Infrared absorbance Fourier transform

B.K. Alsberg. W.G. Wade and R. Goodacre. Chemometric Analysis of Diffuse reflectance-absorbance Fourier Transform Infrared Spectra Using Rule Induction Methods Application to the Classification of Eubacterium Species, Applied Spectroscopy, 52(6) (1998), 823-832. 

Goodacre, R., Timmins, E. M., Rooney, P. J., Rowland, J. J. Kell, D. B. (1996) Rapid identification of Streptococcus species using diffuse reflectance-absorbance Fourier transform infrared spectroscopy and artificial neural networks. FEMS Microbiol. Lett. 140,233-239. 

Fourier transform infrared (FTIR) analyzers can be used for industrial applications and m situ measurements in addition to conventional laboratory use. Industrial instruments are transportable, rugged and relatively simple to calibrate and operate. They are capable of analyzing many gas components and determining their concentrations, practically continuously. FTIR analyzers are based on the spectra characterization of infrared light absorbed by transitions in vibrational and rotational energy levels of heteroatomic molecules. 

Infrared spectroelectrochemical methods, particularly those based on Fourier transform infrared (FTIR) spectroscopy can provide structural information that UV-visible absorbance techniques do not. FTIR spectroelectrochemistry has thus been fruitful in the characterization of reactions occurring on electrode surfaces. The technique requires very thin cells to overcome solvent absorption problems. 

FIGURE 3.5 Fourier Transform infrared (FTIR) spectra of acrylic rubber (ACM)-siUca hybrid nanocomposites. The numbers after ACM (10 and 50) indicate the wt% tetraethoxysilane (TEOS) concentration. The letters preceding the numbers indicate the ACM-silica samples cross-linked from benzoyl peroxide (B) and a mixed cross-linker hexamethylene diamine carbamate and ammonium benzoate (D). The numbers over the absorption peaks are the wave numbers corresponding to absorbance of those peaks. (From Bandyopadhyay, A., Bhowmick, A.K., and De Sarkar, M., J. Appl. Polym. Sci., 93, 2579, 2004. Courtesy of Wiley InterScience.)... 

Recent work in our laboratory has shown that Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) can be used routinely to measure vibrational spectra of a monolayer on a low area metal surface. To achieve sensitivity and resolution, a pseudo-double beam, polarization modulation technique was integrated into the FT-IR experiment. We have shown applicability of FT-IRRAS to spectral measurements of surface adsorbates in the presence of a surrounding infrared absorbing gas or liquid as well as measurements in the UHV. We now show progress toward situ measurement of thermal and hydration induced conformational changes of adsorbate structure. The design of the cell and some preliminary measurements will be discussed. 

The infrared absorbance spectra were recorded at room temperature on a Fourier transform spectrophotometer (Biucker I.F.S. 110) with a resolution of 4 cm To compare the integrated adsorbances of the various samples the weight of the pellet and the Pd content were considered. The samples were placed in a heatable cell where the catalysts were treated "in situ". Different kinds of experiments were carried out ... 

Recently, nonliving biomass of S. cucullata has been described as a low-cost absorbent of Cr(VI).106 Optimum conditions for the Cr(VI) adsorption by acid-treated S. cucullata were found out using a full factorial design. The Cr(VI) removal efficiency of the adsorbent was found to increase with the increase in time, temperature, adsorbate concentration, and stirring speed, and to decrease with increase in pH and adsorbent dose. The Fourier transform infrared spectroscopy (FT-IR) analysis revealed that in addition to electrostatic force, the adsorption may be due to... 

Fourier transform infrared spectroscopic examination of these polymers also supports this conclusion. As can be observed in Figure 6, a peak occurring at 2086 cm->l is seen in all the copolysilanes. This absorbance has been assigned to a Si-H stretching absorption (15). Such a moiety is expected if silyl radical abstraction of a hydrogen occurs. Examination of the infrared spectra for a Si-CHo-Si vibrational peak which should be located between 1000 and 1100 cm-1 is inconclusive due to the presence of a multitude of absorbances in this region. 

IR dichroism has also been particularly helpful in this regard. Of predominant interest is the orientation factor S=( 1/2)(3—1) (see Chapter 8), which can be obtained experimentally from the ratio of absorbances of a chosen peak parallel and perpendicular to the direction in which an elastomer is stretched [5,249]. One representation of such results is the effect of network chain length on the reduced orientation factor [S]=S/(72—2 1), where X is the elongation. A comparison is made among typical theoretical results in which the affine model assumes the chain dimensions to change linearly with the imposed macroscopic strain, and the phantom model allows for junction fluctuations that make the relationship nonlinear. The experimental results were found to be close to the phantom relationship. Combined techniques, such as Fourier-transform infrared (FTIR) spectroscopy combined with rheometry (see Chapter 8), are also of increasing interest [250]. 

In the diffuse reflectance mode, samples can be measured as loose powders, with the advantages that not only is the tedious preparation of wafers unnecessary but also diffusion limitations associated with tightly pressed samples are avoided. Diffuse reflectance is also the indicated technique for strongly scattering or absorbing particles. The often-used acronyms DRIFT or DRIFTS stand for diffuse reflectance infrared Fourier transform spectroscopy. The diffusely scattered radiation is collected by an ellipsoidal mirror and focussed on the detector. The infrared absorption spectrum is described the Kubelka-Munk function ... 

Instrumentation. Fourier transform infrared (FUR) spectra were recorded on a Nicolet 5DX using standard techniques. Spectra were measured from various sample supports, including KBR pellets, free polymer films and films cast on NaCl windows. Spectra for quantitative analysis were recorded in the absorbance mode. The height of the 639 cm 1 absorbance was measured after the spectrum was expanded or contracted such that the 829 cm 1 absorbance was a constant height. In some spectra an artifact due to instrumental response appeared near 2300 cm 1. 

FIGURE 5.15 Fourier transform infrared spectroscopy absorbance spectra at different temperatures for a C22 monomeric stationary-phase. (Reproduced from Sander, L.C., et al., Anal. Chem.,55, 1068, 1983. With permission.)... 

Fourier-transform infrared (FTIR) spectroscopy Spectroscopy based on excitation of vibrational modes of chemical bonds in a molecule. The energy of the infrared radiation absorbed is expressed in inverse centimeters (cm ), which represents a frequency unit. For transition-metal complexes, the ligands -C N and -C=0 have characteristic absorption bands at unusually high frequencies, so that they are easily distinguished from other bonds. The position of these bonds depends on the distribution of electron density between the metal and the ligand an increase of charge density at the metal results in a shift of the bands to lower frequencies. 

An infrared spectrum is a plot of percent radiation absorbed versus the frequency of the incident radiation given in wavenumbers (cm ) or in wave length ( xm). A variation of this method, diffuse reflectance spectroscopy, is used for samples with poor transmittance, e.g. cubic hematite crystals. Increased resolution and sensitivity as well as more rapid collection of data is provided by Fourier-transform-IR (FTIR), which averages a large number of spectra. Another IR technique makes use of attenuated total reflectance FTIR (ATR-FTIR) often using a cylindrical internal reflectance cell (CIR) (e.g. Tejedor-Tejedor Anderson, 1986). ATR enables wet systems and adsorbing species to be studied in situ. 

Because of this mathematical step, the technique is usually called Fourier transform infrared spectroscopy or FTIR spectroscopy. The Fourier transformation is a mathematical procedure that enables one to convert from the results of an interfero-gram back to intensities of a given wavelength. It is performed in a computer connected to the spectrometer. The result is the absorption spectrum of the sample, that is, the intensity of the absorbance as a function of the wavenumbers. 

These features make IR spectrometry a potentially strong technique for the characterization of chromatographic peaks. However, compared with UV-vis absorbance, extinction coefficients in IR are rather low, and the amount of analyte needed for IR detection therefore is often larger that the amounts usually injected into a GC or LC. Nevertheless, enhanced sensitivity can be achieved by the Fourier transform (FT) version of the infrared spectroscopy (FTIR) (136). 

Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT).6 When IR radiation is directed onto the surface of a solid sample, two types of energy reflectance can occur specular and diffuse. The specular component is the radiation that reflects directly off the sample surface (i.e., not absorbed by the sample). Diffuse reflectance is the radiation that penetrates into the sample and then emerges. Diffuse reflectance accessories are designed to optimize the diffuse reflected energy and suppress the specular component. The optics therefore selectively directs the scattered radiation to the IR detector. 

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