Infrared region of the electromagnetic spectrum provides a wealth of information on molecular composition of materials. Within the IR range mid-IR region is of special interest and an earlier article on mid-IR applications has touched upon typical applications in this area. On the other hand contributions of near- IR and far- IR cannot be overlooked and deserve a mention.
Near IR applications
Near IR covers the region from 12500 to 4000 cm -1. This region gives rise to overlapping overtones and combinations of rotational and vibrational modes of hetero atoms linked by single bonds such as C – H, O – H and N-H that occur in mid-IR region. All these bands are overtones of principal bands and inherently have low intensities so sensitive detectors are necessary.
The problem of overlapping vibrational bands that appear to be non-specific or poorly resolved is overcome by using software capable of multivariate calibration algorithms and chemometrics. Traditionally, the region has been used for monitoring raw materials and finished products for moisture or solvent content in drying operations, blending efficiency of blenders and monitoring of fermentation processes in pharmaceutical and food manufacturing operations. In studies on fuels near IR does not give a direct measure of hydrocarbon groups but is correlated by absorptions of functional groups such as methyl, methylene and aromatics which can in turn be related to composition of complex hydrocarbon mixtures.
Near IR because of its higher energy affords deeper penetration than mid IR and far IR regions permitting in situ analysis specially for agricultural and biomedical applications as well as real-time process monitoring.
Far IR applications
Far IR covers the region from 200 to 10cm-1 and is useful for inorganic studies arising in stretching and bending vibrations of bonds between metal atoms and other inorganic atoms or organic ligands. Far IR studies yields useful information on lattice energies of crystals and transition energies in semiconductors. In the gaseous state pure rotational spectra are also observed in the far IR region.
The region presents some problems such as:
- Low energy of sources and low efficiency of beamsplitters often results in lower detection efficiencies
- Interference due to presence of water vapour in the environment
- Lack of high energy transfer sampling matrices and beam splitters
Automatic interchange of beam splitters, detectors coupled with efficient purging of sample compartment results in overcoming such problems..