Electrical Excitation Sources

The prism is meant only to illustrate a dispersive device a diffraction grating is used in all modem spectrometers with a single dispersive device. Echelle spectrometers use two dispersive devices, either a prism and a grating or two gratings. 

The temperature of the arc depends upon the composition of the plasma and varies with the nature of the sample. If the sample is made of material with low ionization energy, the temperature of the plasma will be low if the ionization energy of the material is high, the temperature will be high. In addition, the temperature is not uniform in either the axial or radial directions. This results in matrix effects and self-absorption. Arc temperatures are on the order of 4500 K with a range of 3000-8000 K. Emission spectra from arc sources contain primarily atomic lines with few ion lines. The DC arc can excite more than 70 elements. 

There is a positive aspect to the selective volatility of low-melting elements, in that spectral interferences are likely to be less at the beginning of a burn (low temperature) than at high temperatures when line-rich elements such as Fe start to vaporize. In modern DC arc sources, this problem of selective volatility is addressed by having the polarity of the electrodes reversed for the first minute or so of the burn. With the sample as the cathode, the temperature is lower and the rate of heating lower. This slows the rate of evolution of volatile species and improves sensitivity and precision for the volatile elements. Sensitivity for the nonvolatile elements is somewhat reduced when the sample is the cathode. A common sequence for analysis would consist of a bum of 60 s with the sample as cathode (reverse polarity) at 8-10 A, a 60 s burn of normal polarity (sample as anode) at 8-10 A, and then 60 s at 15 A to volatilize the very refractory elements. 

Element High-Purity Copper High-Purity Copper High-Purity Uranium Oxide 

Copper data The DC arc/CID data were collected using a commercial system no longer in production. Thermo Fisher Scientific (www.thermotisher.com). Used with permission. 

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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. 

DC Arc. The DC arc is the least complex of the electrical excitation-sources discussed here it consists of a low-voltage (10-50 V), high-current (1-35 A) discharge between a sample electrode and a counter electrode. The DC power supply may consist of no more than a full-wave rectifier and a filter. 

The applications of emission spectroscopy with electrical excitation-sources are diverse and extensive. A few examples are selected in this section to illustrate typical analyses. A number of annual and biennial reviews collect and describe new applications as they are published. 

Historically, several types of electrical excitation sources have been used since the early part of the 1900s, among them the DC arc, the AC arc, and the AC spark. Commercial instmments using electrical excitation sources became available about 1940 with PMT detectors prior to this, emission instruments used a photographic plate or film as the detector. Modern instruments are either DC arc emission instruments or high voltage spark... 

Figure 7.13 A Rowland circle polychromator. This configuration is called a Paschen-Runge mount. The source shown is an inductively coupled plasma, hut any of the electrical excitation sources can he substituted. Multiple photomultipler detectors are placed at the appropriate positions on the circumference of the circle to measure dispersed wavelengths. [From Boss and Fredeen, courtesy of PerkinElmer Inc. (www.perkinelmer.com).]...

During the many years that atomic emission spectrometry has been employed for chemical analysis a variety of types of excitation sources have been used. In earlier times electric discharges, dc-arcs and ac-sparks, found considerable favour. The inherent instability of the discharges has meant that as more stable alternatives have been developed they have been progressively replaced by them. Where electrical excitation is still employed it is achieved by an electrically controlled spark with far greater stability and much improved precision for the analysis. 

Luminescence is the emission of light at RT under the influence of various physical agents as mechanical(tribo-l), electrical (electro-1), radiant (photo-1), thermal (thermo-1), or chemical (chemo-1) means. The exciting source also may consist of moving charged particles, such as alpha-, beta-, or gamma-. Certain substances luminesce on crystallization, as,... 

Flames and plasmas can be used as atomisation/excitation sources in OES. Electrically generated plasmas produce flame-like atomisers with significantly higher temperatures and less reactive chemical environments compared with flames. The plasmas are energised with high-frequency electromagnetic fields (radiofrequency or microwave energy) or with direct current. By far the most common plasma used in combination with OES for analytical purposes is the inductively coupled plasma (ICP). 

The source unit must vaporize and excite a portion of the sample, which is generally used as one of the electrodes between which the electric discharge takes place. No single excitation source is ideally suited for all applications of emission spectrochemistry. Trace impurities in metals, alloying constituents in high concentrations, biological substances, ceramics, slags, oils, nonconductors, refractories—all may require different excitation techniques and sample preparation procedures. Table 1 summarizes the important characteristics of the commonly used spectrochemical source units. 

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