Analyzing the red thread of historical tapestries

Weaving science into cultural preservation

The reason why samples have needed to be so large is that tapestries are frequently subject to degradation phenomena. These alter the dyes and reduce the amount to be found in the tapestry. In addition, the presence of chromophores with similar structures can make the identification of the coloring matter difficult. [1]

Researchers at the Department of Chemistry at the University of Milan wondered if there might be a better way to analyze the dyes. They knew that the best technique in terms of selectivity and efficiency is high performance liquid chromatography coupled with mass spectrometry (HPLC-MS) that also allows the resolution of complex mixtures of dyes. [2]. They also knew that the development of tandem mass spectrometers with the possibility to investigate the fragmentation pattern of a specific molecule represented an improvement in the capability of recognizing different compounds, particularly isomers. [3].

Working with other experts specializing in the restoration of textile artefacts, they decided to use these methods to test whether they could use smaller sample sizes to correctly identify the dyes used in tapestries produced between 17th and 19th century, including Profeta, Sogno Arabo, and Lucrezia. Specifically, they decided to focus on the red yarns.

Using LC-MS/MS to identify natural dyes in historical tapestries

Equipment

The analyses were performed using a Shimadzu LCMS-8045. The instrument was equipped with a Nexera series UHPLC with photodiode array (PDA) detector, electrospray ionization (ESI) and triple quadrupole, which operated both in scan and product ion scan mode. A biphenyl column was used (2.1 x 100 mm, 2.7 μm) and the mobile phase selected was gradient of water (eluent A) and methanol supplemented with formic acid (eluent B); the gradient used is reported in Table 1.

Sensitivity & Support

Shimadzu equipment was chosen for two specific reasons. First, the equipment is highly regarded in the scientific community for its advanced technology and overall sensitivity. The latter point was particularly interesting for the researchers at the University of Milan, as they were testing whether a very small sample of degraded material would be sufficient to achieve excellent results. Second, Shimadzu Italia S.r.l. offered dedicated expert assistance in optimizing the ability of the researchers to exploit the full potential of their equipment.

Extraction of fibers

The red dyes were extracted from the textile fibers and prepared accordingly to a methodology previously set up [4, 5] with an acid-methanolic mixture. Samples were treated with 3 mL of CH₃OH and 100 µL of HCl. The solution was kept in a water bath at 70 °C for one hour. Then, the solution was dried under N2 gentle stream. The residual matter was finally dissolved in 120 µL of CH₃OH before injection [5, 6].