Application of Raman spectroscopy for the differentiation of lipstick traces

This study demonstrates that Raman spectroscopy is a valuable tool for discriminating between lipstick samples under a range of forensically relevant situations. Trace amounts of lipstick smears deposited on textile fibres, cigarette butts and paper tissues were analysed. Differentiation of lipstick smears could be achieved with little or no interference from the underlying medium. Lipstick smears on glass slides, cigarette butts and tissues could also be analysed and identified in situ through evidence bags. Using a range of excitation frequencies (473, 633 and 784 nm) was effective in overcoming problems with fluorescent lipstick samples. The majority of the spectra of deposited lipstick samples remained unchanged over a period of up to two years. In some of the aged lipstick spectra, the (C=C) band at 1655 cm(-1) and the (=CH) band at 3011 cm(-1) were found to decrease in intensity and disappear over time. The use of chemometrics for the characterisation of large numbers of lipstick spectra was explored. Thirty spectra each from ten different lipsticks were analysed by Principal Components Analysis (PCA) and classified using the k-Nearest Neighbours (kNN) classifier. Up to 98.7% correct classification was achieved. Spectra from trace amounts of lipstick smears deposited on fibres were also analysed and classified using the same technique. 100% correct classification of these samples was achieved.