Direct analysis in real time-mass spectroscopy for identification of red dye colourants in Paracas Necropolis Textiles

Abstract

The Paracas Necropolis on the south coast of Peru is renowned for the elaborate funerary bundles recovered from that dry sandy region. These bundles contained the remains of male leaders within the Paracas communities surrounded by multiple layers of plain cloth and garments with embroidered designs. The methods and materials used in dyeing the yarns used to embellish these ancient fabrics are still not well understood, and the research presented herein seeks to add to that body of knowledge. To investigate the sources of dye colourants in samples from Paracas Necropolis textiles, we applied direct analysis in real time-time-of-flight mass spectrometry (DART-MS). This new methodology has both advantages (analyses are rapid and require little or no sample preparation) and disadvantages (e.g. inability to identify intact carminic acid) compared to existing ones used in the analysis of dye colourants. Direct analysis in real time mass spectra were collected on chemical standards, botanical materials and comparative reference samples prepared with Relbunium roots and cochineal insects (Dactylopius sp.), and the results are compared to red fibres obtained from several different textiles within two different funerary bundles from the Museo Nacional de Arqueologia Antropologia e Historia in Lima, Peru. The results from the DART-MS analysis of these red fibres show that the compounds present are consistent with the presence of dye obtained from Relbunium species, and cochineal insects were not used to colour these fibres.Statement of significance DART-MS is an efficient and effective method to identify red colourant composition in small samples of fibres removed from archaeological textiles. Of significant note is the ability to differentiate isomers present in Relbunium and other anthraquinone dyes. Colourants in Relbunium are readily distinguished. Although a component of carminic acid from cochineal is less reliably positively identified by DART-MS, it is possible to use a simple extraction and ionization method to confirm or exclude the presence of carminic acid. Relbunium and cochineal are known to have been used in Paracas textiles. Not only does DART-MS aid in identification of colourants using small amounts of material and without significant sample preparation, the information can be used to distinguish fibres that were prepared differently thus indicating past knowledge of dyeing technology. The information can also aid in separating periods of artefact manufacture, and guide development of a conservation strategy.

The analysis of the Saltzman Collection of Peruvian dyes by high performance liquid chromatography and ambient ionisation mass spectrometry

“…Combinations of methods have shed further light on the ornate textiles and other objects from mummy bundles from South America [8]. Mass spectrometry with ambient ionisation [9] has shown that plant dyes from Relbunium species can be identified rapidly in Paracas textiles, confirming results first reported by Saltzman [10]. Burr [11] demonstrated the use of surface enhanced Raman spectroscopy (SERS) to differentiate between the plant and insect red dyes used in ancient Peru using only single fibers.…”

“…As previously observed, DART-MS does not reliably detect carminic acid and generally only traces of compounds characteristic of cochineal dye are observed by DART-MS directly [9]. More readily ionised minor components from cochineal, primarily kermesic acid, are often observed if the fibers are first treated with a few microliters of 88% formic acid, indicating that these compounds can provide some evidence of the presence of insect dyes.…”

“…Application of this approach to mixtures results in complex spectra from overlapping fragments. In-source fragmentation using DART ionisation and the AccuTOF mass spectrometer has been validated for rapid forensic identification of pharmaceuticals [32], and efforts to use this approach for identifying dye components have been reported by the Armitage research group previously [9]. One half of the sample was analyzed directly without any treatment, while the second half was subjected to an addition of approximately 1 µL of formic acid (88%) prior to introduction into the DART ion source gap.…”

“…A form of electrospray ionization, paper spray makes molecules that are not readily ionized with DART accessible in a low-volume format. The Armitage group has pioneered the use of DART-MS for analysis of natural dyes for applications to ancient textiles [9,[19][20][21][22] and other cultural heritage materials [23][24][25]. While paper spray has been used by others for characterizing both natural and artificial colorants in food and beverage [26,27], this is the first instance of which we are aware for its use in heritage science applications.…”

Revealing the Organic Dye and Mordant Composition of Paracas Textiles by a Combined Analytical Approach

“…The reported composition suggests that the dye source employed belonged to Relbunium vegetal species. The comparison of the obtained pro les with those of some Relbunium species employed for dyeing alpaca and sheep bres from the Saltzman collection (Relbunium hypocarpium, Relbunium ciliatum, Relbunium-unknown species), already characterized in the literature (13,18) allowed us to conclude that the most plausible vegetal source for the three samples is Relbunium hypocarpium. The small differences in composition could be due to the use of a mixture of different Relbunium species, different dyeing processes or degradation of the textiles.…”