Monochromators prism

Today s commercially available chromatogram spectrometers usually employ diffraction gratings for monochromation. These possess the following advantages over prism monochromators which are still employed in the Schoeffel doublebeam spectrodensitometer SD 3000 and in the Zeiss chromatogram spectrometer ... 

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

Figure 2. Experimental set-up for Raman spectroscopy. The desired laser line is isolated from other plasma lines by a narrow bandpass filter or broadband prism monochromator, then focused onto a sample in a capillary tube. A collecting lens placed at a 90° angle to the incident beam focuses the scattered light onto the entrance slit of a monochromator with output to a photomultiplier tube (in the case of a scanning instrument) or a diode array detector.

A diagram of the author s original spectrofluorimeter is shown in Figure 3. The light source, S, was either a 1 kw. compact source mercury vapor lamp, or for the measurement of fluorescence excitation spectra, a 375 w. xenon arc. The required frequency of exciting light was isolated by means of a Hilger 1)247 quartz prism monochromator, Mj, and... 

Fig 5. Spectral sensitivity curves for quartz prism monochromator, (a) With EMI 6256 photomultiplier tube (6) with EMI 9558 Q photomultiplier tube. 

Spectral calibration curves for a t)247 quartz prism monochromator with either an EMI 6256 or 9558 photomultiplier are shown in Figure 5. Most of the spectra to be presented later have not been corrected but... 

Fig. 7. Diagram of spectrophosphorimeter.28 L, light source Mi, Ms, Hilger D 247 quartz prism monochromators Di, D2, chopper discs driven by synchronous motors B, silica plat beam splitter F, 0.5-mm. silica optical cell containing fluorescent screen solution Pi, monitoring photomultiplier P2, fluorescence-phosphorescence photomultiplier Q, fused quartz dewar containing sample cell.

Prism monochromators made of quartz have the great disadvantage of not being very useful below about 2500 A. As one goes to shorter wavelengths the transparency decreases and with it the resolving power. Much quartz also fluoresces in its own right. 

Heitmann et al. [11] designed a very compact double monochromator, consisting of a 300 mm prism pre-monochromator and a 400 mm echelle grating monochromator, both in Littrow mounting, which is shown schematically in Figure 4.3. The prism monochromator selects the part of the spectrum that is of interest, and the echelle monochromator provides the high dispersion of the selected spectral interval, which is better than 2 pm per pixel at 200 nm (see Welz et al. [10]). 

Monochromators (a) Czerny-Turner grating monochromator, (b) Bunsen prism monochromator. 

Monochromator almost all equipment presently used is fitted with grating monochromators prism monochromators are only found in old apparatus. Simple equipment, which usually means a single beam apparatus, is often fitted with a single stage monochromator. Higher specification equipment uses double monochromators, and here the beam is usually split for simulta-... 

Fluorometers and spectrofluorometers are used to measure fluorescence. Operationally, a fluorometer uses interference filters or glass filters to produce monochromatic light for sample excitation and for isolation of fluorescence emission, whereas a spectrofluorometer uses a grating or prism monochromator. 

Figure 25-6 Types of monochromators (a) grating monochromator (b) prism monochromator. The monochromator design in (a) is a Czemy-Turaer design, while the prism monochromator in (b) is a Bunsen design. In both cases,...

One of the advantages of a monochromator with an echellette grating is that in contrast to a prism monochromator, the dispersion of radiation along the focal plane is, for all practical purposes, linear. Figure 25-9 demonstrates this property. The linear dispersion of a grating greatly simplifies the design of monochromators. 

CD spectrometers in the IR region use mirrors instead of lenses. Unlike the ECD dispersive spectrometers that use prism monochromators, the dispersive VCD technique is based on the technology of gratings. Semiconductor cooled detectors are used instead of photomultipliers. 

Although many noble efforts at fabricating a microscope for infrared spectrometry using a prism monochromator were made during the 1940s and 1950s [5-10], and Perkin-Ehner actually advertised a microscope that could be installed in one of... 

An alternative approach recently proposed by Sommer et al. is the use of a single or double-pass prism monochromator rather than a grating system [8], This provided a large bandwidth, up to 3000 cm, at the expense of a reduced spectral resolution, especially in the C—H stretching region. Another recently described approach to extend the spectral coverage was to stack two gratings with a low and... 

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. 

A rather simple microspectrophotometer which permits accurate absorption measurements using a capillary cell with a content of only 10 pi was described by Wallach and Surgenor (Wl). The cells are made from borosilicate glass tubing of 1-mm bore. They are precisely 10 mm long and are closed by quartz disks. These are held in position by capillary forces. A monochromatic light beam 0.4 mm in diameter is obtained from a glass prism monochromator. The cell is positioned... 

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