On the spectroscopic investigation of lipstick stains: Forensic trace evidence

In forensic science, lipsticks are considered as crucial trace evidence because it helps in the linking of the criminal with the crime scene. In the present work, twenty-five lipstick samples are characterized and discriminated by using ATR-FTIR spectroscopy coupled with multivariate statistical methods. The utilized approach is non-destructive, fast, and provides reproducible results. It is observed from the FTIR spectra that lipstick contains various aliphatic and aromatic compounds e.g. Propyl ester of Hexanoic acid, Silicates, etc. Further, the discrimination power is calculated by using three approaches i.e. visual examination, cluster analysis (HCA and k-means) and factor analysis method. The multivariate method combined with t-statistics delivered a higher value of discriminating power i.e. 100% which is an improvement on the 99.00% discrimination power of visual comparison method. The developed method is validated by analyzing five duplicate samples and predicted them to their respective brands significantly. This study establishes a method which provides proof of concept discrimination of the lipstick samples. In the future, it can be quite possible to create an FTIR database of more lipstick samples for the identification of unknown/suspected lipstick samples.