The Direct Current Arc

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. 

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It is worthwhile noting that the arc-gap temperature in this case is considerably lower than the direct-current arc, due to the stop-and start nature of the source, which ultimately offers a much lower sensitivity. 

In addition to fullerenes, many other carbon structures formed of six-, five- and seven-membered rings (predominantly six) are likely to be discovered. Special mention must be made of carbon nanotubes formed in the direct-current arc evaporation of graphite (lijima, 1991). The nanotubes are essentially made up of graphite sheets and have an inner core of around 1 nm with a variable number of graphite sheaths (Fig. 

FIGURE 8-3. Effect of potassium on the spectral line density of phosphorus. [From W. G. Schrenk and H. E. Clements, Influence of Certain Elements on Line Intensity in the Direct Current Arc, Anal. Chem., 23, 1467 (1951). Used by permission of the American Chemical Society.]... 

Qualitative and semiquantitative analysis of glasses having complex composition is performed by atomic emission spectrometry (AES), with the excitation of the spectrum in the direct-current arc. This implementation of AES does not require dissolving the sample and enables one to detect 10-15 elements simultaneously with a detection limit of 10 -10. ... 

Direct-current arcs into which no material is introduced have many appHcations as heat sources. Industrial processing of metals using plasma torches has been carried out in the former USSR (126). Thermal plasmas also are used in surface and heat treatment of materials (127,128). Metals can be... 

Direct Current Arc It is considered to be one of the most versatile excitation modes used extensively for quantitative spectrochemical emission analysis. Figure 24.2 represents the different essential components of the circuit for a direct current are... 

Electro-Pyrolysis, Inc. (EPI) has developed the direct current (DC) graphite arc furnace vitrification technology for the ex situ treatment of wastes. The arc furnace can be operated as an oxidation or reduction process. The vendor states that DC arc melter treatment produces a leach-resistant solid and reduces the volume of wastes that require disposal. 

Apparatus. Plasma Jet. A section of the plasma arc gun is shown in Figure 1. A direct current arc was struck between the thoriated tungsten rod, which acted as the cathode, and the copper tube anode. Both electrodes were... 

Winge, R. K., Fassel, V. A. Simultaneous determination of oxygen and nitrogen in refractory metal by the direct current carbon-arc, gas chromatographic technique. Anal. Chem. 37, 67(1965). 

In CG-direct-current plasma (DCP), a direct-current arc is struck between two electrodes as an inert gas sweeps between the electrodes carrying the sample. Carrier gases such as helium, argon, and nitrogen have been used. 

Carbon sulfide telluride was prepared by allowing a direct-current arc to burn between a graphite and a graphite/tellurium electrode. The electrodes were immersed into carbon disulfide kept at 0°3-5. 

Alternating or direct current arcs and spark discharge are common methods of excitation for emission spectroscopic analysis of rare earth elements. Emission spectra of rare earth elements contain a large number of lines. The three arbitrary groups are (i) spectra of La, Eu, Yb, Lu and Y, (ii) more complicated spectra of Sm, Gd and Tm, (iii) even more complicated spectra of Ce, Nd, Pr, Tb, Dy and Er. Rare earths have been analyzed with spectrographs of high resolution and dispersion up to 2 A/mm. Some salient information is presented in Table 1.36. 

Emission spectrography includes an excitation source (in this instance a direct-current arc), an optical unit using a dispersion system to provide monochromatic images of the input slit on its focal surface, and a detection system (in this instance, a photographic emulsion). 

The arc is an electrical discharge that must be triggered by an auxiliary self-ionising discharge. The direct-current electric arc (300 V, 20 A) is created between two solid electrodes. One of the electrodes contains the sample in powder form. This is compacted in a crater hollowed out in the end of the electrode consisting of a graphite rod (see Fig. 3.2). 

Figure 3.2 Direct-current arc (after G.L. Moore, Introduction to Inductively Coupled Plasma. Atomic Emission Spectrometry, p. 35. C 1989, with the permission of Elsevier Science, Amsterdam).

The elements, arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver, are listed as contaminants for characteristics of toxicity by the EPA [2J. Chromium, mercury, and selenium were not detected in the phosphogypsum. The detection limits for direct-current arc emission spectrograf ic analysis are 0.001% for chromium, 0.05% for mercury, and 0.10% for selenium. Barium, cadmium, lead, and silver were detected at concentrations far less than allowable by EPA requirements, even assuming that 100% of these elements would be... 

The decomposition is accomplished using electrically heated filaments, microwave plasma discharge, or direct-current arc discharge. Polycrystalline diamond is deposited as a thin, hard film. 

A number of electrical excitation-sources are available for emission spectroscopy. In most commercial spectrochemical instruments, more than one excitation source is contained in a single power-supply cabinet a typical combination may include a spark, a direct-current arc, and an alternating-current arc. A list of the various electrical excitation-sources, some of their characteristics, their approximate cost and the types of samples generally required is given in Table 11.1. Because of the actual or potential widespread use in emission spectroscopy, only the arc, spark, and inductively coupled plasma discharges will be described here in detail. 

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