Rapid non-destructive techniques to identify the traces of Kajal using chemometrics; A comparison of ATR-FTIR and Raman spectroscopy

Analytical chemistry finds application in forensic science when traces of cosmetic products are left on various substrates at crime scenes, and it is important to link them to the source. A rapid and more importantly, nondestructive approach is needed, that can be used to associate these trace samples to the source. We demonstrate for the first time, the application of two novel vibrational spectroscopy techniques namely; Attenuated Total Reflectance -Fourier Transform Infrared (ATR-FTIR) and Raman spectroscopy, combined with chemometric modeling using Partial Least Squares Discriminant Analysis (PLS-DA) for the discrimination of 45 kajal/ kohl samples. The trained PLS-DA model could classify all the samples with high degree of accuracy using ATRFTIR data, but the performance on the Raman data was found to be perfect. Importantly, when the spectral data of all these traces on different substrates (wet tissue, wall, plastic, nylon, dry tissue, cotton, and polyester) commonly found in crime scenes, were analysed with the trained PLS-DA model, 100% accuracy in predictions of source was obtained in case of the Raman data. However, this figure was slightly lesser for the ATR-FTIR data. Overall, this is a proof of concept study demonstrating that Raman and ATR-FTIR spectroscopy combined with chemometric model (PLS-DA) has great potential for the discrimination of the source of kajal/kohl samples. When the traces of Kajal/kohl are recovered as an evidence from the crime scene, the identification of these traces could provide a significant association between the trace and the source and therefore will be helpful as corroborative evidence in forensic context.