Unveiling the composition of native Asian dye plants by EGA/MS and Py-GC/MS

Plants have always played an important role in Asian culture and the great variety of vegetal species has been and still is widely employed in traditional medicine given their bioactive content, but also in food flavoring, and textile dyeing. With regards to dyeing processes, recipes have been passed from generation to generation and the meaning behind the choice of dyes and mixtures to attain a desired hue are still relevant aspects in traditional Asian textile artistry. The development of methods aiming at identifying the dyeing source in historical textiles, and the build-up of suitable and wide databases of native Japanese dyeing materials are fundamental to preserve such information. In this work, the suitability of thermal analysis to identify major and minor constituents of several selected Asian dye sources in their original raw form (i.e. roots, bark, leaves, dried flower buds and fruits) was investigated. Evolved gas analysis-mass spectrometry (EGA/MS) was firstly used to assess the thermal behavior of the samples and establish desorption/pyrolysis temperatures. The data were then used to perform multi-shot analytical pyrolysis-gas chromatography-mass spectrometry (multi-shot Py-GC/MS), with a customized set of shots for each sample. This allowed to separately detect desorption and pyrolysis products of each plant. The results provided a highly detailed description of the chemical content of the plants under study, most of which were never characterized with thermal techniques and some neither with any other analytical technique. This will contribute to the enrichment of the botanic sources database, and provide complementary information to that usually associated with Asian colorants and dyestuffs.