FT-NIR Microspectroscopy: A Method for Quantitatively Mapping One-Dimensional Moisture Penetration Into Sugar Glasses

In this methods paper, one-dimensional moisture penetration into thin films of sugar glass exposed to different relative humidity was measured by use of FT-near infrared microspectroscopy working in a spectrum range of wavelength from 1.35 to 2.50 μm. The peak height ratio of characteristic peaks for H--OH (primarily water) and O--H (primarily sugar) bands, respectively, were highly correlated to moisture content for both sucrose and lactose systems and were used for calibration of moisture content. Moisture content along one-dimensional locations was accurately measured using microscopic mapping. Sorption penetration profiles of moisture content vs. penetration depth were obtained. Depending on the sorption time and storage conditions, moisture penetration profiles typically showed a fairly sharp boundary between intact glass in the interior and a high moisture content surface layer, which slowly moved into the sample interior over time. Crystallization was observed to occur at the surface under certain conditions when insufficient inhibitors were present. In these cases, a moisture peak was seen to penetrate into the glass. Microspectroscopic mapping proved to be an excellent technique for studying moisture penetration behavior and kinetics, providing quantitative data for future diffusion modeling.