Fourier transform absorbance subtraction

The EXAFS function is obtained from the X-ray absorption spectrum by subtracting the absorption due to the free atom. A Fourier transform of the EXAFS data gives a radial distribution function which shows the distribution of the neighbouring atoms as a function of internuclear distance from the absorbing atom. Shells of neighbours, known as coordination shells, surround the absorbing atom. Finally, the radial distribution function is fitted to a series of trial structural models until a structure which best fits the... 

Figure 23-3 Infrared absorbance spectra of the amide regions of proteins. (A) Spectra of insulin fibrils illustrating dichroism. Solid line, electric vector parallel to fibril axis broken line, electric vector perpendicular to fibril axis. From Burke and Rougvie.24 Courtesy of Malcolm Rougvie. See also Box 29-E. (B) Fourier transform infrared (FTIR) spectra of two soluble proteins in aqueous solution obtained after subtraction of the background H20 absorption. The spectrum of myoglobin, a predominantly a-helical protein, is shown as a continuous line. That of concanavalin A, a predominantly (3-sheet containing protein, is shown as a broken line. From Haris and Chapman.14 Courtesy of Dennis Chapman.

ATR spectroscopy in the infrared has been used extensively in protein adsorption studies. Transmission IR spectra of a protein contain a wealth of conformational information. ATR-IR spectroscopy has been used to study protein adsorption from whole, flowing blood ex vivo 164). Fourier transform (FT) infrared spectra (ATR-FTIR) can be collected each 5-10 seconds165), thus making kinetic study of protein adsorption by IR possible 166). Interaction of protein with soft contact lens materials has been studied by ATR-FTIR 167). The ATR-IR method suffers from problems similar to TIRF there is no direct quantitation of the amount of protein adsorbed, although a scheme similar to the one used for intrinsic TIRF has been proposed 168) the depth of penetration is usually much larger than in any other evanescent method, i.e. up to 1000 nm water absorbs strongly in the infrared and can overwhelm the protein signal, even with spectral subtraction applied. 

Fourier transform infrared spectroscopy FT-IR. The measurement of individual degradation products with FT-IR is very simple, quick and precise. A reference sample spectrum of new oil is required to subtract electronically from the oil sample spectrum. The spectra of the fresh oil and the used oil sample are obtained individually in the same cell. The results - both spectra and the "differential" spectrum are stored in the computer in absorbance format, a form that varies linearly with concentration. 

Degradation was followed by measuring the infrared absorption intensities of the aliphatic and sulfone groups in the chain as a function of dose. Measurements were made on a Perkin Elmer Model 257 grating spectrophotometer and by Fourier Transform infrared spectrometry using a Nicolet 5DX FTIR spectrometer operating at 2 cm-1 resolution. Absorbance spectra of PMPS in novolac/PMPS blends were corrected for the contribution due to novolac absorption by subtraction of an appropriately scaled absorbance spectrum of pure novolac. 

Micro-Fourier transform infrared (FT-IR) spectroscopy was employed to examine the chemical structures by observing absorption bands at 1716 cm 1 (carbonyl group), 964 cm"1 (trans- inylene) and 910 cm 1 (end-vinyl). After 2 months from the irradiation, samples were sliced into 100-150 pm films along the direction of ion-beam penetration and the FT-IR spectra were measured as a function of depth from the surface [6]. We obtained the net absorbance AAbs. for the three bands for carbonyl group, /raws-vinylene and end-vinyl at each depth by subtraction of the measured spectrum from that of the unirradiated sample. Sliced samples were stored in the dark at room temperature. They were repeatedly measured after 4, 6 and 12 months from the irradiation to observe the effect of long-term storage. 

The strength of the Fourier transform method lies in the throughput of the spectrometer that allows very minute quantities of impurities to be detected. Furthermore, the absorbance spectral subtraction can compensate for sample and reference thickness differences. The automated cryogenic system is capable of handling a sample every few minutes.56... 

If a Fourier transform (FT) IR instrument is used, a reference cell is not required because the IR spectrum of the pure solvent is previously stored in the memory of the spectrometer. The software that controls the instrument then substracts those absorptions that are due to the solvent from the spectrum. However, as is evident from the IR spectrum in Figure 8.7a, this subtraction may be imperfect this is most notable at about 750 and 1220 cm , where the solvent chloroform absorbs strongly. 

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