Author
Satoru Nakashima, Research Institute for Natural Environment, Science and Technology (RINEST)
Keywords
attenuated total reflection infrared (ATR-IR) spectroscopy, variable angle (VA), ATR correction, water structure, water contents, skin surface
Abstract
Attenuated total reflection infrared (ATR-IR) spectroscopy is an useful method for characterization of various solid and liquid materials without sample preparation. However, raw ATR specra (pATR＝-log I/I0) are distorted due to differences in penetration depths with wavenumbers and an ATR correction is needed for their quantitative treatments. The authors have developed a quantitative ATR correction program and it was converted to an option program in Spectra Manager of Jasco. In order to test the validity of this quantitative ATR correction program, the author first conducted variable angle ATR-IR (VA-ATR-IR) spectroscopy of pure water on Ge and Si ATR crystals for incident angles of 30 to 80 degrees. OH stretching bands of pure water on Ge and Si converted to absorption spectra for 1 μm thickness after the quantitative ATR correction were fitted by 3 Gaussian water components (free, medium, and bound). The obtained results show mostly constant total band areas and area percentages of 3 components between 30 to 75 degrees, indicating the validity of the program. The author next conducted VA-ATR-IR of human skin before and after hydration by moisturing water. Before hydration (dry) water decreased toward the skin surface, while after hydration (wet) water increased toward the surface. In dry condition, free water decreased and bound water increased to the surface, while in wet condition, free and bound water ratio do not change greatly. Peak height ratios of free water / NH (amide A) and bound water / NH are added to calculate total water /NH before and after the ATR correction. They are converted to water contents in skin using our calibration curve of water contents in keratin by IR/QCM/RH method. They are quite similar before and after the ATR correction indicating the validity of the peak height rations for quantitative treatments. As a result, water contents in skin by VA-ATR-IR decreased from 18 wt% at 1250 nm depth to 4-5 wt% at 250 nm near the surface.