Gel substrates and ammonia-EDTA extraction solution: a new non-invasive combined approach for the identification of anthraquinone dyes from wool textiles

The identification of dyes from ancient textiles represents a challenging issue requiring high proficiency and
versatility, that must be guided by the principles of non-destructiveness. The complexity of the dye/textile
system influences both the choice of sampling and extraction procedures and the analytical methodology. For
instance, in order to preserve both the fibre and the original chromophores on it, the extraction procedures
commonly used for natural dyes have evolved in last few decades towards increasingly mild conditions. Along
with this approach, the use of solid substrates for sampling has become prominent in several fields of Cultural
Heritage diagnostics.With reference to these two aspects, the aim of this work is to combine gel substrates and a
mild basic solution for the extraction of anthraquinone dyes (madder and cochineal) from wool textiles, in order
to evaluate the versatility of the extraction procedure for different analytical techniques. Because of their wide
use in the field of Conservation, agar and nanorestore gel substrates soaked in an ammonia-EDTA solution were
tested. The dye-extracting substrates were then analysed with Raman and SERS spectroscopy, whilst the fraction
re-extracted from them was examined with HPLC-HRMS. Finally, scanning electron microscopy was used to
monitor the integrity of the wool after the extraction treatment with the ammonia-EDTA solution.HPLC-HRMS
results show that the basic solution in both gels is effective for the dye extraction, and no alteration of wool fibres
was evidenced by SEM-EDS. The Raman spectra give no information about the dyes, but only substrate signals
are obtained. Conversely, SERS spectroscopy makes it possible to distinguish the two dyes, especially using the
633 nm laser on the agar substrate.