Grating monochromator spectroscopy

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). 

A dispersive element for spectral analysis of PL. This may be as simple as a filter, but it is usually a scanning grating monochromator. For excitation spectroscopy or in the presence of much scattered light, a double or triple monochromator (as used in Raman scattering) may be required. 

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

The besl isolation of radiant energy can he achieved with flame spectrometers that incorporate either a prism sir grating monochromator, those with prisms having variable gauged entrance and exii slits. Both these spectrometers provide a continuous selection of wavelengths with resolving power sufficient lo separate completely most of the easily excited emission lines, and afford freedom from scattered radiation sufficient lo minimize interferences. Fused silica or quartz optical components are necessary to permit measurements in Ihe ultraviolet portion of the spectrum below 350 nanometers Sec also Analysis (Chemical) Atomic Spectroscopy Photometers and Spectra Instruments. 

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 dispersion of the refractive index of alkali halides and of other materials at energies above ujq has been used to produce reasonable monochromatic radiation. In prism monochromators, a parallel beam of polychromatic radiation incident on a prism made from these materials is dispersed, with angular deviations depending on the dispersion uj2dn/duj of the refractive index with the photon energies. Before the advent of grating monochromators and Fourier transform spectrometers, the prism monochromators were widely used in optical spectroscopy and they are still used for specific experiments. 

Presently, grating monochromators are used every time a sample must be illuminated with quasi-monochromatic radiations that are tunable in a broad spectral range or for experiments in the visible-UV range. Another interest of dispersive monochromators is the possibility of wavelength modulation of the output of these monochromators in order to get the first derivative of the transmission spectrum. This has the advantage of increasing the sensitivity, and this technique is also used in laser spectroscopy. 

Fig. 6. Experimental arrangement for photocurrent spectroscopy. C, chopper E, electrode F, optical filters I, interface L, tungsten or xenon lamp M, grating monochromator PC, microcomputer with peripheral hardware PSD, phase sensitive detector P, potentiostat WG, waveform generator.

In contrast to the well-known difficulties of traditionally applied quantitative IR spectroscopy of mixtures in solid (powdered) samples, the near-infrared reflectance analysis (NIRA) technique [32] has gained importance over the last decade and can now be implemented on a variety of commercially available Instruments In a number of applications to Industrial, agricultural and pharmaceutical analyses. Both the NIRA instruments equipped with grating monochromators and those fitted with filter systems feature built—In microprocessors with software suited to the Intrinsic characteristics of this spectroscopic alternative. Filter Instruments generate raw optical data for only a few wave-... 

Recall that Raman spectroscopy produces a spectrum of a sample by frequency analysis of the light scattered from a sample. The sample is excited by an intense, monochromatic light-source such as a laser, and the frequency is analyzed with a grating monochromator. Table 8.3 lists some lasers and their characteristics. 

The electron affinities listed in the table below agree within the error limits given. They were obtained by laser photoelectron spectroscopy (LPES) on the PHg ion [19] and PH2 photodetachment using a tunable laser (LPD) [20, 21] or an Xe arc lamp (with a grating monochromator) [21] and ion cyclotron resonance (ICR) spectrometry [20, 21]. All values are reported in two reviews on electron affinities [22] and electron photodetachment [23], and may be considered as adiabatic (see the remarks below the table) ... 

Figure 2.1 Schematic of the optical path of a double-beam infrared spectrometer with a grating monochromator. Reproduced from Brittain, E. F. H., George, W. O. and Wells, C. H. J., Introduction to Molecular Spectroscopy, Academic Press, London, Copsnight (1975), with permission from Elsevier.

For a more detailed discussion of special designs of grating monochromators, such as the concave gratings used in VUV spectroscopy, the reader is referred to the literature on this subject [4.12. 17]. An excellent account of the production and design of ruled gratings can be found in [4.12]. 

Three major advances have moved the technique closer to equal utility with chromatographic methods for release testing. The first advance was the development of suitable hardware. While useful for food and agricultural products, early interference-type filter instruments and scanning grating monochromators were neither sensitive nor quiet (instrument noise, not operating noise) enough for certain pharmaceutical applications. The development of stable, sensitive, and precise instruments has obviated the inherent problems with reflectance spectroscopy. 

Fig. 14.10 Schematic of modular components for emission spectroscopy. The standard fluorimeter arrangement is a Xe arc lamp as source single grating monochromators for excitation and emission selection and analogue or photon counting PMT as detector...

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. 

It must be emphasized that IR spectroscopy has undergone major improvements in terms of light sources, detectors, and data systems, and particularly in developments of the Fourier transform technique which utilizes interferometers instead of salt prisms and grating monochromators. Modern commercial spectrometers operating with the aid of a Michelson interferometer produce interfero-grams which, upon mathematical decoding by means of Fourier transformation, deliver simultaneously whole absorption spectra that are commonly referred to as Fourier transform infrared FTIR) spectra. 

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