Effects of Water on the Spectra of Model Compounds in the Short-Wavelength near Infrared Spectral Region (14,000–9091 cm −1 or 714–1100 nm)

Abstract

The spectral region from 14,000 to 9091 cm⁻¹ (714–1100 nm) is increasingly being investigated for the analysis of high moisture systems due to its low absorption by water. The objective of this work was to determine if the effects of water on model compounds seen in the 7140–4000 cm⁻¹ (1400–2500 nm) near infrared region occurred in this short wavelength region. Spectra were obtained by diffuse reflectance and transmission using a Fourier transform spectrometer. Spectra were obtained for a variety of organic liquids, liquid/water solutions, solids, wet solids and solutions of solids in water. Solutions included ethanol, acetic acid, acetone, pyridine, sugars, starch, cellulose, gums, amino acids and proteins. The spectral results showed that the effects seen in the 7140–4000 cm⁻¹ (1400–2500 nm) region were also common in the 14,000–9091 cm⁻¹ (714–1100 nm) region (i.e. peak shifts, loss of spectral features etc.). For example, in the long wavelength near infrared region, sugars, such as sucrose and glucose, were distinctively different as crystalline solids, but very similar in solution. In addition, molten glucose and urea appeared virtually identical to their dissolved counterparts indicating a loss of crystallinity to be the source of the changes. Finally, changes in the spectra of other materials, such as acetone, n-butylamine and ethanol (while similar in nature to those previously found in the near infrared) were not identical. Thus, while some shifts in peaks were found to occur with acetone/water mixtures, the dominant effects were changes in the relative intensities of peaks within the acetone spectrum, something not seen in the long wavelength region. Therefore, while the type of spectral effects caused by the presence of water may be similar across various spectral regions, the degree and exact nature of those effects vary with the material in question, the amount of water present and the region in question. Thus, the choice of the spectral region to be used for a specific problem should consider the materials in question, as well as other factors such as the usable pathlength.