Dispersive element Echelle grating

Fig. 1 Optical diagram of the prototype system model. The drawing is not to scale and is not to be considered an optical ray diagram. The principal dispersing element is a coarse echelle ruled grating 20 x 40 cm wide. Theoretical double-pass resolution at four normal slits is approximately 0.095 cm-1 actual achievable resolution is approximately 0.009 cm-1.

Due to its versatility, robustness, and relativel low cost, ICP-AES has started to be considered as routine instrumentation for elemental composition measurements in many laboratories involved in food analysis. Unquestionably, the acceptance of the technique has been further advanced by the commercial availability of rapid, simultaneous, and flexible, sequential instruments based on echelle grating crossed dispersion and solid state detectors. It is expected that novel nebulizer devices for liquid sample introduction will find increased applications in beverage analysis. 

In plasma atomic emission spectrometry the dispersing element mosdy used is a grating. Two types of grating are employed (i) conventional gratings, and (ii) echelle gratings. A fore-prism as an order-sorter is an essential feature for use with an echelle grating. The operation of conventional gratings is discussed in Section 1.6. 

Since spectra of different orders may coincide or overlap, some method of separating orders is required. Separation of orders is achieved by use of a second dispersing element, either a prism or conventional grating, mounted so its dispersion is in a direction perpendicular to that of the echelle. This combination of dispersing elements produces a two-dimensional, spectral pattern. An example of such a pattern is shown in Figure 3-18, which was... 

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... 

Apart from gratings used at low orders, so-called echelle gratings can be used. Their groove density is low (a up to 1/100 mm) but therefore order numbers of up to 70 can be used [50]. Here, the orders overlap and must be separated by using a second dispersive element (e.g. a prism) either with its axis parallel to that of the echelle grating or in so-called crossed-dispersion mode. In the latter case, the spectrum occurs as a number of dots with a height equalling that of the entrance slit. 

In addition, echelle spectrometers are often used [50]. By combination of an order-sorter and an echelle grating either in parallel or in crossed-dispersion mode, high practical resolution (up to 300 000) can be realized with an instrument of fairly low focal length (down to 0.5 m) (Fig. 103). Therefore, the stability as well as the luminosity are high. By using an exit slit mask with a high number of pre-adjusted slits, highly flexible and rapid multi-element determinations are possible. 

To locate the specific radiations of an element to be quantified (the different analytical lines) an optical bench of very high quality is required. These apparatuses can be divided into spectrographs and spectrometers (Figure 14.8 and see also Figure 9.12). As above pointed out, the source illuminates a slit which becomes a luminous object, which will be analysed by a dispersive system containing a grating (planar, concave or echelle). 

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