USDA, ARS, Cotton Quality Research Station, P.O. Box 792, Clemson, South Carolina 29633, USA.
Appl Spectrosc. 2010 Dec;64(12):1355-63. doi: 10.1366/000370210793561556.
Two-dimensional (2D) correlation analysis was applied to characterize the attenuated total reflection (ATR) spectral intensity fluctuations of native cotton fibers with various water contents. 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 changes resulting from water diffusion in fibers and resultant density dilution. Next, a new spectral set was subjected to principal component analysis (PCA) and two clusters of hydrated (≥13.3%) and dehydrated (<13.3%) fibers were obtained. Synchronous and asynchronous 2D correlation spectra from individual ATR spectral sets enhanced spectral resolution and provided insights about water-content-dependent intensity variations not readily accessible from one-dimensional ATR spectra. The 2D results revealed remarkable differences corresponding to water loss between the hydrated and dehydrated fibers. Of interest were that: (1) the intensity of the 1640 cm(-1) water band remains in a steady state for hydrated fibers but decreases for dehydrated fibers; (2) during the desorption process of adsorbed water, small and water-soluble carbonyl species (i.e., esters, acids, carboxylates, and proteins) begin to accumulate on the cotton surface, resulting in possible changes in the coloration and surface chemistry of native cotton fibers that were rained on prior to harvesting; (3) intensities of bands in the 1200 to 950 cm(-1) region exhibit a more apparent intensity increase than those in the 1500 to 1200 cm(-1) region, indicating the sensitivity of the 1200 to 950 cm(-1) infrared (IR) region to intra- and inter-molecular hydrogen bonding in fiber celluloses; and (4) the 750 cm(-1) band, ascribed to the unstable I(α) phase in amorphous regions, might originate from the cellulose-water complex through hydrogen bonding.
二维（2D）相关分析被应用于描述具有不同含水量的天然棉纤维的衰减全反射（ATR）光谱强度波动。在进行 2D 分析之前，先将光谱在 1800cm-1 的峰值强度处调零，然后在 660cm-1 的峰值强度处归一化，以主观地校正纤维中水分扩散和密度稀释导致的变化。接下来，对新的光谱集进行主成分分析（PCA），得到两个水分化（≥13.3%）和脱水（<13.3%）纤维簇。从各个 ATR 光谱集中提取的同步和异步 2D 相关光谱增强了光谱分辨率，并提供了有关水分依赖性强度变化的见解，这些变化从一维 ATR 光谱中不易获得。2D 结果显示了与水损失相关的显著差异，反映在水化和脱水纤维之间。有趣的是：（1）水带的 1640cm-1 强度在水化纤维中保持稳定，但在脱水纤维中下降；（2）在吸附水的解吸过程中，小的水溶性羰基物质（如酯、酸、羧酸盐和蛋白质）开始在棉纤维表面积累，导致收获前遇雨的天然棉纤维的颜色和表面化学性质可能发生变化；（3）1200 至 950cm-1 区域的带强度比 1500 至 1200cm-1 区域的带强度表现出更明显的增加，表明 1200 至 950cm-1 红外（IR）区域对纤维纤维素中分子内和分子间氢键的敏感性；（4）750cm-1 带归因于非晶区中不稳定的 I(α)相，可能通过氢键起源于纤维素-水复合物。
