Forensic investigation of red lipstick residue using ATR-FTIR spectroscopy and chemometrics-A comprehensive wearing and storage effect study.

Lipstick traces can be recovered from the crime scene on various substrates and linked to the lipstick worn by the suspect or victim. These samples are usually collected using the swabbing method from the lips. Sometimes, the same samples are stored in forensic laboratories for years due to a backlog of cases, which affects the samples' originality. Therefore, in this study, a comprehensive study of the wearing and storage effects was conducted to assess the period up to which lipstick traces can be linked to their source by analyzing 20 brands of red lipsticks using ATR-FTIR spectroscopy and chemometric tools. The study was divided into two phases. In Phase I, samples were collected using the swabbing method from the lips of volunteers after 5 min, 1, 3, and 5 h of application, followed by ATR-FTIR spectroscopic analysis. Similarly, in Phase II, the same swabs were kept for 1 year to investigate their aging process, followed by ATR-FTIR spectroscopic analysis. The results showed that all the pristine samples can be discriminated with 100% LDA accuracy. The built training model was also validated, which showed 95% accuracy. Chemometric prediction of the worn samples yielded 80% accuracy for Phase I. However, a reduced accuracy (45%) was observed for Phase II due to the drying effect and loss of sample over time. It can be concluded that these types of samples must not be stored for a longer duration in real-case scenarios, so that the authenticity of the evidence is retained.