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Reference (natural and synthetic) indigo samples were analysed by Raman spectrometry and only indigotin was detected. ‘Jeans’ refers to a specific garment with respect to both the dye, indigo, and the garment cut; from reference spectra of blue-dyed fibres and threads from garments, it is straightforward to observe by Raman microspectrometry the spectra of dyes fixed on threads without chemical pretreatment. On these supports, the spectrum of indigotin shows extra bands assigned to vibrational modes of Bu symmetry, indicating a loss of the centrosymmetry of the molecule and an alteration of its planarity due to the dyeing process. © 1997 by John Wiley & Sons, Ltd.
Coupry et al. (Sat,) studied this question.