Forensics teams working within police departments frequently need to analyze and identify samples for various reasons. For example, chips of paint can often be deposited at the scene of a crime and the identification of their origin can help rule suspects in or out of an investigation.
A case of a hit and run for instance, may leave tiny chips of paint from the car at the scene and identifying these paint pigments can help identify the make and model of a suspect’s vehicle, or even determine if a car in question was likely involved or not.
Paint pigments can come from all sources, not just cars, and for this reason, developing effective and reliable methods for the identification of paint pigments forms an essential part of the forensic detective’s toolbox. Below we discuss the various methods available for identifying paint pigments in forensic examinations.
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The transfer, identification, classification, association, and reconstruction of a piece of evidence, a paint pigment, for example, can fill in gaps that help detectives figure out the unknowns of a court case, it can also help to implicate suspects or even to clear them. Unsurprisingly, analytical chemistry techniques are heavily relied upon to resolve these questions surrounding a piece of evidence.
Raman spectroscopy is one of the most established analytical chemistry techniques in the forensic field, and it is particularly useful in the identification of paint pigments. The Raman spectroscopy technique is non-destructive, meaning that it does not damage the sample, which can be important for criminal cases where only a small pigment of paint may have been salvaged from the scene and it may be required for future testing. Also, with Raman spectroscopy only a small sample is required to generate results, meaning that if the paint pigment left at the scene if very small, it can still be used to identify its source.
Three main factors make up any given paint, and these are the binder, pigment, and solvent. Forensic identification is mostly focused on the identification of the pigment or binder. Information on pigment is most commonly attained through Raman spectroscopy.
An area of paint pigment identification in with Raman spectroscopy particularly excels is in the identification of certain organic pigments and dyes at low concentrations, because organic pigments have high visible absorption coefficients. As a result, their absorptions may be too weak to be observed in IR spectra, but they may result in strong Raman bands.
Fourier transform infrared (FTIR) spectrometry
Fourier-transform infrared spectrometry is used by forensic labs to determine the paint type by gaining information on the chemicals, pigments, and binders from the way the various components of paint absorb infrared light. Certain compounds reflect light in specific ways, allowing for their identification.
The infrared spectra of samples can be used to determine pigments and fillers such as titanium dioxide, kaolin, talc, barium sulfate and chalk, all which have visible absorption bands. Other inorganic components, such as oxides and chromates, can be identified by their weak absorption of infrared light. Finally, organic compounds within the paint can be determined by specific spectra composed of many sharp absorption bands.
Solvent tests are less elegant than Raman spectroscopy or FTIR, and they are also destructive, which is a major drawback to using them as it may mean that the only sample collected from the crime scene is destroyed by analyzing it.
During solvent tests, the paint sample is exposed to various chemicals to elicit certain reactions such as softening, swelling, curling, or changes in color that can indicate the presence of specific components, helping to identify the paint. Commonly, acrylic paint lacquers are soluble in both chloroform and acetone; and nitrocellulose lacquers are soluble in acetone and insoluble in chloroform. Finally, enamels are insoluble in both acetone and chloroform. These rules help to identify what type of paint the sample was.
Pyrolysis gas chromatography/mass spectrometry
Information about the paint binder is can be attained found using pyrolysis gas chromatography. In this process, the paint sample if broken into smaller fragments with heat, and then separated into its various components. The pyrograms produced in pyrolysis can detect minor components that are not visible by FTIR. However, the downside is that the technique is not suitable for all samples, such as those that are contaminated.
Finally, the technique of X-ray fluorescence (XRF) spectroscopy is also occasionally used to identify paint pigments. However, it is seldom used on its own but is usually used alongside another method, such as FTIR or Raman spectroscopy. Most often, XRF spectroscopy is used to confirm the presence of pigments identified in other analytical techniques, helping to enhance the reliability of a given result.
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Fikiet, M., Khandasammy, S., Mistek, E., Ahmed, Y., Halámková, L., Bueno, J. and Lednev, I. (2018). Forensics: evidence examination via Raman spectroscopy. Physical Sciences Reviews, 4(2). https://www.degruyter.com/view/j/psr.2019.4.issue-2/psr-2017-0049/psr-2017-0049.xml
Zięba-Palus, J. and Borusiewicz, R. (2006). Examination of multilayer paint coats by the use of infrared, Raman and XRF spectroscopy for forensic purposes. Journal of Molecular Structure, 792-793, pp.286-292. https://www.sciencedirect.com/science/article/abs/pii/S0022286006003565