Moisture effects on diffuse reflection infrared spectra of contrasting minerals and soils: A mechanistic interpretation

作者：Janik Leslie J^*; Soriano Disla Jose M; Forrester Sean T; McLaughlin Michael J

来源：Vibrational Spectroscopy, 2016, 86: 244-252.

DOI：10.1016/j.vibspec.2016.07.005

• Soil
• In-field
• Mechanism
• Diffuse reflection
• Mid-infrared
• Near-infrared
• Optical constants

Moisture can impact on infrared spectra of soils, with implications for field analysis. The basis for these effects needs to be explained. This study aims to describe the basis for the impact of moisture content on soil spectra and to use this information to suggest a method to predict the limiting moisture contents in various soil types. Reference minerals and soils were spiked with water, typical of field conditions. Their infrared spectral band intensities were non-linearly related to moisture contents and showed severe distortion in the mid-infrared (MIR) near 3400 cm(-1) at critical moisture contents. Distortion was not observed in the near-infrared (NIR) spectra due to the lower spectral absorbances compared to the MIR. The extent of spectral distortion appears to depend on the refractive index of water, associated with specular reflection from free water films on the particle surfaces and on the total absorbance from internal water in major soil components. The key mechanisms impacting on the spectral distortion were the limiting absorbance near 1.5 absorbance units being reached and the rapidly changing refractive index of water films in the MIR region. It was shown that the onset of spectral distortion could be predicted for a range of soils from specific soil properties such as quartz and smectite contents, particle size, and moisture retention. From this study, it could be concluded that high clay soils cause minimal distortion of infrared spectra but high sand soils pose significant risk of distortion for moisture contents typical of field conditions.

• 出版日期2016-9