Fourier-transform IR spectrometer

Most modem IR facilities will use a Fourier Transform IR Spectrometer (FTIR), rather than a dispersive instrument. The essential feature is that all of the light from the source falls on to the detector at any instant, which thus leads to increased signal levels, thereby automatically improving the signal-to-noise ratio at all points on the spectrum. 

IR absorbance was measured with a Fourier-transform IR spectrometer. The absorbance at wave number a is defined as (1 /TV) In [F(U0)/ F(U)], where N 10 is the number of useful reflections at the electrochemical interface, F(U) the light intensity at wave number a reaching the detector at potential U, and F(U0) the same but under reference conditions at potential U0. 

Figure 10.11—Optical arrangement of a Fourier transform IR spectrometer, a) A 90c Michelson interferometer including the details of the beam splitter (expanded view) b) optical diagram of a single beam spectrometer (based on a Nicolet model). A weak intensity HeNe laser (632.8 nm) is used as an internal standard to measure precisely the position of the moving mirror using an interference method (a simple sinusoidal interferogram caused by the laser is produced within the device). According to the Nyquist theorem, at least two points per period are needed to calculate the wavelength within the given spectrum.

Figure 10.12—Sequence of events necessary to obtain a pseudo-double beam spectrum with a Fourier transform IR spectrometer. The instrument records and stores in its memory two spectra representing the variation of lu (blank) and / (sample) as a function of wavenumber (emission spectra 1 and 2 above). Then, it calculates the conventional spectrum, which is identical to that obtained on a double beam instrument, by calculating the ratio T — /// — f(A) for each wavenumber. Atmospheric absorption (CO2 and H20) is thus eliminated. The figure illustrates the spectrum of a polystyrene film.

One of the major advances in the past decade has been the maturation of the electronic revolution. This has had its effect on surface spectroscopy, with regard to instrumentation for transmission IR, but particularly for sensitivity gains that have made reflectance techniques the preferred alternative for fundamental studies. In the transmission mode, the commercial development of the Fourier transform IR spectrometer has led to significant advantages in the determination of the vibrational spectra of adsorbed species. This is covered in the chapter by Bell. 

The authors gratefully acknowledge the financial support of the National Heart and Lung Institute under Grant HL20079-01. Partial support to one of the authors (C.S.P.S.) from the Health Sciences Fund is also acknowledged as well as the support of the National Science Foundation for the Fourier transform IR spectrometer. 

Figure 9.2. (a) UV/visible diode array spectrometer (Hewleu-Packard HP8450A) using the reversed-optics configuration (courtesy of Hewlett-Packard), (b) Fourier-transform IR spectrometer (Nicoiet brochure)... 

An advantage of PM-IRAS over SFG is that PM-IRAS provides IR surface spectra (p-s) and gas-phase spectra (s) in parallel. Thus, the catalytic turnover can be followed not only by gas chromatography, but also by PM-IRAS (cf. Figs 45 and 53). The efficiency of PM-IRAS has been demonstrated both with dispersed and Fourier-transformed IR spectrometers in the characterization of various metal surfaces (114,164-177). 

FTIR spectra were measined with a IFS-113v Bruker Fourier-transform IR spectrometer at 300 K. In Fig. 2 experimental FTIR transmittance spectrum of... 

In addition to the studies cited above, Fourier Transform IR-spectrometers have been used for the study of magnetic excitations since the 1960s. Thus, collective excitations were observed in, among others, FeF2 [91, 92], Fe dithiocarbamate... 

Fig. 4.19. IR spectra for the product (bottom) and fi-CD (top). NEXUS fourier transform IR spectrometer DTGS detector scanning frequency 32 resolution 4cm [35].

Equipment manufacturers now offer Fourier-transform IR spectrometers as standard. These use the multiple-scan technique to analyze about 1 pg of... 

Figure 2 (A) Fourier transform IR spectrometer with (B) Spectratech IR plan microscope attachment. (C) Infrared spectrum from a singie poiyester fiber measured by using the Spectratech iR pian microscope.

The complete set of the three normal vibrations (0-0 stretch), vg (bend), and vg (0-F stretch), has only been observed by IR absorption of the radical isolated in various matrices. In the gas phase, the band center (at 1489 cm" ) was found with a Fourier transform IR spectrometer [2]. Only a small section of the band around 1480 cm" was observed by tuning the Zeeman splittings of vibration-rotation transitions to selected lines of a CO laser (laser magnetic resonance, LMR) [1]. Vg = 579.31839 0.00035 cm (band origin) was measured with a tunable IR diode laser and using Zeeman modulation of absorption lines [3]. 

The biggest changes that have occurred in recent years have been in instrumentation, and the chapter on IR experimental techniques has been revised extensively. We have avoided detailed descriptions of instruments rather we have described in broad terms the basic features of instrumentation. These include discussions of interferometers that are used in Fourier transform IR spectrometers, and discussions of grating monochromators that are used in dispersive IR spectrometers. Infrared spectra can be run on gases, liquids, or solids most of the techniques used in running these samples are discussed. Solid sampling techniques are the most diversified. Sections are included on quantitative analysis, internal reflection spectroscopy, and the use of polarized IR radiation. 

Most spectroscopical analysis is a data intensive process. A single sample scan across a wavelength region of interest can produce a two- or three-dimensional set of data containing 2000 or 3000 data points. A few dozen scans of that sample can yield a data set that was quite cumbersome to handle just a few years ago. It is essential to acknowledge that all forms of spectroscopy and spectrometry have been greatly enhanced by the development of personal computers. When Fourier transform IR spectrometers were first commercialized they filled a room. Today they can fit into a suitcase and be operated in the heart of the wilderness. To be sure, there are trade-offs to be made in instrument performance within these extremes, but computer miniaturization has played a significant part in this development. 

With beam condensers and Fourier transform IR spectrometers or GC-IR combinations, spectra can be obtained from less than 1 ng (e. g. 44). IR spectra are useful in identifying certain functional groups such as carbonyl or hydroxyl the band at about 970cm is especially useful for confirming the -configuration of the carbon-carbon double bonds. 

Infrared (IR) spectrum of adsorbed carbon monoxide was recorded with a JASCO FT/IR-3 Fourier-transform IR spectrometer. The construction of the vacuum IR cell used for the measurements was similar to that reported by Peri and Hannan (ref. 3). The sample was pressed into a thin self-supporting wafer and pretreated in the cell. After reduction with hydrogen at 673 K for 2 h, the sample was evacuated at the same temperature for 30 min and the temperature of the sample was lowered to room temperature for adsorption of carbon monoxide. The adsorption was carried out at a constant pressure of 40 Torr for 15 min at room temperature. Carbon monoxide in the gas phase was evacuated at room temperature for 5 min before IR measurement. The spectrum taken before carbon monoxide adsorption was used as the background spectrum. 

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