 different binder systems with a terephthalic or orthophthalic alkyd being present in 95% of the samples. Inorganic components such as barium sulphate and calcium carbonate were found in two-thirds of the paint samples, with their presence being strongly influenced by the chosen binder system. The overall DP of FTIR for tool paint is 0.982, confirming its strong discriminating potential. Additionally, the DP is calculated for different subsets of the database and compared to literature values. A practical method to calculate a likelihood ratio (LR) for source level evaluation, based exclusively on the chemical composition of tool paint samples, is presented. Using this approach and the study's results, LR values ranging from 5 to 400 were obtained. This study provides valuable population data for tool paints in Belgium and highlights the need for studies in different geographical areas. The developed database and the discussed findings can serve as a foundation for future research and will support forensic practitioners to interpret their findings in a more consistent manner using Bayesian reasoning.
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