Infrared (IR) spectroscopy is a universal, versatile analytical technique for the structure elucidation and quantification of a large variety of organic, inorganic and biological samples. The technique is based on the measurement of IR radiation, absorbed or emitted by a sample, as a function of the wavelength in the region 2.5–100 µm, or 4000–100 cm
−1
in wavenumbers. The absorption or emission of IR radiation is related to discrete vibrational and rotational transitions in a molecule and, therefore, represents structural information. The analytical value of IR spectroscopy is based on the fact that the IR bands occur at more or less localized positions in the spectrum which are correlated to the presence of characteristic structural features of the sample under study. This similarity and transferability of spectral characteristics and the corresponding structural features, makes IR a powerful tool for functional group analysis. On the other hand vibrations, and thus the exact peak positions, are sensitive to the local environment which also makes IR spectra unique molecular fingerprints, highly suited for the unambiguous identification of a sample by comparison with reference spectra. Finally, IR spectroscopy obeys Beer's Law and can thus be used for quantitative purposes too.
The major advantage of IR over other spectroscopic techniques is that practically all compounds show absorption/emission and can thus be analyzed both quantitatively and qualitatively. Besides, IR spectroscopy is nondestructive and admits in situ and remote measurement of almost any sample, irrespective the physical state and without elaborate preparations. A broad variety of advanced accessories and technologies, such as IR microscopy, optical fibers, remote sensing, reflection and emission equipment and the combination with gas chromatography (GC), liquid chromatography (LC) and supercritical fluid chromatography (SFC) sampling and measurement techniques are available to tackle a diversity of problems. The absolute sensitivity is about 50–500 picograms, which is sufficient to solve many problems in trace analysis.
IR spectroscopy is used in a broad range of environmental applications. The technique is a viable tool in remote measurements of gases, liquids and solids and as a fast and reliable method for the characterization and indicative structure classification of a wide variety of samples. Besides, IR is widely applied for identification purposes either to confirm results of other techniques or for reasons of quality assurance (QA).
