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17.3: Near-Infrared and Far-Infrared Spectroscopy

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    From LibreTexts, under CC BY-NC-SA 4.0, as the source states it; its equations are written in TeX, which the PDF shows typeset. This page at LibreTexts.

    At the beginning of this chapter we divided infrared radiation into three areas: the near-IR, the mid-IR, and the far-IR. The mid-IR, which runs from 4000 cm–1 to 670 cm–1 (2.5 µm to 15 µm) is the most analytical useful region and was the subject of the previous two sections. Here we briefly turn our attention to applications using the near-IR and the far-IR.

    Near-Infrared (NIR) Spectroscopy

    The near-IR extends from approximately 13,000 cm–1 (a wavelength of 770 nm or 0.77 µm, the upper wavelength limit of visible light) to 4000 cm–1 (a wavelength of 2,500 nm or 2.5 µm). Earlier we noted that absorption bands in the region that extends from 1500 cm–1 to 4000 cm–1 are called group frequencies. The absorption bands in the near-infrared often are overtones and combination bands of these group frequencies. Of particular importance are functional groups that include hydrogen: OH, CH, and NH are examples. Absorption bands generally are less intense and less broad. Compared to mid-IR, the NIR is more useful for a quantitative analysis of aqueous samples because the OH absorption bands are much weaker. The instrumentation for NIR spectroscopy, both in transmission mode and in reflectance mode, is similar to that for UV/visible spectrometers and for mid-IR spectrometry.

    Far-Infrared (FIR) Spectroscopy

    The far-IR extends from approximately 670 cm–1 (a wavelength of 15 µm) to 10 cm–1 (a wavelength of 1000 µm or 1 mm). FIR spectroscopy finds applications in the analysis of materials that include metals, including metal oxides, metal sulfides, and metal-ligand complexes. FIR spectroscopy has also been applied to the analysis of polyamides, peptides, and proteins. Because the FIR merges into the microwave region, it also finds use in the analysis of the rotational energies of gases.

    This page titled 17.3: Near-Infrared and Far-Infrared Spectroscopy was last modified on Sun, 28 Aug 2022 20:54:46 GMT and is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by David Harvey.