An alternative spectral library is provided by the National Institute of Advanced Industrial Science and Technology (AIST).The use of near infrared spectroscopy for soil science Comparison of test spectra to NIST spectra should be limited to the relative presence of absorbances and should not be an exact comparison of absorbance frequency or intensity. The NIST spectral library contains many of the standard-grade reference materials listed in this book. A standard-grade reference material will pass the comparison test if all absorbances found in the NIST spectrum are present in the test spectrum. Each entry of selected standard-grade reference materials to be analyzed by IR spectroscopy will contain a minimum of three absorbances that must be present to pass the test. The criteria under which standard-grade reference materials pass identity tests are based on observation of characteristic absorbances and by comparison to a spectral library, if one is available. IR spectroscopy is used in selected standard-grade reference materials specifications as a technique for identity confirmation. In some instances, vapor-phase analysis provides differences in absorbance frequency and intensity when compared with solid- or liquid-phase analysis. It is common to see absorbances for moisture when using potassium bromide disks. Care must be taken when preparing the disk to protect it from moisture. The mixture is then pressed in a die at 10,000вЂ“15,000 psi to form a small transparent disk and analyzed. The materials are ground together using a mortar and pestle or a small ball mill. The ratio of analyte to potassium bromide is usually about 1:100. A second method of forming a mull involves grinding the powdered analyte with dry potassium bromide and forming a disk. The disadvantage of Nujol is that hydrocarbon bands may interfere with analyte absorbances. The liquid mull is analyzed between salt plates as described above. Liquid mulls have been formed by combining the powdered analyte with Nujol. The technique consists of grinding the material into a fine powder and then dispersing it into a liquid or solid matrix to form a mull. Michelson interferometer FTIRs offer significant sensitivity improvement over other types of instruments.ДЄ method commonly used for analysis of neat solid samples is the mull technique. The interferogram obtained is then decoded by Fourier transform thus providing the spectrum of the target radiation. The beams first split and then recombine again thus undergoing interference that depends on the difference in the path lengths. FTIR instruments utilize a Michelson interferometer to modulate the incoming optical radiation by using two mirrors that are perpendicular to each other (one fixed and one movable) and a beam splitter placed on the path of the incoming radiation. The use of an interferometer offers an advantage when using background subtraction techniques. The frequency data from the FTIR instrument is obtained using mirrors in a Michelson interferometer to produce a signal that is transformed to data with a much lower frequency while containing the same information as the original IR signal. In spectra produced by Fourier transform infrared instruments, all wavelengths emitted by the source are present. FTIR instruments work in the time domain. These instruments operate in the frequency domain. A brief discussion of the theory will be followed by a discussion of instrumentation, sample handling techniques, and qualitative analysis.Дispersive IR instruments record energy absorption as the instrument scans through the IR energy spectrum. Instrumental operating procedures are not given because they will vary depending on instrument design. The IR technique is discussed here primarily for application in identification of organic compounds and will focus on the mid-infrared region. It can be used to quantitate analytes in complex mixtures and is used extensively in detection of industrial pollutants in the environment. IR spectroscopy is also used in quantitative techniques because of its sensitivity and selectivity. The IR spectrum of an organic compound is a unique physical property and can be used to identify unknowns by interpretation of characteristic absorbances and comparison with spectral libraries. The infrared region of the spectrum includes electromagnetic radiation that can alter the vibrational and rotational states of covalent bonds in organic molecules. Infrared (IR) spectroscopy is an absorption method widely used in both qualitative and quantitative analyses.

Infrared spectra analysis