Identification of orcein and selected natural dyes in 14th and 15th century liturgical paraments with high-performance liquid chromatography coupled to the electrospray ionization tandem mass spectrometry (HPLC-ESI/MS/MS)

Abstract

In this work, a novel method for identification of orcein derivatives from orchil is presented. It was developed, similarly to other methods used to determine natural dyes, with the use of liquid chromatography coupled to tandem mass spectrometry (LC/MS/MS). Apart from presenting a novel method for orcein derivatives determination, this work also aims to review currently existing method for identification of blue (indigo), red (anthraquinones and brazilwood), and yellow (flavonoids) dyes. In described research, selected reaction monitoring (SRM) conditions were optimized for a number of natural colorants, including the characteristic components of blue (indigo), red (anthraquinones, orchil and brazilwood) and yellow (flavonoids) dyes. For the first time, the major colorants of orchil (orcein derivatives) were analyzed with high performance liquid chromatography (HPLC) coupled to the triple quadrupole mass analyzer. This, made the developed analytical method very sensitive and selective with LOD's values in the range of 0.002 and 0.09μg/ml (when pure standards were available). HPLC coupled to the time-of-flight mass analyzer (TOF/MS) was used to define elemental composition of the orcein derivatives at given retention times. The analysis of natural indigo allowed to conclude that indigo prepared using Isatis tinctoria and Indigofera tinctoria contained similar amounts of indirubin. However, the isatin content was significantly higher for Indigofera tinctoria than for Isatis tinctoria. Before analyzing historical samples, analysis of the natural indigo were carried out in order to evaluate if isatis to indigotin ratio can be used to elucidate the plant source of indigo detected in historical samples. Commercial Indigofera indigo powder showed significantly higher value of this ratio (above 0.5) than Isatis tinctoria for which this ratio took values below 0.2. This proportion was used for the identification of blue dye's source. Therefore, the difference in isatin content may be used to identify indigo plant source. This approach requires careful and accurate analysis because isatin is not only a component of natural indigo but also its decomposition product. In this work, the developed analytical protocol was used to successfully identify natural dyes: indigo, brazilwood and orchil in medieval silk textiles (14th and 15th century) from collection of liturgical vestments from Saint Mary's Church in Gdańsk (Poland), currently stored in National Museum in Gdańsk.