Paint it Back
From colors to proteins: modern technologies and ambient MS for cultural heritage objects.
As an analytical scientist, I believe that what the field needs most are modern and dedicated methodologies to tackle highly complex and severely degraded samples, and that is the main focus for my work.
My postdoc, with Garry Corthals at the University of Amsterdam, used a new ambient MS technique called surface acoustic wave nebulization-MS (SAWN-MS) for the first time in the field of cultural heritage. SAWN-MS allows much smaller samples and simplified sample treatment procedures, plus results within a minute. I applied the new technique first in the identification of organic colorants in wool samples, followed by fatty acids profiling in oil paint swatches that show different degrees of water sensitivity. In collaboration with the Rijksmuseum, Amsterdam, I also studied the effect of different cleaning procedures on oil paints.
Here at the Cultural Heritage Agency of the Netherlands in Amsterdam my role is to adapt and improve existing MS-based technologies and enable their implementation in the cultural heritage field. We need techniques that are gentle enough to preserve the fragile chemistry within the sample, yet powerful enough to characterize their complex system. Without this, further research on conservation treatments remains impossible, impeding our ability to protect cultural artifacts.
Modern tools and modernist art
The team in our Amsterdam laboratory relies on this type of technology to answer questions posed by conservators from different museums on the composition and degradation state of different historical materials. A recent example was three paintings by Marc Chagall from the collection of Stedelijk museum Amsterdam (Selfportrait with Seven Fingers; The Fiddler and The Pregnant Woman/Maternity). We analyzed microsamples using a combination of different techniques to gain information on the organic materials used by the artist. Specifically, we used LC-PDA-HRMS to identify organic pigments from different regions of interest on the paintings – information that contributes to a better understanding of the objects and helps conservators make decisions.
My own work involves developing methodologies to identify organic colorants and understand their degradation mechanisms in textiles and paint swatches. One very interesting project involves the study of the organic pigments used in different artists’ colors and commercial paints from the late 19th and beginning of the 20th century.
Another project involves identifying the proteinaceous materials used in cultural heritage objects. For this, we are working on adapting the extraction and digestion strategies used in modern proteomics to different types of historical materials. Due to the very small amount of proteinaceous material, the extreme complexity and high levels of degradation, samples from artworks are very different to those typically encountered by proteomics labs.
The techniques traditionally used in field of cultural heritage for the identification of organic materials (such as GC-MS, pyrolysis GC-MS, HPLC-PDA, direct temperature timeresolved MS) involve time-consuming preparation steps, chemical derivatization, high temperatures, high ionization energies and long analysis times with poor sensitivity for small samples. Moreover, traditional extraction methods may lead to chemical modification through hydrolyzation and esterification, and could even cause the breakdown of the compounds of interest. One big problem, especially for small molecules, is source-induced fragmentation, which can cause confusion about whether the detected fragments are due to degradation of the object or the analytical technique itself. To avoid source-induced fragmentation and reduce ion suppression as much as possible, we need to minimize stress on the sample.
Do no harm
It’s clear that for analysis of historical samples we need modern minimally invasive methodologies that provide detailed and reliable information. This is highlighted in Article 10 of the Venice Charter for the Conservation and Restoration of Monuments and Sites (the internationally recognized framework for the conservation and restoration of historic buildings): “Where traditional techniques prove inadequate, the consolidation of a monument can be achieved by the use of any modern technique for conservation and construction, the efficacy of which has been shown by scientific data and proved by experience.”
Excellent progress is being made in several areas; for example, the introduction of ambient MS techniques, which eliminate the need for chromatographic separation and minimize harsh and timeconsuming sample preparation steps. These techniques are minimally invasive, incredibly fast and require extremely low samples sizes.
Paint swatches from the RCE reference collection.
SAWN power source and android controller.
The SAWN device is a straightforward portable device, in which a small chip containing a piezoelectric substrate is placed directly in front of the mass spectrometer without the need for a nebulization gas or the use of electrodes and voltages for the ionization. The SAWN creates plumes of droplets with ionized molecules that are further identified by the mass spectrometer. When applying this technology for cultural heritage materials, we noticed a significant reduction in samples needed for analysis, which is critical in art analysis.
Another example of an ambient MS technique introduced in the field of cultural heritage is direct analysis in real time MS (DART-MS). Alba Alvarez-Martin (Smithsonian Museum, Washington DC, USA) was the first to use an enclosed solid-phase microextraction DART-MS interface to identify specific volatile organic compounds that are hazardous to museum objects. She has also used this setup to identify the breakdown products and effects of mixed inorganic pigments on eosin degradation in oil paints.
Mix it up
Considering the complexity and often severe degradation state of cultural heritage objects, we should not rely on one analytical platform or one technology, but explore and combine multiple techniques and analytical strategies that give complementary information. Innovative techniques suitable for the analysis of complex samples and fragile molecules in both liquid and solid form are becoming available and progress is being made in speeding up and simplifying the entire analytical workflow. However, one unmet need is smart, automated data analysis tools for faster data interpretation and I hope to see advances in the is arena in the near future.
A final thought: my work focuses on translating technological advances in other fields to cultural artifacts, but it’s also true that the knowledge and insight gained by studying complex materials within the cultural heritage field are valuable for scientific progress elsewhere. Many of the processes studied – including degradation mechanisms of organic colorants, lipids and fatty acids oxidation, polymerization processes, protein analysis and molecular interactions – are of fundamental interest for environmental and food research, polymer science and biochemistry.
Research Scientist, Cultural Heritage Agency of the Netherlands, and Assistant Professor, Analytical Chemistry Group, HIMS, University of Amsterdam, the Netherlands.