A collaborative study on Raman spectroscopy and microspectrophotometry (MSP) was carried out by
members of the ENFSI (European Network of Forensic Science Institutes) European Fibres Group (EFG) on
different dyed cotton fabrics. The detection limits of the two methods were tested on two cotton sets
with a dye concentration ranging from 0.5 to 0.005% (w/w).
This survey shows that it is possible to detect the presence of dye in fibres with concentrations below
that detectable by the traditional methods of light microscopy and microspectrophotometry (MSP). The
MSP detection limit for the dyes used in this study was found to be a concentration of 0.5% (w/w). At this
concentration, the fibres appear colourless with light microscopy. Raman spectroscopy clearly shows a
higher potential to detect concentrations of dyes as low as 0.05% for the yellow dye RY145 and 0.005% for
the blue dye RB221. This detection limit was found to depend both on the chemical composition of the
dye itself and on the analytical conditions, particularly the laser wavelength.
Furthermore, analysis of binary mixtures of dyes showed that while the minor dye was detected at
1.5% (w/w) (30% of the total dye concentration) using microspectrophotometry, it was detected at a level
as low as 0.05% (w/w) (10% of the total dye concentration) using Raman spectroscopy.
This work also highlights the importance of a flexible Raman instrument equipped with several lasers
at different wavelengths for the analysis of dyed fibres. The operator and the set up of the analytical
conditions are also of prime importance in order to obtain high quality spectra. Changing the laser
wavelength is important to detect different dyes in a mixture.