Molar Absorptivities of Glucose and other Biological Molecules in Aqueous Solutions over the First Overtone and Combination Regions of the Near-Infrared Spectrum

Abstract

Molar absorptivities are measured for water, glucose, alanine, ascorbate, lactate, triacetin, and urea in the near-infrared spectral region at 37 °C. Values are based on the Beer–Lambert law and cover the first overtone (1550–1850 nm; 6450–5400 cm⁻¹) and combination (2000–2500 nm; 4000–5000 cm⁻¹) spectral windows through aqueous media. Accurate calculations demand accounting for the impact of water displacement upon dissolution of solute. In this regard, water displacement coefficients are measured and reported for each solute. First overtone absorptivities range from 2 to 7 × 10⁻⁵ mM⁻¹mm⁻¹ for all solutes except urea, for which absorptivity values are below 0.5 × 10⁻⁵ mM⁻¹mm⁻¹ across this spectral range. Molar absorptivities over the combination spectral region range from 0.8 to 3.2 × 10⁻⁴ mM⁻¹mm⁻¹, which is a factor of four to five greater than the first overtone absorptivities. Accuracy of the measured values is assessed by comparing calculated or modeled spectra with spectra measured from standard solutions. This comparison reveals accurately modeled spectra in terms of magnitude and position of solute absorption bands. Both actual and modeled spectra from glucose solutions reveal positive and negative absorbance values depending on the measurement wavelength. It is shown that the net absorbance of light is controlled by the magnitude of the absorptivity of glucose compared to the product of the absorptivity of water and the water displacement coefficient for glucose.