Chemometric analysis on ATR-FT-IR spectra of spray paint samples for forensic purposes

Forensic investigation of cases involving spray paints requires comparison of paint such as those recovered from graffiti with the references sample seized from the suspect's possession site. This study describes the use of Attenuated Total Reflectance Fourier Transform Infrared (ATR-FT-IR) spectroscopy for nondestructive analysis of 20 red spray paints of different manufactures, which could be encountered at a crime scene, particularly in case of vandalism. Spray paints could be encountered on a number of different substrates, and therefore, the samples were placed on simulated substrates and were analyzed in order to check the effect of substrates on sample analysis. Results showed that all samples could be successfully differentiated using ATR-FT-IR spectroscopy assisted with principal component analysis. Analysis of spray paints on a paper and fabric substrate resulted in poor spectra and hampered the comparative examination between the spray paint deposited on fabric/paper surface and the control sample. In case of sample deposited on cemented wall, the substrate got scraped out along with the sample and caused interferences in the spectrum. On the other hand, for sample deposited on the floor, gloves, metal, plastic, leather shoes, tile, wood, and hair substrates, all the significant peaks were observed to be present and hence it was concluded that comparative studies can be done between neat spray paint and spray paint deposited on the aforesaid substrates. Additionally, blind validation test was conducted and it resulted in linking the blind samples with the source or origin with 100% accuracy.