NIR Vibrational Bands and Spectral Interpretation

Bands for S—H, P—H, C=N, and C=0 also appear in the NIR region. Water has several distinct absorption peaks at 1400, 1890,2700, and 2750 nm. These bands enable the determination of hydrocarbons, amines, polymers, fatty acids, proteins, water, and other compounds in a wide variety of materials. 

There is some correlation between molecular structure and band position for certain bands, but because these are often overtone and combination bands, their positions are not as structure-dependent as the fundamental bands in the mid-IR. For example, primary amines, both ahphatic and aromatic, have two absorption bands, one at about 1500 nm and the second at about 1990 nm. Secondary amines have only one band at about 1500 nm. As expected, a tertiary amine has no NH band. Amides with an — NH2 group can be distinguished from R— NH—R amides by the number and position of the N—H bands. The reference by Goddu has a detailed table of NIR structure-wavelength correlations. 

The molecular absorption coefficients (molar absorptivities) for NIR bands are up to three orders of magnitude lower than the fundamental bands in the mid-IR. This results in reduced sensitivity. Greater sample thickness can be used to compensate for this, giving more representative results with less interference from trace contaminants. Sample path-lengths of 0.1 mm-10 cm are common. 

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