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Technological Advances Make Low-Cost Infrared Microspectroscopy Routine


Bethany Renstrom,1


Oxford, OH 45056 2


Kelly Minuskin,1 Bryce Egan,1 David Schiering,2 and Gregg Ressler2


1Molecular Microspectroscopy Laboratory, Department of Chemistry and Biochemistry, Miami University, 651 E. High St., Czitek, 6 Finance Dr., Danbury, CT 06810


*sommeraj@miamioh.edu


Abstract: A low-cost infrared microscope is evaluated in both trans- mission and attenuated total internal reflection (ATR) sampling modes. Transmission spectra of 60 to 250 micrometer samples exhibited an excellent signal-to-noise ratio. Signal-to-noise in transmission mode also correlated well with theoretical predictions. The quantitative capa- bility of the microscope was demonstrated by measuring the colorant concentration of red, yellow, and blue polypropylene fibers. These lat- ter measurements were conducted using the ATR sampling mode of the microscope. Colorant concentrations could be measured to 1% by weight, and calibration curves with excellent R2


values were obtained over the dye concentration range of 1 to 10% by weight colorant.


Keywords: Infrared microspectroscopy, FTIR, ATR, colored fiber analysis


Introduction Te development of infrared microspectroscopy dates back


to the late 1940s and early 1950s at a time when infrared spec- troscopy was in its early infancy [1]. Since its development, infra- red microspectroscopy has been used to characterize additives in synthetic fibers important in many different fields, including textiles, fabrics, forensics, and archeology. Molecular spectros- copy is an excellent tool to carry out fiber characterization, and the necessity of characterizing fiber additives such as colorants is multifaceted. For example, fiber and textile industries may need to ensure proper quality of materials and products. Te


ability to identify different fibers and colorants in a fiber found at a crime scene can greatly aid in solving a crime. Figure 1 illustrates infrared spectra of an acrylic fiber and a nylon fiber collected on one of the first commercially available infrared microscopes [2]. Te spectra demonstrate that different fiber types can clearly be distinguished using infrared microscopy. Further, the identification of pigments and dyes used in


ancient works of art can provide insights into the origin and the technology of a civilization. Figure 2 illustrates the infrared spec- trum of indigo dye and a spectrum produced from the subtrac- tion of an undyed wool fiber spectrum from that of a blue Paracas Indian archeological fiber. Te blue fiber was removed from a funerary bundle of the Paracas culture that existed from 400 BC to 400 AD in Peru [3]. Infrared techniques have been used in sin- gle-fiber analyses ranging from identification of single fibers and its additives to the characterization of polymer blends [4]. One of the first infrared microscopes to become


commercially available was the Perkin Elmer model 85 (illus- trated in Figure 3), which enabled micrometer-size samples to be studied [2]. Te prototype of this microscope was built by V. J. Coates, A. Offner, and E. H. Siegler aſter consultation with several potential users of such a piece of equipment. Tis report presented infrared spectra of single acrylic and Nylon fibers whose diameters were 17 and 20 micrometers, respectively (see Figure 1). However, to obtain spectra of sufficient quality, the length of the fibers had to be increased to 650 micrometers. Te minimum sample size that could be


observed by this microscope varied but was, in summary, dependent on source brightness, optical geometry, transmission efficiency of the optical system, and detector sensitivity [2]. However, this method saw little use due to 1) the limited throughput associated with the sample size and 2) the serial data collection associated with the use of prisms/diffraction gratings and slits in spectrometers of the day [2,5]. Depending on the spectral resolution, a typical spectrum required 5 to 30 minutes per scan to collect, leading to extremely long data collection times. With the introduction of Fourier trans-


Figure 1: Infrared spectra collected on single fibers using the first commercially available infrared microscope (adapted from reference 2).


26 doi:10.1017/S1551929520000012


form infrared (FTIR) spectrometers in the 1970s, spectra could be collected in seconds, leading to significantly shorter collection


www.microscopy-today.com • 2020 March André J. Sommer,1 *


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