Basic Studies on the Identification of Excavated Archaeological Textile Fibers Using Polarized FT-IR Micro-spectroscopy - The Identification of Bast Fibers -

To preserve excavated archaeological textile fibers, a preliminary investigation of materials and the degraded state of samples is indispensable. The excavated samples are valuable because the remaining amounts are usually scarce and often heavily degraded. In the past, infrared microscopic analysis has been used to overcome the above-mentioned limitation and has sufficient sensitivity for the analysis. However, the identification of bast fibers, such as hemp and ramie, using FT-IR, is rather difficult because their infrared spectra resemble each other. In this report, we investigated a procedure using a polarized radiation beam for the FT-IR microscope at JASRI (SPring-8) Beamline BL43IR for the identification of bast fibers. A minute amount of sample fibers was pressed by diamond plates to make a flat thin layer. The polarized absorbance spectrum of the sample on the diamond plate was measured. The polarizer was rotated from 0 degrees to 90 degrees, measuring the spectrum at 15 degrees intervals. Both of the bast fibers, hemp and ramie, showed a decrease in the absorption peaks at 1428,1371,1160,1110, and 1060cm(-1) with increase in the rotation angle of polarizer. Since the absorption peak intensity at 2900cm(-1) of both fibers remained constants, the decreases were expressed as the ratios to the 2900cm(-1) values of hemp and ramie. The plotted curves showing the relation between peak intensity versus polarizer angle are distinctly different between hemp and ramie. The infrared dichroism could be due to the difference in molecular orientation of the fiber components (cellulose and hemicelluloses) of hemp and ramie.