Ebert monochromator

Figure 10. Experimental set-up for Raman spectroscopy with the evanescent wave. For excitation, an argon ion laser was used ( 488 nm, output power 1.5 W cw). The material of the reflection plate is fused silica or super dense flint glass. Monochromator Ebert mount Jarrell-Ash 30 cm focal length diffraction gratings 150 or 600 g/mm.

The analysis system consisted of a Shimsdzu QC-6A gas chromatograph, a chemically deactivated four-way valve for solvent ventilation, a heated transfer tube interface, a Beenakker-type TM0i0 microwave resonance cavity, and an Ebert-type monochromator (0.5m focal length). 

The radiation of the primary source (a) is lead through the absorption volume (f) and subsequently into the monochromator (h). As a rule the radiation densities are measured with a photomultiplier (i) and the measured values are processed electronically. Usually a Czerny-Turner or an Ebert monochromator with a low focal length (0.3-0.4 m) and a moderate spectral bandpass (normally not below 0.1 nm) is used. 

Figure 9.12 Monochromator gratings, (a) Ebert assembly incorporating a single concave spherical mirror M3. Able to compensate for aberrations this monochromator gives excellent image quality, (b) Of similar design to Ebert the Czerny-Turner assembly contains two spherical concave mirrors Mj and M4. (c) The concave grating in this design permits hoth dispersion and focussing of the radiation. The spectral bandwidth of these monochromators depends upon the width of the entrance and the exit Fj slits, respectively.

The Littrow monochromator has a prism as a splitting system while the Ebert monochromator has a diffraction grid for the same purpose. The principles of the schemes of both monochromators are presented in Figures 2.40 and 2.41, respectively. 

The diverged infrared radiation from the input slit is directed to a parabolic mirror and returned toward the splitting system (prism or gird). Depending on the type of optical principle, the parallel reflected infrared light passes through the prism or split by the diffraction gird. It is then reflected back by a plane mirror at the same parabolic reflector for the Littrow monochromator or at a second parabolic reflector for the Ebert monochromator. After this, the monochromic infrared radiation is directed to the output slit. 

The two most popular monochromator designs are the Littrow and the Ebert-Fastie, shown schematically in Figure 23. The merits of the two systems have been compared for many years for atomic absorption purposes, there seems to be little to choose between them. 

The spectrometers used are adapted either for sequential or simultaneous multi-element measurements. Commonly used grating spectrometers in plasma AES include (i) spectrometers with the Paschen-Runge mount, (ii) echelle spectrometers, (iii) spectrometers with Ebert and Czerny-Turner mounts, (iv) spectrometers with Seya-Namioka mounts, and (v) double monochromators. Also Fourier transform spectrometers may be used in plasma AES. 

In an Ebert monochromator the entrance slit and exit slit are either side of the grating and a single concave spherical mirror is used as a collimating and focusing element (Figure 125). Wavelength scanning and selection of a... 

Monochromator Czerney-Turner or Ebert Echelle, dispersion of A, s/prism, dispersion of diffraction orders... 

The near-infrared (NIR) region of the electromagnetic spectrum was discovered by William Her-schel [8] in 1800. However, it was not until the 1950s that suitable sources and sensitive lead sulfide detectors [9] were available for NIR instrumentation. In 1954, engineers at DuPont developed an NIR process analyzer using the newly discovered lead sulfide detector and an Ebert monochromator for dispersion. The same group developed a workhorse NIR process analyzer using the lead sulfide detectors and optical interference filters [10]. 

Figure 4 Schematics of different monochromator types (A) Ebert, (B) Czerny-Turner, (C) Littrow. From Ebdon L (1998) Introduction to Analytical Atomic Spectrometry. Reproduced by permission of John Wiiey Sons Limited.

A conventional monochromator (either Ebert, Czerny-Turner, Littrow or Echelle) may be used (see AAS sections for details), but some of the more basic instrumentation uses interference filters. These are optical filters that remove large bands of radiation in a nondispersive way. A dispersion element such as a prism or a grating is therefore not required. Only a relatively narrow band of radiation is allowed to pass to the detector. The disadvantage with such devices is that they are not particularly efficient and hence much of the fluoresced light is lost. An alternative development is the multi-reflectance filter. This is shown diagramatically in Figure 3, and has the... 

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