Abstract
Effective analytical approaches for the identification of natural dyes in historical textiles are mainly based on high-performance liquid chromatography coupled with spectrophotometric detection and tandem mass spectrometric detection with electrospray ionization (HPLC-UV-Vis-ESI MS/MS). Due to the wide variety of dyes, the developed method should include an adequate number of reference color compounds, but not all of them are commercially available. Thus, the present study was focused on extending of the universal analytical HPLC-UV-Vis-ESI MS/MS approach to commercially unavailable markers of red, purple, and blue dyes. In the present study, HPLC-UV-Vis-ESI MS/MS was used to characterize the colorants in ten natural dyes (American cochineal, brazilwood, indigo, kermes, lac dye, logwood, madder, orchil, Polish cochineal, and sandalwood) and, hence, to extend the analytical method for the identification of natural dyes used in historical objects to new compounds. Dye markers were identified mostly on the basis of triple quadrupole MS/MS spectra. In consequence, the HPLC-UV-Vis-ESI MS/MS method with dynamic multiple reaction monitoring (dMRM) was extended to the next 49 commercially unavailable colorants (anthraquinones and flavonoids) in negative ion mode and to 11 (indigoids and orceins) in positive ion mode. These include protosappanin B, protosappanin E, erythrolaccin, deoxyerythrolaccin, nordamnacanthal, lucidin, santalin A, santalin B, santarubin A, and many others. Moreover, high-resolution QToF MS data led to the establishment of the complex fragmentation pathways of α-, β-, and γ- aminoorceins, hydroxyorceins, and aminoorceinimines extracted from wool dyed with Roccella tinctoria DC. The developed approach has been tested in the identification of natural dyes used in 223 red, purple, and blue fibers from 15th- to 17th-century silk textiles. These European and Near Eastern textiles have been used in vestments from the collections of twenty Krakow churches.
Highlights
High-performance liquid chromatography (HPLC) is the most common technique used for the separation of colorants present in natural dyes and historical textiles [1,2,3]
The most intense precursor and product ion pairs of the identified dye markers were used to develop the final method in dynamic multiple reaction monitoring mode, which provides superior sensitivity and selectivity for targeted compounds in complex samples
Tandem mass spectrometric detection provided information on the structures of unknown colorants eluted from the HPLC column
Summary
High-performance liquid chromatography (HPLC) is the most common technique used for the separation of colorants present in natural dyes and historical textiles [1,2,3]. Since the amount of available research material is usually limited, it is extremely important to use a sensitive detector that is both selective and versatile for a variety of coloring compounds. The best separations to date have been achieved using reversed phase columns, mostly C18. Molecules 2020, 25, 3223 use columns with phenyl-type stationary phases capable of exploiting pi–pi interactions to achieve separation. The correct identification of a dye requires, at first, the true positive identification of analytes, which has to be based on the comparison of their retention times and their other physicochemical properties (such as m/z values and characteristic absorption) with those of reference compounds determined under identical experimental conditions. It is important to prepare a wide/extensive database of markers covering the largest possible number of dyes
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