Mid-infrared absorption spectroscopy

Allen MG (1998) Diode laser absorption sensors for gas-dynamic and combustion flows. Meas Sci Technol 9 545-562 Anderson MS, Andringa JM, Carlson RW, Conrad P, Hartford W, Shafer M, Soto A, Tsapin AI, Dybwad JP, Wadsworth W, Hand K (2005) Fourier transform infrared spectroscopy for Mars science. Rev Sci Instrum 76 034101 Ashizawa H, Yamaguchi S, Endo M, Ohara S, Takahashi M, Nanri K, Fujioka T (2003) Development of a nitrogen dioxide gas sensor based on mid-infrared absorption spectroscopy. Rev Laser Eng 31 151-155 Baldini F, Chester AN, Homola J, Martellucci S (eds) (2006) Optical chemical sensors. Springer, Dordrecht Bansal L (2004) Development of a fiber optic chemical sensor for detection of toxic vapors. Ph.D. thesis, Drexel University, Philadelphia, PA... 

Cavity ring down (CRD) spectroscopy, having proven to be a very sensitive method for detecting molecular species in a wide variety of environments, has also been applied to the mid infrared vibrational spectroscopy of hydrogen-bonded clusters of water " and alcohols.As a direct absorption method, it can be used to quantitatively measure important molecular properties, such as absorption cross sections and coefficients. Knowing these properties, as a function of cluster size and structure, is useful in making the connection to the condensed phase. The sensitive detection of methanol clusters, as shown in Fig. 13, is of considerable importance. These particular measurements nicely complement the action spectra of methanol clusters, detected by depletion of mass-detected signal via vibrational predissociation. 

Raman provides easy sampling, whereas IR spectroscopy frequently needs some form of sample preparation. Materials which are difficult to handle in IR (highly viscous liquids, solids requiring pellets, mulls, or diffuse reflectance) are often easily measured by Raman. Unlike IR reflectance spectra, Raman spectra of solid samples are not affected by sample properties such as particle size. A significant difference with infrared absorption spectroscopy is that the Raman signal is emitted from the sample. Consequently, matrix effects are seldom as severe in RS as they are with mid-IR and NIR. Water may be used as a solvent with no loss in signal or resolution. Glass, even tinted, does not interfere with the Raman spectra. 

The carbonyl index is not a standard technique, but is a widely used convenient measurement for comparing the relative extent and rate of oxidation in series of related polymer samples. The carbonyl index is determined using mid-infrared spectroscopy. The method is based on determining the absorbance ratio of a carbonyl (vC = 0) band generated as a consequence of oxidation normalised normally to the intensity of an absorption band in the polymer spectrum that is invariant with respect to polymer oxidation. (In an analogous manner, a hydroxyl index may be determined from a determination of the absorbance intensity of a vOH band normalised against an absorbance band that is invariant to the extent of oxidation.) In the text following, two examples of multi-technique studies of polymer oxidation will be discussed briefly each includes a measure of a carbonyl index. 

Infrared spectroscopy has proven to be a very informative and powerful technique for the characterization of zeolitic materials. Most infrared spectrometers measure the absorption of radiation in the mid-infrared region of the electromagnetic spectrum (4000-400 cm or 2.5-25 xm). In this region of the spectrum, absorption is due to various vibrational modes in the sample. Analysis of these vibrational absorption bands provides information about the chemical species present. This includes information about the structure of the zeolite as well as other functional... 

Infrared spectroscopy has been applied to ambient air measurements since the mid-1950s (Stephens, 1958). Indeed, PAN was first identified in laboratory systems by its infrared absorptions and dubbed compound X because its identity was not known (Stephens et al., 1956a, 1956b). It was subsequently measured in ambient air (Scott et al., 1957). Since then, IR has been applied in many areas and has provided unequivocal and artifact-free measurements of a number of compounds. Because of its specificity, it has often been used as a standard for intercomparison studies (e.g., for HNO see later). 

Modern infrared (IR) spectroscopy is a versatile tool applied to the qualitative and quantitative determination of molecular species of all types. Its applications fall into three categories based on the spectral regions considered. Mid-IR (MIR) is by far the most widely used, with absorption, reflection, and emission spectra being employed for both qualitative and quantitative analysis. The NIR region is particularly used for routine quantitative determinations in complex samples, which is of interest in agriculture, food and feed, and, more recently, pharmaceutical industries. Determinations are usually based on diffuse reflectance measurements of untreated solid or liquid samples or, in some cases, on transmittance studies. Far-IR (FIR) is used primarily for absorption measurements of inorganic and metal-organic samples. 

If near-infrared diode lasers have low-noise characteristics similar to those of mid-infrared diode lasers, and thus minimum absorbances of 10 5 or less are possible, then an approximate detection limit can be calculated for an absorption experiment. For a 200-m optical path, the calculated detection limit is 5 x 1010 molecules/cm3, which is well above levels of H02 expected to be found in the atmosphere. An absorption experiment in this spectral region apparently would require extremely long optical path lengths, and, indeed, a calculation with a 5-km path yields a calculated detection limit of 2 x 109 molecules/cm3, still rather high for tropospheric measurements. Other issues associated with the use of diode lasers in absorption spectroscopy are discussed in the next section. 

Jensen PS, Bak J, Andersson-Engels S. Influence of temperature on water and aqueous glucose absorption spectra in the near- and mid-infrared regions at physiologically relevant temperatures. Applied Spectroscopy 2003, 57, 28-36. 

Vibrational Spectroscopy [Infrared (mid-IR, NIR), Raman]. In contrast to X-ray powder diffraction, which probes the orderly arrangement of molecules in the crystal lattice, vibration spectroscopy probes differences in the influence of the solid state on the molecular spectroscopy. As a result, there is often a severe overlap of the majority of the spectra for different forms of the pharmaceutical. Sometimes complete resolution of the vibrational modes of a particular functional group suffices to differentiate the solid-state form and allows direct quantification. In other instances, particularly with near-infrared (NIR) spectroscopy, the overlap of spectral features results in the need to rely on more sophisticated approaches for quantification. Of the spectroscopic methods which have been shown to be useful for quantitative analysis, vibrational (mid-IR absorption, Raman scattering, and NIR) spectroscopy is perhaps the most amenable to routine, on-line, off-line, and quality-control quantitation. 

For the assay of enzymes with products and reagents that have no absorption, fluorescence or luminescence in the ultraviolet or visible region, developments in analytical infrared spectroscopy can be used. In particular, mid-Fourier transform infrared (mFTIR) spectroscopy has been successfully applied to the determination of enzyme activities and kinetics, e.g. of /i-fructosidasc, phosphoglucose isomerase and polyphenol oxidase [90]. The method could very well be a tool that may also be applied to a variety of other enzyme classes. The potential of high-throughput applications, however, has yet to be demonstrated. 

The common range for infrared spectroscopy is 10-12,800 cm (780-10 nm). Absorption spectra are described as a function of the wavenumber of the incident the wavenumber () is the reciprocal of the wavelength and has the advantage of being linear with energy. The infrared region can be divided into near-infrared, mid-infrared, and far-infrared regions. 

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