Arc wander

The addition of 5% (w/w) of Specpure barium nitrate to the sample-graphite mixture improved the precision of the spectrometric analysis. Use of a small diameter, thin-walled crater electrode further improved the precision by reducing the amount of space covered by arc wander. 

The evaporation of sample from the electrode and the consumption of the electrode during arcing also cause the arc column to shift its position on the electrode surface. This movement or wander of the arc relative to the optical axis of the spectrograph drastically reduces the precision of arc analysis. The Stallwood jet also aids in reducing arc wander. 

Several authors indicate that modern spectrochemical analysis should be dated with the introduction of the concept of the internal standard by Gerlach in 1925. A variety of difficult to control operating parameters influence spectral line intensity. Among them are arc wandering, arc length, arc current, failure to time the exposure exactly, small errors in sample weight, and small losses of sample in transfer to the electrode. 

It is difficult to decide which of the criteria listed earlier for selection of an internal standard element is most important however, the rate of volatilization certainly is one of the more important. If the distillation rates of the internal standard and the analysis element differ greatly, the reproducibility of data will be adversely affected. If one element appears in the arc soon after the arc is ignited and the other appears later, the arc temperature will differ and variations in electrode spacing, arc current, and arc wandering will occur. Figure 8-2 illustrates volatilization rates of some selected elements. 

Figure 22 Examples of arc wander (b) and (c) show the front and back faces of a full penetration weld (d) is a magnified section of (b) showing presence of slag spots.

B8.6 The equipment and, in particular, the shielding gas nozzle should be kept clean and free of weld spatter. A dirty nozzle adversely influences the gas shielding. This contributes to improper gas flow patterns and arc wandering, which can result in poor weld quality. It may also contribute to excessive electrode consumption. 

Electrodes for d.c. arc. The two electrodes are shown in Fig. 20.9. They are conveniently shaped on a lathe from graphite electrodes (Johnson, Matthey 30 cm long JM 3B, 10 mm diameter JM 4B, 6.5 mm diameter). The maximum depression on the lower electrode is 3 mm the small projection in the centre helps to ensure that the arc passes between it and the upper electrode and does not wander appreciably to the edges of the electrode. A small quantity (about 20 mg) of the alloy or powder is placed on the lower electrode. 

Whereas the dogma of uniform circular motion demanded that the moving planet traverse equal arcs in equal intervals of time, Kepler discovered the new uniformity of equal areas in equal times to be valid for any elhpse, including the circle. By adding an additional focus to planetary orbits the need of epicycles was eliminated and the wandering planets were shown, for the first time, to follow harmonious paths around the sim. This discovery should have destroyed the Ptolemaic system for good. It failed. Not even Kepler s discovery of a new supernova, that remained visible for 17 months, was sufficient to shake the world s faith in a permanent sphere of fixed stars. 

However, these types of plasmajets were prone to several disadvantages. The arcs were unstable and wandered about on the electrode surfaces. The electrodes, eroded by the arc, contributed to spectral emission as did the coolant and carrier gases. The mixing of the sample aerosol was not satisfactory. 

Figure 6 Force-extension of semiflexible filament. If one end of a filament is fixed, the filament tends to wander in a way that can be characterized by the transverse displacement o(x). For a fixed arc length of the filament, thermal fluctuations result in a contraction /xl of the end-to-end distance.

The dc arc excitation is primarily thermal in nature. The temperature in the arc varies across the arc gap and increases as the current increases. Temperatures of 4000-8000°C can be obtained using a dc arc. The dc arc is subject to considerable wandering and thus reproducibility is not as good as with some other excitation sources. Selective volatilization of the sample into the arc also is a problem. The dc arc is very sensitive and can be used to detect very low concentration levels. Spectra contain primarily atom lines although some ion lines are observed. 

Since the dc arc is the most sensitive excitation source, it will be the excitation source of choice if extremely low detection limits are required. It also is the most difficult source to use in terms of reproducibility. The arc column wanders in the arc gap, the gap length changes during arcing, the background usually is high, and selective vaporization of the sample elements into the arc occurs. However, the dc arc is applicable to solid or liquid samples, sample, preparation is a minimum, and the power source is inexpensive. 

Upon arc ignition (Section II. A), the space between the electrodes quickly fills with a diffuse plasma consisting of partially iottized metal vapor. At high currents this plasma expands into the volume surrounding the electrodes and their supports. At low currents the positive electrode collects electron current from the plasma uniformly over its surface. The metal shield or vacuum envelope that surrounds the arc also collects charges from the plasma and metal vapor. At high currents one or more distinct anode spots may appear. These spots always form on the end of the anode, which faces the cathode, in contrast with the cathode spots, which some-times wander off the end of the cathode and move about on the sides of the electrode. The anode spot also tends to remain in one position, in contrast with the mobility of its counterparts on the cathode. 

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