(The following is a post by Sarah Fong, Preservation Science Intern, Preservation Research and Testing Division. She is a rising junior at the University of Virginia, pursuing a Bachelor of Science in chemistry with biochemistry specialization.)
This summer, I had the opportunity to work with the Preservation Research and Testing Division (PRTD) on building a database for their reference colorant collection.
The reference colorant collection houses a variety of materials including pigments and allows PRTD to work with materials similar to those of collection items and implement suitable techniques to preserve them. Over the course of my internship, I collected and compiled data from 50 pigment reference samples using three portable FTIR modules.
FTIR, or Fourier-Transform Infrared Spectroscopy, is an analytical technique that uses infrared light to create transmission and reflectance spectra associated with specific compounds. This is a vital technique used in preservation research to determine the identity of a given pigment or material. A lot of information can be gained from an infrared spectrum such as what material components are present, provide a timeframe of a painted object (if there is a known start or end date to the common use of the pigment used), and inform the best preservation treatments.
In the preservation science world, modern technology has eased the process of collecting data through improving the instruments used. What’s neat about PRTD’s array of instruments is its addition of the portable FTIR instrument.
The ability to capture an IR spectrum using an instrument that’s the size of a laptop is revolutionary, especially in the field of preservation. The portable FTIR instrument allows for an easier and more efficient data collection process. Rather than transporting a fragile and extremely valuable collection item down to the sub-basement lab, which can be risky and potentially cause unnecessary damage, the instrument can travel to the collection item instead and even be mounted above an object. Even using the instrument in lab and stationery on a lab bench is much easier compared to its bulkier alternative.
The portable FTIR comes with four attachments: ATR, DRIFTS, Transmission, and ER. Each attachment has a different way of orientating the infrared light towards the sample, which ends up producing a different spectrum for the same pigment. The goal of my project was to conduct a comparison study among the different attachments, or modules, which will eventually be used to build a reference database for FTIR spectra. With each new module, there came a different set-up as well as sample preparation.
The three modules I used – ATR, DRIFTS, and Transmission – involved a destructive sample collection, meaning a sample of the pigment must be harvested and used up in order to create a sample ready for testing. Luckily, these methods don’t use up much of the sample, which is good when working with items that can’t be replicated. The fourth module, ER or External Reflectance, is non-destructive and more commonly used by the department when collecting data from collection pieces.
The first method, ATR or Attenuated Total Reflectance, uses a diamond crystal to reflect the infrared light from the instrument between the sample and the crystal to create the reflectance spectrum.
ATR did not require preparation of a sample, so collecting the spectra was quick and simple. The next two methods required the colorant pigments to be diluted to a 1-2% concentration in potassium bromide (KBr) and, therefore, required more involved sample preparation processes.
DRIFTS, or Diffuse Reflectance Infrared Fourier transform spectroscopy, uses a series of gold mirrors to reflect any infrared light beam scattered from the surface of the sample that sits inside of a metal cup back to be captured by the detector.
To prepare the sample for DRIFTS, I needed to create a mixture of the pigment sample and KBr, that serves as a carrier substance inert to infrared analysis and does not interfere with the measurements taken. This involved grinding the pigment and KBr together into a fine powder, followed by carefully filling up and leveling off the metal cup.
Transmission was the most time-consuming module to collect data on, mostly due to the sample preparation that came in the form of pellets. Transmission creates an IR spectrum by shining light directly through a translucent sample pellet that sits vertical in the apparatus. In order to create the pellet, the beginnings of the sample preparation were similar to DRIFTS in that a pigment and KBr powder must be made. However, transmission required an extra and more physically taxing step of applying a great deal of pressure to the powder in order to form a thin disc, or pellet, specifically designed to fit into the transmission module.
Creating 50 of these pellets for data collection was an arm workout! Luckily, PRTD’s hydraulic press did most of the physical labor by applying the 10 metric tons needed to seal these samples. These three modules involved different sample preparations and different experimental samples. Unsurprisingly, each module measured a different IR spectrum for the same pigments.
The unique thing about having three IR spectra of the same pigment is that it allows for the cross-comparison of each FTIR module with one another to determine which is the overall “best” or most informative FTIR method. It also provides a more holistic approach to analyzing given pigments and serves as a point of reference when all of the information has been compiled into a database.
This summer, I not only had a chance to work with the Preservation Research and Testing Division, but observe the works of other divisions within the Library of Congress as well. I’ve had the opportunity to observe some of the rare collection pieces that are housed in the Library, such as one of Thomas Jefferson’s drafts of the Declaration of Independence and a draft of the Virginia Declaration of Rights.
The work of PRTD is crucial to the continued efforts of preservation science in order to determine the best preservation techniques for the current and future pieces that are part of the Library’s collection. This database is also important for the preservation science community since, as of now, publicly accessible reference databases containing IR spectra of colorant pigments from the DRIFTS or External Reflectance methods are difficult to find.
I’d like to thank Amanda Satorius and the entire Preservation Research and Testing Division for their guidance and efforts in creating this memorable learning experience.
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