Analysis Strategy
The development of a strategy for analysis of organic dyes on yarns and organic pigments in paint required consideration of the following parameters: level of destructiveness to both the object and the sample, diagnostic value, sensitivity, and reproducibility. Liquid Chromatography-Photodiode Array-Mass Spectrometry (LC-PDA-MSn) is routinely used for analysis of organic dyes and pigments present in artifacts produced in Europe and the Americas. Its use in an Asian or, more specifically, Chinese context cannot necessarily be extrapolated because the majority of biological sources identified in Chinese artifacts were different from those found in the European and American artifacts. LC-PDA-MSn, however, remained the core analytical technique because of its high diagnostic value, sensitivity, and reproducibility.
The method used by the Getty Conservation Institute was optimized for the analysis of Asian organic dyestuffs and pigments and resulted in:
a library of Ultraviolet-Visual (UV-Vis) spectra for color-contributing components
an Electro-Spray Ionization–Negative Ion Mode (ESI-NIM) mass spectra database of the same diagnostic components of organic colorants including both MS and MS-MS spectra.
Once the analytical strategy was verified, historical samples from wall paintings at the Mogao Grottoes were taken and analyzed. The strategy is also applicable to Asian dyestuffs used in textiles and Asian organic pigments used in paintings.
The following techniques were also investigated for their ability to identify important information not detected through LC-PDA-MSn and for their ability to be less destructive on the object and sample.
- In-situ UV induced fluorescence illumination
- Fourier transform infrared spectroscopy
- Gas chromatography-mass spectroscopy of carbohydrates to explore a possible link between sugars detected in a paint layer and the presence of organic pigments
- Raman spectroscopy
- Direct temperature-resolved mass spectrometry
- 3D-UV-Vis fluorescence spectroscopy