Forensic characterization of transparent traces: A spectroscopic and chemometric approach to discriminate clear lip glosses using ATR-FTIR spectroscopy.

Clear or unpigmented lip gloss, in particular, has become quite popular among people of all genders. Their unpigmented characteristics make them less noticeable at the crime scene; thus, they are less likely to be intentionally removed by the perpetrators. Therefore, in the present study, 20 brands of clear lip gloss were analyzed using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. Five out of 20 brands exhibited distinct spectral patterns and could be differentiated visually; however, the remaining 16 brands were grouped into five categories. Trend analysis was performed using principal component analysis (PCA), and samples within each group were found to be either clustered or located close to each other on the PCA plot. Further, for the discrimination of lip gloss samples, the support vector machine (SVM) method was employed. 97.5% of training as well as cross-validation accuracy was achieved. To ensure the reliability of the model, external validation and a blind test were also performed, which yielded 90% and 100% prediction accuracy, respectively. Moreover, substrate study on various porous and non-porous substrates in the time interval of 1 day, 7 days, and 15 days was also conducted. Non-porous substrates (ceramic, steel, plastic) did not affect the classification of samples and allowed for accurate sample classification (100%) for up to 15 days. However, samples on porous substrates were poorly classified. Therefore, ATR-FTIR spectroscopy and chemometric classification can potentially be used in real-world cases where questioned samples could be linked to their source and offer investigative leads.