Monochromators infrared spectroscopy

Fig. 19.2 Layout of an infrared spectrophotometer employing a diffraction grating for monochromation. Reproduced by permission from R. C. J. Osland, Principles and Practices of Infrared Spectroscopy, 2nd edn, Philips Ltd, 1985.

Near-infrared Spectroscopy for Process Analytical Technology 119 5.3.2 The scanning grating monochromator and polychromator diode-array... 

E.A. DeThomas and P.J. Brimmer, Monochromators for near-infrared spectroscopy, in Handbook of Vibrational Spectroscopy, J.M. Chalmers and PR. Griffiths (eds), vol 1, John Wiley Sons, New York, 2002. 

DeThomas, F.A. and Brimmer, P.J., Monochromators for Near-Infrared Spectroscopy. In Chalmers, J.M. and Griffiths, P.R. (eds), Handbook of Vibrational Spectroscopy, vol 1 John Wiley 8c Sons New York, 2002, pp. 383-392. 

Between the source and the detector is put either monochromators used in dispersive instruments or interferometers used in Fourier transform infrared (FT-IR) instruments. In a dispersive instrument the intensity at each wavenumber is measured one by one in sequence and only a small spectral range falls on the detector at any one time. In a FT-IR instrument the intensities of all the wavenumbers are measured simultaneously by the detector. Fourier transform infrared spectroscopy offers some advantages compared to dispersive instruments, namely (i) higher signal-to-noise ratios for spectra obtained under conditions of equal measurement time, and (ii) higher accuracy in frequency for spectra recorded over a wide range of frequencies. Therefore we will give below a brief picture of the principle of FT-IR spectroscopy, based on a Michelson interferometer (Fig. 2). 

Detailed experimental procedures for obtaining infrared spectra on humic and fulvic acids have been reported previously 9,22,25-26) and will be briefly described here. Infrared spectra were taken on the size-fractionated samples by using a Fourier transform infrared spectrometer (Mattson, Polaris) with a cooled Hg/Cd/Te detector. Dried humic and fulvic materials were studied by diffuse reflectance infrared spectroscopy (Spectra Tech DRIFT accessory) and reported in K-M units, as well as by transmission absorbance in a KBr pellet. Infrared absorption spectra were obtained directly on the aqueous size-fractioned concentrates with CIR (Spectra Tech CIRCLE accessory). Raman spectra were taken by using an argon ion laser (Spectra-Physics Model 2025-05), a triple-grating monochromator (Spex Triplemate Model 1877), and a photodiode array detector system (Princeton Applied Research Model 1420). All Raman and infrared spectra were taken at 2 cm resolution. 

The powder X-ray diffraction patterns were measured in a D-500 SIEMENS diffractometer with a graphite seeondary beam monochromator and CuKoj contribution was eliminated by the DIFFRAC/AT software to obtain a monochromatic CuKa,. The Unit Cell Size (UCS) was measured following the ASTM D-3942-90 procedure. The Surface areas were measured by nitrogen adsorption at 75 K on a Micromeritics Accusorb 2100 E equipment using the ASTM method D-3663-78. Temperature Programmed Desorption (TPD) of ammonia and pyridine adsorption by Infrared Spectroscopy (IR) were used to characterize the acidity of the zeolites. For IR-Pyridine the spectra were recorded each 100°C and the characteristic bands of Lewis and Bronsted acid sites (1444 cm" and 1540 cm, respectively) were integrated in order to obtain the total acid sites. 

Almost exclusively, only diffraction grids are used as monochromators in fluoremeters. The functioning principle of this device was described in detail in the section in this chapter devoted to infrared spectroscopy. 

McDowell and coworkers (15J studied the high resolution infrared spectrum of UF5 at ambient and low temperatures. This work was followed by a series of vibrational and electronic spectroscopic studies of matrix isolated UFg (16,17,18,19,20). In the first experiments, UFg deposited in Ar or CO matrices was vibrationally characterized by infrared spectroscopy and then exposed to broadband UV radiation at 10°K. In argon, photoreduction proceeded rapidly the 619 cirri UF5 infrared peak decreased in intensity while two new peaks grew in at 584 cirri anc 561 cirri. The new peaks were assigned to the expected UF5 photolysis product and a tentative C4V structure assignment was made. The wavelength dependence of the photoreduction was studied using a monochroma-tized UV source (1 kw Hg-Xe lamp, Schoeffel 6M-250 monochromator). The relative quantum efficiency of the UF5 dissociation per unit absorbance of UFg was found to be relatively constant in the allowed B-X absorption band (250-300 nm) (T7). Radiation in the... 

FT-IR). As in convcniional infrared spectroscopy, FT-IR dclccus absorptions in the infrared region (4000 - 4(X) cm ) but detection involves the use of an interferometer rather than a monochromator. The penetration depth is of the order of a few micrometers. A combination of surface analysis techniques (c.g. XPS, SIMS and FT-IR) is often required to elucidate the chemical sinicture in the top layer. 

Non-dispersive spectrometry is an alternative to the classic, dispersive technique where monochromators are used, as depicted schematically in Fig. 2.67. Non-dispersive techiuques, used mainly in infrared spectroscopy, are based generally on interferometry. The sample is irradiated by polychromatic radiation. After transmitting the sample, the fight forms an interference pattern, which contains aU pieces of information that could be obtained alternatively by dispersive methods. Several types of interferometers can be built in integrated optics having miiuature dimensions. They are useful for chemical sensors. 

Sohd-state multi-element detector arrays in the focal planes of simple grating monochromators can simultaneously monitor several absorption features. These devices were first used for uv—vis spectroscopy. Infrared coverage is limited (see Table 3), but research continues to extend the response to longer wavelengths. Less expensive nir array detectors have been appHed to on-line process instmmentation (125) (see Photodetectors). 

The only X-ray source with sufficient intensity for surface measurements is synchrotron radiation. Synchrotron radiation is white light, including all wavelengths ftom the infrared to X rays. A spectroscopy experiment needs a particular wavelength (photon energy) to be selected with a monochromator and scanned through... 

In the early work of Bewick and Robinson (1975), a simple monochromator system was used. This is called a dispersive spectrometer. In the experiment the electrode potential was modulated between two potentials, one where the adsorbed species was present and the other where it was absent. Because of the thin electrolyte layer, the modulation frequency is limited to a few hertz. This technique is referred to as electrochemically modulated infrared reflectance spectroscopy (EMIRS). The main problem with this technique is that data acquisition time is long. So it is possible for changes to occur on the electrode surface. 

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