Vibrational spectroscopic techniques have shown to be highly suitable for the identification and comparison of textile fibres and clothing fabrics. On the other hand, new chemical imaging modes based on these spectroscopic techniques are becoming useful in multiple fields. This is particularly important to, for instance, chemically visualize and screen different samples including forensic evidence (crime scene investigation), chemical and food products (quality control), biological tissues and living beings (medical imaging), among others. This study explores the forensic examination and selective chemical visualization of textile fibres and clothing fabrics using Raman imaging. Four experiments were performed, which were focused on the screening of (i) white different materials made of 100 % cotton (gauze, cotton wool, t-shirt, and swab), (ii) polyester and cotton fabrics evidence of the same colour, (iii) five different coloured cotton fabrics, and (iv) textile fibres of different materials (acrylic, cotton, nylon, polyester, and silk). Several methods of multivariate chemometric analysis including principal component analysis (PCA), multivariate analysis of variance (MANOVA), and multivariate curve resolution (MCR) were applied to enhance the limited visual comparison of the spectra accomplished with the unaided eye. The results evidenced the suitability of Raman imaging to statistically discriminate textile fibres and fabrics due to the chemical composition of both the clothing material and the dyestuff.
Textile fibres are usual evidence in crime scene investigations, which usually provide useful information to recreate the crime events since their presence may prove the contact between people and a crime scene [1–8]. Particularly, textile fibres are highly relevant in crimes such as kidnapping, rape and homicide, in which there is a contact between the victim and the aggressor that may facilitate their transference from one to each other and to the crime scene [1–8]. In fact, when the fibres found in the crime scene, and especially on the victim, are the same as those from the suspect, that evidence is usually a convincing proof about the physical contact between both.
The visual microscopic examination of textile fibres is recommended by forensic experts because it allows a rapid differentiation of the fibres based on their morphological characteristics. However, the discrimination of fibres based on their perceptible colour and other non-specific visual properties is not always possible. In those cases, spectroscopic techniques are also used [8–10]. In fact, the forensic identification and comparison of fibres usually imply their characterization through different spectroscopic techniques such as ultraviolet–visible (UV–Vis) [11–16], infrared (IR) [17–20] and Raman spectroscopy [21–26]. IR and Raman spectroscopy are particularly suitable for this purpose since they enable a highly specific characterization of samples through their spectral ‘‘fingerprint”, based on the chemical vibrations that molecules undergo during IR and Raman analysis [1,8,9]. In addition, these vibrational spectroscopic techniques can be coupled to microscopy, which make them highly useful in forensics.
4. Conclusions and future trends
In this work, Raman micro-spectroscopy showed enough selectivity to discriminate fabrics and fibres. In fact, it was verified how the Raman spectrum was determined by both the fabric’s material and the colour (dyeing agents) present within the fibres. Thus, the chemical bonds from the material composition and the dyeing substances are both responsible for the Raman spectra of the fibres, depending on which one is more Raman active. As an example, slight but significant spectral differences were found among the white different materials made of 100 % cotton (gauze, cotton wool, swab, and t-shirt), and among the five 100 % cotton Tshirts of different colour. This implies additional chemical differences besides those given by the manufacturer about the material composition.