The Interface of Chemistry, Art, & Archaeology


Organic Pigment Identification and Spectroscopy

Improved detection and identification of organic compounds can especially impact our understanding of many of the pigments used in artwork. Of particular interest is the study of microscopic art cross sections containing individual layers of dye. This research can shed light on painting techniques, source materials, cross-cultural trade routes, as well as pigment degradations processes. 

 

This project is done in collaboration with Dr. Catherine Schmidt-Patterson at the Getty Conservation Institute in Los Angeles, CA. Funding acknowledgement to National Science Foundation, Award CHE-1241779.

 

 


Organic Pigment Identification and Spectroscopy

Exposure to light can lead to the degradation of many molecules by the absorption of ultraviolet and visible wavelength photons placing them into an energetically excited state prone to chemical and physical changes. Whereby the canonical nucleobases have ultrafast relaxation as a protective mechanism to UV absorption, organic dyes must have similar mechanisms to maintain molecular structure, termed lightfastness, when repeatedly absorbing visible light. Here we study that potential for protective photomechanisms in a subset of naturally occurring organic dyes, red anthraquinone dyes, which were used prominently prior to the 20th century in daily life and in antiquity but due to structural variation degrade in unpredictable ways. We study the isolated chromophores in the gas phase by 2-color Resonance Enhanced 2 photon ionization (R2PI) and time resolved pump-probe spectroscopy to understand how functional substitution affects excited state lifetimes.

Madder chromophores and n-hydroxy substituted analogues included in this study of Excited State Proton Transfer (ESIPT) as applied to color-steadfastness. Arrow indicates hyper-resonance which when less diffuse, as in the case with alizarin, ESIPT seems to occur much more rapidly.

 

 


 

Archaeological Studies: Identifying Ancient Beverage Residues 

 

 

The old adage you are what you eat (and drink) is perhaps the greatest indicator of culture, status and ethnic identity. In anthropological terms this is referred to as foodways analysis. We will use resonant ionization mass spectrometry to provide direct evidence of foodways from ancient ceramic drinking vessels. By determining the presence of wine in ancient Egyptian and Nubian vessels, we hope to shed new light on the cultural dynamics between these two rival ethnic groups and how this changed across time and political power shifts.

Anthocyanins in wine have been shown to survive over time and their associated phenolic acids can be recovered by alkaline fusion for analysis. 1,2 Figure 1 shows the R2PI spectra for five phenolic acids, which we measured in our laboratory. Although they normally all absorb in the same wavelength region, they each have unique resonance wavelengths under the jet-cooling conditions of our experiment. These specific resonances allow us to selectively ionize each of these components without ionizing any other compounds. Furthermore, comparison of on- and off-resonance mass spectra firmly establishes molecular identification by optical spectroscopy in addition to mass spectrometry.

We have successfully detected these wine markers in a model experiment using modern wine absorbed in a clay vessel. We have also addressed issues of contamination during our preliminary study to exclude any cross-contamination when sampling liquid extracts by employing disposable graphite discs as a desorption substrate. The next phase of this research will focus on analyzing archaeological vessel fragments. Resonant ionization mass spectrometry is highly specific and sensitive and its application would be a first for analyzing archaeological samples.

 

This project is in collaboration with Prof. Stuart Smith in the department of anthropology.

This work is funded by the National Science Foundation, BCS-1018804

References:

(1) Singelton, V. I. (1996) in The origins and ancient history of wine, eds. McGovern, P. E., Flemming, S. J., & Katz, S. H. (Gordon and Breach, Luxembourg), pp. 67-78.

(2) Guasch-Jane, M. R., Ibern-Gomez, M., Andres-Lacueva, C., Jauregui, O., & Lamuela Raventos, R. M. (2004)Analytical Chemistry 76, 1672-1677.