Spark stand

The length of the instrument from the slit to the end of the plate holder is about 1.2 metres, and it is supported on a massive base which raises the optical parts about 30 cm above bench level. An optical bar of steel is attached to the base of the instrument, from which it projects about 90 cm it is parallel with the optical axis. The bar serves to carry lenses, an arc and spark stand (Gramont stand) for holding samples, and other ancillary equipment. 

As a number of elements have their most sensitive lines at VUV wavelengths, such as N, O, P, S, Cl and H, access to the VUV region is very important, as is possible through the use of sealed spectrometers with air-tight flushed spark stands. For elements that are highly relevant metallurgically, such as C, O, H and N, efforts to lower the blank values due to the environment gases are still required to be able to replace classical C, H, N, O analyzers based on sample combustion. 

The Wadsworth optical system is used in several commercially available spectrometers with a variety of focal lengths. A 1.5-m Wadsworth spectrometer produced by the Jarrell-Ash Division, Fisher Scientific Company is shown in Figure 4-17 along with an excitation source and arc-spark stand. Photographic film recording of spectra is used. With a grating of 590 lines/mm a linear reciprocal dispersion of 10.8 A/mm is obtained in the first order. 

Many spectrometers are available as packaged units. For example, small emission spectrographs are available as complete units including power source, arc-spark stand, etc. Those units used as direct readers include... 

Modern arc and spark stands are enclosed systems constructed so fumes and volatile substances enter an exhaust system and are withdrawn from the laboratory. Electrical connections are shielded, doors have safety interlocks, a focusing lens usually is part of the arc-spark stand, electrodes can be positioned laterally and horizontally by external means, and electrical, gas, air, and water facilities usually are available within the excitation enclosure. 

FIGURE 5-15. Photograph of an arc-spark stand for emission spectroscopy. [Courtesy Spex Industries, Inc.]... 

Arc-spark stands are designed to accept carbon or metal rod electrodes. The special devices that have been developed to aid with some spectroscopic problems include the following. 

A Retry stand is designed to sample a flat surface spectroscopically. This design is especially useful to study metal or metal alloy surfaces. The counter electrode usually is a sharp, pointed graphite or carbon rod and spark excitation is most common. Some Retry stands have a motor-driven turntable so the flat sample can be rotated during excitation. The system also may include provision to flow a gas across the analytical gap. Retry stands are also constructed so they may be mounted in the electrode holder jaws of an arc-spark stand. 

Several other devices to use with arc-spark stands are available but are not so frequently used as the three that have been described. Some of these include controlled-atmosphere chambers, briquette holders, rotating platform assemblies, and water-cooled electrode holders. These devices are available and are very useful for special applications. 

Of particular concern is the safe removal of the vaporized products of excitation. An enclosed arc-spark stand is mandatory, as is an efficient exhaust system. The exhaust system should be constructed so air can be removed but the particulate matter retained in a filter or some other collection device. 

Spark sources produce lower average temperature than arcs, but the local temperature can be as high as 40 000 K. Like arc sources, sparks produce atomic lines, but also more pronounced lines for ions, which are known as spark lines. The emission source consists of a sparking stand and a spark generator. A spark forms between the cathode and the sample (which acts as the anode). The adjustable gap... 

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