The analysis of colored acrylic, cotton, and wool fibers using Raman spectroscopy

There has been a resurgence in Raman spectroscopy for its application in forensic science. This method allows for the measurement of the inelastic scattering of light due to the vibrational modes of a molecule when irradiated by an intense monochromatic source such as a laser. The Raman technique presents advantages such as its non-destructive nature, its fast analysis time, and the possibility of performing microscopic in situ analyses. In its forensic application, it is a versatile technique that covers a wide spectrum of samples such as drugs of abuse, trace evidence, and inks. Its potential was also demonstrated for the analysis of textile fibers. Raman spectroscopy allows for the detection of dyes used in the coloration of such samples. In this study, 180 samples were randomly collected: 60 acrylics (20 blue, 20 red, and 20 black), 60 cotton (20 blue, 20 red, and 20 black), and 60 wool samples (20 blue, 20 red, and 20 black). The Raman instrument was equipped with four laser excitation sources: argon ion laser at 514 nm, helium-neon at 633 nm, and two near infrared (NIR) diode array lasers at 785 and 830 nm. The importance of using different laser wavelengths was also emphasized in the forensic Raman analysis of fibers. Several aspects were investigated according to the best analytical conditions for every type/color fiber combination; particular attention was placed on the discriminating power of the technique. Based on the results from the Raman analysis, it was possible to obtain different classes of fibers according to the general shape of spectra, more particularly on the basis of additional bands. Raman spectroscopy was not considered as an isolated technique but rather as a step in the conventional analytical scheme for textile fibers. Thus, in order to optimize the analytical sequence, it was determined at which stage Raman spectroscopy should be integrated and under which conditions. For this purpose, the collected samples were also analyzed by light microscopy (bright field, double polarization, and fluorescence), UV-Vis microspectrophotometry (MSP), and thin layer chromatography (TLC). This approach allowed for the determination of which situation requires Raman spectroscopy to be used in conjunction with the conventional methods for fiber dye analysis.