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Preservation with Instrumentation: Raman Spectroscopy

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The Preservation Research and Testing Division (PRTD) is equipped with many wonderful analytical tools to investigate the Library’s vast collection. Material identification is key for preservation; knowing what materials make up a particular object will guide preservation policies and treatments. PRTD has several different laboratories, including a non-invasive optical properties lab, which is generally where collections items come to be examined. The optical properties lab includes a wide range of spectroscopic instrumentation, imaging techniques, and microscopes. More recent spectroscopic instrumentation purchases have included relatively small and portable instruments to supplement the larger, benchtop instruments typically associated with scientific analyses. Portable equipment makes it possible to take analyses to the collection items, wheeling our instruments to the Rosenwald room, Kislak collection, secure storage vaults, conservation, Jefferson galleries, and even to the labs of our colleagues at other institutions.

A photograph of an analytical instrument.
Photograph of a portable Raman instrument with a Raman spectrum on the screen. Peaks corresponds to the energy needed to set certain bonds in motion. Photo Credit: Tana Villafana

In general terms, optical spectroscopy encompasses a range of techniques that involve bouncing light off items or areas of interest and collecting the light that is reflected back to a detector in the instrument. Spectroscopy aids in material identification because, depending on the color (or wavelength) of the light, different information can be gleaned from the light that bounces back. As an example, imagine a piece of paper painted with a red spot. Illuminating the red spot with x-rays gives information about the metallic elements in the paint, infrared light can identify the molecular vibrations in the paint’s chemical bonds, while ultraviolet and visible light highlights the electronic structure of the paint’s chemical compounds. In other words, these various spectroscopic techniques can tell us almost everything we want to know about this red paint. To illuminate this example further, let’s pretend our sample is vermilion painted in animal glue on a piece of paper (and also pretend you don’t know what the red paint is, but you desperately want to). X-ray fluorescence will reveal the presence of mercury and sulfur, infrared light will find the nitrogen and hydrogen bonds that make up the protein portion of animal glue, as well as all the carbon and water peaks associated with a wood-based paper, and ultraviolent and visible light will be able to measure a distinct bandgap for vermilion, as it is a semi-conductor pigment. In other words, we would have identified the pigment and its binding medium without physically sampling the paint on the object.

Another form of optical spectroscopy that we use to look at dyes, pigments, paints, etc. is Raman spectroscopy. In Raman spectroscopy, the light doesn’t exactly bounce back to the detector, but rather scatters off of the chemical bonds in the sample/object. A small amount of light is absorbed as energy that causes, for example, a carbon and hydrogen atom that are bonded together to start bending, stretching, and vibrating. The energy it takes to cause this motion is specific to each type of bond and the molecule of which that bond is a part. Using specialized Raman instrumentation, we can record this unique energy signature to identify materials. Many Raman spectra have been recorded on historic pigments, serving as a useful guide and reference point for our own analyses. PRTD operates two Raman spectrometers, a benchtop Raman microspectrometer and a small, portable instrument. The benchtop Raman is essentially a Raman spectrometer coupled to a microscope, making it possible to focus in on single strands of fiber or individual, small pigment particles, identifying those of interest or deeper studies into material’s molecular degradation pathways. The portable instrument can be held by hand and scans areas several millimeters in diameter, making it ideal for quickly examining objects across the Library, often in areas where they are normally housed.

A photograph of two laptops and an analytical instrument.
At home set of portable Raman spectroscopy, complete with cat assistant. Photo Credit: Tana Villafana

Portable instruments can be packed up and go anywhere. As we navigate the waters of creating a new hybrid work environment for preservation research and researchers, portable instruments can provide some unique opportunities for us. Just as they can be taken to collections, many can be set up right in the living room during telework! The Raman spectrometer, for instance, was easy to set up at home. I took a plastic worktable from the shed, cleaned it, and set it up where I can see my garden through the window while taking spectra. The instrument requires only enough space for itself and its operating laptop, with samples held in place in front of the focusing optics with a simple clamp and holder set-up. If I were somewhere in the field, I could collect data by using the touch interface of the instrument’s screen, but as we often want to have more precise control over the instrument and its operational parameters, I prefer using a laptop, tooling around with the different scan parameters until I find the optimal parameters for the particular pigment.

Photograph of pigments, created from historic recipes, used as reference materials. Each pigment has been painted in six to seven different binders to study the effects different materials will have on the spectral properties of the pigments. Photo Credit: Tana Villafana

You might be wondering just what kind of samples I am studying with the Raman spectrometer from home. While I may not be able to interact with the Library’s collection items at the moment, there is still a perfect way to move the research of the Library’s collection forward. Reference materials are a hallmark of many types of scientific research and even more so in cultural heritage research. Reference materials that we make in the PRTD labs or obtain outside of the Library’s collections for testing can be studied, aged, destroyed, and tested for any number of analytical tests. The goal of having such materials is to enable us to know how particular materials are affected by various environment conditions, as well as all the spectroscopic identifying markers and established safe testing parameters. Then, when the time comes to study the Library’s treasures, researchers can do so non-invasively with useful knowledge of what to look for in order to identify the materials.

Preservation scientists at the Library have found a lot of success in working from home; creating new reference samples, deep diving into data analysis, writing reports and publications, and, with portable instruments, continue developing non-destructive analysis protocols and reference data from telework-enabled home laboratories.

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