Submitted to: Applied Spectroscopy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/8/2011
Publication Date: 2/2/2012
Citation: Liu, Y., Thibodeaux, D.P., Gamble, G.R. 2012. Characterization of attenuated total reflection infrared spectral intensity variations of immature and mature cotton fibers by two-dimensional correlation analysis. Applied Spectroscopy. 66(2):198-207. Interpretive Summary: Cotton fibers are natural plant products and their end-use qualities depend on their stages of development. In general, the quantity of cellulose increases rapidly in developing cotton fibers, and this leads to a number of significant differences in compositions and structures between mature and immature fibers. Given the obvious distinctions between two types of fibers, a number of techniques have been proved to be effective in describing the fiber growth. FT-IR spectroscopy could be a potential consideration, as it requires minimal sample preparation, permits routine analysis at both laboratory and on-field environments, is easy to operate, and is sensitive to structure on cellulose. Despite the efforts of extracting useful information from relatively sharp IR bands of cotton celluloses, their spectral features have not been well understood, mainly due to slight spectral differences between fiber celluloses and noncellulosic polysaccharides and also between the amorphous and crystalline celluloses. To examine the IR spectral differences between immature and mature fibers, we applied generalized two-dimensional (2D) correlation spectroscopy to analyze the attenuated total reflection (ATR) infrared bands in the 1800-650 cm-1 region. Prior to 2D analysis, the spectra were offset, normalized, and then subjected to PCA for confirming the assignment of either immature or mature fibers and also for setting the spectra in an order of increasing PC1 scores. For the first time, the normalized ATR spectra clearly demonstrated the intensity increase or decrease of the bands ascribed to different C-O confirmations of primary alcohols in the 1050-950 cm-1 region, which was not apparent from raw ATR spectra. Furthermore, most of significant 2D observation lies in the identification of several bands that are indicative of cotton fiber development and could be used to develop 3-band ratio algorithm for assessing the degree of fiber maturity.
Technical Abstract: Two-dimensional (2D) correlation analysis was applied to characterize the ATR spectral intensity fluctuations of immature and mature cotton fibers. Prior to 2D analysis, the spectra were leveled to zero at the peak intensity of 1800 cm-1 and then were normalized at the peak intensity of 660 cm-1 to subjectively correct the variations resulted from ATR sampling. Next, normalized spectra were subjected to principal component analysis (PCA) and two clusters of immature and mature fibers were confirmed on the basis of the first principal component (PC1) negative and positive scores, respectively. For the first time, the normalized spectra clearly demonstrated the intensity increase or decrease of the bands ascribed to different C-O confirmations of primary alcohols in the 1050-950 cm-1 region, which was not apparent from raw ATR spectra. The PC1 increasing-induced 2D correlation analysis revealed remarkable differences between the immature and mature fibers. Of interest were that: (1) Both intensity increase of two bands at 968 and 956 cm-1 and the shifting of 968 cm-1 in immature fibers to 956 cm-1 in mature fibers, together with the intensity decreasing and shifting of the 1048 and 1042 cm-1 bands, are the characteristics of cotton fiber development and maturation. (2) Intensities of most bands in the 1800-1200 cm-1 region decreased with the fiber growth and suggested they are from either noncellulosic components or CH and OH fractions in amorphous celluloses. (3) The reverse sequence of intensity variations of the bands in the 1100-1000 cm-1 and 1000-900 cm-1 region of asynchronous spectra indicated a different mechanism of compositional and structural changes in developing cotton fibers at different growth stages.