The following is a guest post by Oliver Ding, summer intern in the Preservation Research & Testing Division (PRTD). Oliver is an incoming freshman who will be studying chemistry at Brown University this fall.
Paper is a commonality in our everyday lives. Calendars, journals, notebooks, and more all rely on this special material. Most importantly, various historical objects, such as books and manuscripts, keep knowledge alive across centuries through the versatility of printed materials. During my summer working at the Library of Congress with PRTD, I gained valuable insight into the world of paper chemistry and the analytical techniques used by scientists studying this material. External reflectance Fourier-transform infrared spectroscopy (ER-FTIR) is one of those techniques, but a recent question had arisen from a larger project: How does surface texture affect ER-FTIR spectroscopy? Cue me, brought on to conduct research for the division through the American Chemical Society’s summer internship program known as “Project SEED.”
My first impression of this research project came from discussions with Megan Zins, a dedicated and passionate preservation technician working at the Library, who also served as my co-mentor. She introduced me to ER-FTIR, an analytical technique used to uncover what kinds of molecular compounds are present in a material. When a material, whether it is a solid, liquid, or gas, is illuminated with infrared light of a specific wavelength, specific chemical bonds will absorb this energy, and the resulting light will be reflected back towards a detector. Depending on which wavelengths of light are absorbed, we can then identify what kinds of compounds are in a particular sample. ER-FTIR spectroscopy is such a valuable non-invasive analytical technique and PRTD frequently uses it to get a sense for what something is made of. However, ER-FTIR spectra of smoother, glossier paper samples are more difficult to interpret, an issue believed to be caused by the exterior of the paper.
ER-FTIR spectroscopy is a non-invasive technique, meaning that samples do not need any physical or chemical preparation compared to other types of FTIR techniques. This technique was great for my project, as my samples consisted entirely of paper, both loose-leaf and bound inside books. All I had to do was set up the instrument, boot up the interfacing software, and prop up the book or paper sample against the detector to collect my data. Through this process, I was able to generate an IR spectrum of my sample (shown below), which contains many “peaks” that correspond to what chemical groups or atoms make up something.
For each paper sample I analyzed, I would first assign a texture rating through a thorough, tactile assessment of its surface via dermal resources—in other words, I would feel the paper and see how coarse or smooth it was. Then, I would take an ER-FTIR spectrum of the sample and see if there were any broad trends or patterns in the data.
In terms of “wet” lab work, or work involving chemical solutions, I focused on using a phloroglucinol solution to identify the presence of a common biopolymer in wood pulp papers: lignin. When lignin is exposed to phloroglucinol and acid, it produces a product that is red to violet in color. This not only creates colorful pictures but is also an easy way of telling whether a paper has lignin in it. I then compared the results of my spot tests to the ER-FTIR spectra I captured from my samples to see if they each showed similar results. For this investigation, my paper samples originated from PRTD’s own collection of reference materials, named the “Center for Heritage Analytical Reference Materials,” or CHARM. CHARM objects serve as important reference materials that are allowed to be destructively tested in the wet lab, as opposed to collection items.
This was one of two primary batches of data I collected to investigate surface texture and ER-FTIR spectra. We weren’t exactly concerned if our samples contained lignin; we wanted to see if certain samples, maybe of a specific texture, had, say, differing spectral results from samples of another texture.
My second batch of data resulted from a discussion on whether colorants would show up differently in ER-FTIR spectra when on different textured papers. My paper samples also originated from CHARM for this investigation. CHARM objects have been analyzed for their composition and thus serve as important reference materials. For my project, they served as another group of samples for me to take ER-FTIR spectra of.
Generally, I found that paper samples with a smoother surface were more likely to have unusual ER-FTIR spectra. In addition, I discovered that this trend was more common with coated papers, or paper that had some kind of finishing layer of material. Though I was not able to find an exact relationship, the data I collected will assist preservation scientists at the Library in determining the next step in this investigation. Currently, this may involve continuing to take ER-FTIR spectra of more coated papers or testing to see if other materials are present but also cannot be found through ER-FTIR spectroscopy.
I’d like to give huge thanks to Dr. Andrew Davis and Megan Zins, who were my mentor and co-mentor, respectively. They were amazing instructors who guided me every step of the way and gave me an excellent introduction to the world of analytical chemistry through this project. I’d also like to mention three PRTD staff who assisted my research. Amanda Satorius for training me on how to use the ER-FTIR instrument, Chris Bolser for providing me with raking light images of my samples, and Cindy Connelly Ryan for our discussion of colorants and organic dyes. Without the help of the PRTD, I don’t think my pre-college summer would have been as informative or interesting.
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