Towards the identification of the lichen species in historical orchil dyes by HPLC-MS/MS

Orchil is a purple dye extracted from lichens, widely used since the 6th century for dyeing parchments, yarns and textiles, in particular as a substitute for the rare and expensive Tyrian Purple dye; additionally, it was used as a colourant in illuminated manuscripts. The dye was produced since antiquity, through a slow process which could take three weeks or more, by soaking lichens in ammonia in order to promote the transformation of the chemical precursors into a mixture of phenoxazonic compounds, collectively known as orceins. Orchil can be obtained from different lichen species, which are native to specific regions or geographical areas. Since each species can have a typical set of chemical precursors, the final set of orceins could be characteristic of the relative species, even if the macroscopic features of the dye would be similar in terms of hue and spectral responses upon analysis. In this work, a preliminary study is carried out in order to try to identify a diagnostic pattern of three common lichen species (Lasallia pustulata, Ochrolechia tartarea, Roccella tinctoria) by means of HPLC-MS/MS analysis, so as to develop a tool useful to identify the origin of real samples dyed with orchil, among which are featured textiles and purple parchments from ancient manuscripts. Such identification may be exploited in order to assess the relationship between an artwork and the geographical origin of its raw materials.