Chemometric Analysis on ATR-FT-IR Spectra of Spray Paint Samples for Forensic Purposes

Spray paint is often used by vandals for creating graffiti, as well as for criminals to leave signs, messages, and blots to conceal the left traces at the scene of their efforts. Rajinder Singh and his colleagues in the Department of Forensic Science at Punjabi University (Punjab, India) have used attenuated total reflectance Fourier transform infrared (ATR-FT-IR) spectroscopy for nondestructive analysis of 20 red spray paints of different manufacturers, which could possibly be encountered at a crime scene, particularly in case of vandalism. Singh spoke to Spectroscopy about the findings, and the paper that resulted from their efforts (1).

What inspired you to research the ATR-FT-IR method for the purpose of your analysis?
These days, spray paints are often used in criminal offenses. The most obvious cases are vandalism and graffiti where spray paints could be utilized to destroy or blot different types of objects. Additionally, in many criminal attempts, criminals leave any kind of messages or signs using spray paint at the scene of the crime. However, these marks can sometimes be the only available valued evidence deserving a precise analytical method for the identification and for discrimination purposes. In forensic casework conditions, the samples are often encountered in only trace levels; thus, the trace amount of available sample make their analysis very difficult. The traditional methods are destructive to the sample which makes the preservation and further analysis of sample not possible. The technique used in this present work is completely non-destructive, rapid, reliable, and eco-friendly in nature. Therefore, following ATR-FT-IR analysis, samples can be preserved and further analyzed using other analytical methods, if needed. Henceforth, the non-destructive nature of this method inspired us to move ahead with the analysis of various trace evidence such as spray paints. ATR FT-IR spectroscopy, in combination with chemometric tools, yielded valuable results leading to a high degree of discrimination between samples.

What benefits did you anticipate by using ATR-FT-IR spectroscopy over other analytical techniques for your intended analysis?
Generally, forensic examination of paint samples includes a comparison of their chemical content. In this aspect, various techniques such as optical microscopic analysis, scanning electron microscopy with energy dispersive X-ray analysis (SEM–EDX), micro-spectrometry in the ultraviolet (UV) and visible (vis) spectral ranges, pyrolysis gas chromatography-mass spectrometry (GC–MS), micro-X-ray fluorescence (μ-XRF), and so forth, are used for the determination of its chemical composition. These methods are routinely employed in the analysis of paint traces. The limitation that exists with these methods is that they are destructive in nature, time-consuming, and sometimes require various chemical processes for extraction and sample preparation. In forensic casework conditions, samples are often encountered that have been applied to different substrates. Unfortunately, it is often not possible to separate the paint traces entirely from the substrate, and this is because of the obvious fact that the paint sticks very firmly to the base material. Therefore, it usually is necessary to try and examine the paint traces directly so that the substrate has a diminished influence on the obtained results. Therefore, in situ non-destructive analysis is highly preferred and desirable for these examinations. ATR FT-IR spectroscopy meets all the requirements needed for the analysis of paint traces. Samples can be analyzed non-destructively in a rapid manner without any sample preparation, and directly measured from the different substrates.

Briefly discuss your findings and their implications. Were you surprised by the results?
ATR FT-IR spectroscopy is rapid and non-destructive; is able of analyzing small quantities of samples, and provides rapid analysis time. The results obtained showed good reproducibility and repeatability. The chemometric tools assist significantly in the interpretation of results in an objective manner. Principal component analysis (PCA) successfully conveyed 100% of discriminating power for the differentiation of different brands of red spray paints, which is highly significant. To check the reliability of the PCA model, the blind validation test was also carried out and 100% accurate classification of unknown samples was observed in this study. Furthermore, the results of the substrate study suggested that it is possible to analyze the type of spray paints on various substrates; however, the accuracy of discrimination depends mainly on the paint coat, material type, and thickness of the substrates. We observed that paint on floor, gloves, metal, plastic, leather shoes, tile, wood, and hair substrates all demonstrated significant chemical peaks; and all were present allowing comparative studies between neat spray paint and spray paint deposited on these various substrates. We, the authors, were then able to obtain the results as we expected in our hypothesis.

What are the biggest challenges that you have encountered in developing this method? What options or alternative developments are available to overcome these challenges or to improve your approach?
The biggest challenge faced in the current work was to analyze the spray paints directly on a variety of substrates, such as paper, fabric, and cemented wall using the ATR FT-IR spectroscopy technique. Sampling can be rather complicated, and thus, could severely hamper the comparative study. We are trying our best to improve our methodologies for the extraction of the paint sample from porous surface substrates.

What sort of feedback did you receive from this paper and the study results?
This is a good approachable methodology for the rapid, eco-friendly, and non-destructive analysis of spray paints on various fabric substrates.

Are there any additional analytical approaches or methods where your findings might be beneficial?
This technique can be used as a complementary tool for other available analytical methods. The findings of this approach will definitely provide a complement to Raman spectroscopy.

Are you aware of the various research work that has been done for forensic analysis of automotive paints using FT-IR? 
There is an article recently published in Talanta (2). There have also been several papers written describing the forensic analysis of automotive paints using ATR-FT-IR.

What are your next steps regarding this or related research?
In the future, we foresee undergoing further investigations on the applicability of ATR FTIR spectroscopy for the identification and discrimination of automotive paints and architectural paints of different brands and manufacturers under simulated forensic casework conditions.


(1) S. Sharma, R. Chophi, C. Kaur, and R. Singh, J. Forensic Sci. 66(6), 2190-2200 (2021). doi: 10.1111/1556-4029.14806.

(2) J.M. Duarte, N.G.S.S. Sales, J.W.B. Braga, C. Bridge, M. Maric, M.H. Sousa, and J.A. Gomes, Talanta 240, 123154 (2022). doi: 10.1016/j.talanta.2021.123154

Rajinder Singh Chandel, earned his Master's degree in Forensic Science with a Gold medal from the Department of Forensic Science, Punjabi University, Patiala, India in 2001. He carried out his PhD research work on “Hair Characterization of Schedule-1 Felids of Wildlife (Protection) Act-1972” in collaboration with the Wildlife Institute of India (WII), Dehradun, which led him to attain his PhD in 2008 from Punjabi University. He has been actively involved in the research related to the forensic characterization of body fluids, trace cosmetics evidence, inks, paints, diatoms, grasses, poisonous plants, hair of various species protected under Wildlife (Protection) Act-1972, and other evidentiary materials of biological origin using morphological, chromatographic, spectroscopic, and DNA-based techniques.

Chandel has been involved in regular teaching and research in the area of forensic biology and forensic chemistry since 2003 at Punjabi University. He has published 68 research papers in various national and international journals and has contributed reference mtDNA sequences for the identification of highly endangered and protected cats of India in NCBI (GenBank). He has been involved in many training courses on wildlife forensics and illegal wildlife trade at LNJN National Institute of Criminology and Forensic Science, Ministry of Home Affairs, Govt. of India, New Delhi, and has also been a resource for many conferences, symposia, and workshops of national and international repute.