Raman spectroscopy of dye mixtures on cotton fibres

Raman spectroscopy is becoming a routine technique in numerous forensic labs since it is fast and non-destructive. Recent work has shown that Raman spectra of dyes depend on the excitation laser wavelength used (resonance effects) and on the scattering ability of the dye molecule itself. Both factors together with fluorescence emission may affect the detection of a dye, especially within a mixture. In order to obtain a better understanding of their Raman behavior binary mixtures at various ratios have been prepared using five known dyes showing different scattering and fluorescent abilities. Their Raman investigation at 514 and 785 nm highlights the complementarity of these two resonant and non-resonant sources and the limitations of the Raman technique in the detection of both major and minor components of a dye mixture. Other investigations have been performed on binary and ternary known dye mixtures on cotton fibres by Raman spectroscopy and MSP-Vis. The combination of two laser sources leads in most cases to the detection of both or two out of three dye components. This Raman information reinforces clearly the confidence in MSP-Vis results. Indeed the contribution of the minor dye component is sometimes very small in the MSP spectrum and a visual inspection of the spectra in addition to inhomogeneous dyeing on cotton may result in difficult interpretation. For these reasons Raman spectroscopy is a very convenient technique to confirm or maybe specify the MSP results, especially for fibre types with common MSP spectra. Furthermore, MSP-Vis also showed some limitations with very light or very dark colored fibres whereas Raman spectroscopy could still discriminate between fibre types