Chemometrics based ATR-FTIR spectroscopy method for rapid and non-destructive discrimination between eyeliner and mascara traces

Abstract Eye cosmetics are used by most females and thus, can be an important corroborative forensic evidence in cases of crimes against women. These cosmetics can also sometimes be used to write suicide notes, threatening letters, and anonymous letters on a variety of substrates like skin, fabrics, paper (tissue paper or any writing paper) etc. Their traces can establish link of the perpetrator with the victim and the scene, and thus be an important corroborative evidence in various crimes. This paper reports the analysis of 102 cosmetic samples (62 different eyeliner samples from 37 brands and 40 mascara samples from 24 brands) using Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) Spectroscopy. Chemometric analysis was performed utilizing the Principal Component Analysis (PCA) and Partial Least Squares Discriminant Analysis (PLS-DA). These methods have a higher discrimination ability than that of the visual spectral comparisons. In addition, the model is validated using 15 unknown eyeliner and mascara samples. The validation results show 95% accuracy making the model significant. Further, the sensitivity and specificity of the model are 90% and 100% respectively which makes the model apt for forensic casework. We used differences in peak patterns, their intensities, and the presence or absence of particular frequency in the fingerprint region for the discrimination of eyeliners and mascaras. The present work demonstrates a quick, non-destructive, robust and cost-effective method for the discrimination of eye cosmetics in cases of crime against women, as well as other forensic case work.