Fourier transform-NIR instruments

These are extremely fast with the highest possible resolution, although the scan averaging required for adequate signal-to-noise may provide ultimate scan times that are similar to those of other instrumentation. They are the best bet for analyzing gases or materials where resolution is critical (due to a sharp Many active pharmaceutical ingredients (APIs) are crystalline and have sharp peaks, even in the NIR. In such cases, the resolution of a FT instrument is a plus. 

The usual advantages of FT-IRs do not necessarily apply in the NIR, however. The throughput and multiplex advantages (of mid-range IR) only apply where the source is weak, absorbencies are strong, and the detector is not too sensitive. 

In this scenario (mid-range infrared), the extra energy is needed to (1) penetrate the sample and (2) raise the signal-to-noise (S/N) ratio to a level where small peaks are visible and quantitation is possible. 

With NIR, the source is quite strong, the absorbencies weak, and the detector(s) quite quiet and sensitive. The extra energy can, in some cases, burn the sample. Lactose is an example of a pharmaceutical excipient browned by the energy of FT-NIR. In addition, since most NIR peaks are broad and relatively featureless, high resolution usually adds noise (which is, by nature, high frequency). 

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An NIR method has been developed for measuring the hardwood content of bleached hardwood and the lignin content of vmbleached hardwood pulp. Work has also been carried out using NIR reflectance spectra of hardwoods to discriminate between the different species. Using a Fourier transform NIR instrument carbonate measurements have been made on soil samples. NIR analysis of forest humus samples has provided satisfactory calibrations for microbial basal respiration, based on the organic polymer content of the humans. NIR analysis of the lake sediments for pH has been used to construct the lake water history. 

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