Direct Analysis of Silk Dyes from the Murong Zhi Tomb from the Tang Dynasty Using Desorption Electrospray Ionization High-Resolution Mass-Spectrometry Imaging (DESI-MSI)

The identification of dyes in ancient textiles is crucial for provenance research and scientific conservation. However, the extremely significant value of these cultural relics necessitates the use of non-destructive analytical techniques. To establish a non-destructive, in-situ, accurate, and rapid method for identifying natural dyes in ancient silk fabric samples, we employed desorption electrospray ionization high-resolution mass-spectrometry imaging (DESI-MSI). By optimizing key instrumental parameters—including sample pretreatment method, DESI spray solvent composition, and DESI heated transfer line (HTL) temperature—we determined the optimal mass-spectrometry imaging conditions. The optimal conditions for achieving the highest mass-spectrometry ion peak signal intensity and the best imaging quality were as follows: employing sample pretreatment using double-sided adhesive tape; a spray solvent composed of methanol (100%, v/v) with 0.1% formic acid and 0.1 μg/mL of leucine enkephalin; and an HTL temperature of 400 °C. The characteristic compound in the G42 silk fabric sample was successfully separated. Based on the characteristic mass-to-charge ratio of the major component, the compound was preliminarily identified as berberine. This result was further verified by tandem mass-spectrometry imaging and tandem mass spectra and finally confirmed by comparison with the mass spectrum of a reference standard. Consequently, the source of the dye in the sample was determined to be amur cork tree. The experiments confirmed the applicability and accuracy of the DESI-MSI method for the non-destructive analysis of precious textiles. This work underscores the urgent need to use such non-destructive techniques to provide technical support for the identification of high-value, inaccessible, or fragile silk artifacts and guide the historical tracing and preservation of these cultural relics.