Arc and Spark Excitation

Winge R. K., DeKalb E. L. and Fassel V. A. (1985) Comparative complexity of emission spectra from ICP, DC arc and spark excitation sources, Appl Spectrosc 39 673—676. 

Plsko E (1988) Analysis of water using optical emission spectrometry with arc and spark excitation. In Butler LRP and Strasheim A, section editors. Atomic-, mass-, X-ray-spectrometric methods, electron paramagnetic and luminescence methods. In West TS and Niimberg HW, eds. The determination of trace metals in natural waters. 

There are different techniques in atomic emission spectroscopy that are based upon the types of excitation and detection used. Under this heading arc and spark excitation and photographic and multiphotometric detection will be discussed. Flame photometry although by principle belonging to this group will be discussed together with atomic absorption spectrometry. 

The relative accuracy and precision obtained by arc and spark emission spectroscopy is commonly about 5%, but may be as poor as 20-30%. Arc emission is much more prone to matrix effects than spark emission due to the lower temperature of the discharge. Both arc and spark excitation may require matrix matching of sample and standards for accurate analyses, and usually require the use of an internal standard. 

Emission spectroscopy with arc and spark excitation has been used since the 1930s for many industrial analyses. In metaUurgy, for example, the presence in iron and steel of the elements nickel, chromium, sihcon, manganese, molybdenum, copper, aluminum, arsenic, tin, cobalt, vanadium, lead, titanium, phosphoms, and bismuth have been determined on a routine basis. Modem instruments can also measure oxygen, nitrogen, and carbon in metals, which used to require separate measurements with dedicated high-temperature... 

Arc and spark excitation sources are still widely used in the analysis of solid materials by emission spectroscopy, especially if the solids are difficult to dissolve or rapid analysis for quality control and production is required. They are still the mainstay of analysis in foundries, where samples are easily cast into electrodes and the range of compositions analyzed is well known. 

The early use of a flame as an excitation source for analytical emission spectroscopy dates back to HerscheF and Talbot, who identified alkali metals by flame excitation. The work of Kirchhoff and Bunsen also was basic to the establishment of this technique of atomic excitation. One of the earliest uses of flame excitation was for the determination of sodium in plant ash (1873) by Champion, Pellet, and Grenier.Thus use of the flame paralleled that of arc and spark excitation in the 1800 s. 

Analysis of rare earth mixtures 4.1. Arc and spark excitation... 

Prior to the use of plasma excitation, arc and spark sources were used on multichannel spectrometers, the so-called direct-reading instruments. 

Principles and Characteristics Arc and spark discharges have widely been used as excitation sources for qualitative and quantitative emission spectrometry since the 1920s commercial instruments became available during the 1940s. 

Spark sources are especially important for metal analysis. To date, medium-voltage sparks (0.5-1 kV) often at high frequencies (1 kHz and more), are used under an argon atmosphere. Spark analyses can be performed in less than 30 s. For accurate analyses, extensive sets of calibration samples must be used, and mathematical procedures may be helpful so as to perform corrections for matrix interferences. In arc and spark emission spectrometry, the spectral lines used are situated in the UV (180-380nm), VIS (380-550nm) and VUV (<180 nm) regions. Atomic emission spectrometry with spark excitation is a standard method for production and product control in the metal industry. 

Compared to flame excitation, random fluctuations in the intensity of emitted radiation from samples excited by arc and spark discharges are considerable. For this reason instantaneous measurements are not sufficiently reliable for analytical purposes and it is necessary to measure integrated intensities over periods of up to several minutes. Modern instruments will be computer controlled and fitted with VDUs. Computer-based data handling will enable qualitative analysis by sequential examination of the spectrum for elemental lines. Peak integration may be used for quantitative analysis and peak overlay routines for comparisons with standard spectra, detection of interferences and their correction (Figure 8.4). Alternatively an instrument fitted with a poly-chromator and which has a number of fixed channels (ca. 30) enables simultaneous measurements to be made. Such instruments are called direct reading spectrometers. 

Emission spectrometry (ES). Emission spectrometry is based on the excitation of an element to an upper electronically excited state, from which it returns to the ground state by the emission of radiation. As discussed in Chapter 3, the wavelength emitted is characteristic of the emitted species, and, under the approximate conditions, the emission intensity is proportional to its concentration. Means of excitation include arcs and sparks, plasma jets (see ICP), and lasers. 

For spectra corresponding to transitions from excited levels, line intensities depend on the mode of production of the spectra, therefore, in such cases the general expressions for moments cannot be found. These moments become purely atomic quantities if the excited states of the electronic configuration considered are equally populated (level populations are proportional to their statistical weights). This is close to physical conditions in high temperature plasmas, in arcs and sparks, also when levels are populated by the cascade of elementary processes or even by one process obeying non-strict selection rules. The distribution of oscillator strengths is also excitation-independent. In all these cases spectral moments become purely atomic quantities. If, for local thermodynamic equilibrium, the Boltzmann factor can be expanded in a series of powers (AE/kT)n (this means the condition AE < kT), then the spectral moments are also expanded in a series of purely atomic moments. 

Atomic emission spectrometry (AES) is also called optical emission spectrometry (OES). It is the oldest atomic spectrometric multielement method which originally involved the use of flame, electric arc or spark excitation. Recently there has been considerable innovation in new sources plasma sources and discharges under reduced pressure. Littlejohn et al. (1991) have reviewed recent advances in the field of atomic emission spectrometry, including fundamental processes and instrumentation. 

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. 

In addition, spectral line tables, in which the wavelengths of the spectral lines together with their excitation energy and a number indicating their relative intensity for a certain radiation source are tabulated, are very useful. They are available for different sources, such as arc and spark sources [330-332], but also in a much less complete form for newer radiation sources such as glow discharges [333] and inductively coupled plasmas [334],... 

Apart from the high power of detection, also the realization of the highest analytical accuracy is very important. This relates to the freedom of interferences. Whereas the interferences stemming from influences of the sample constituents on the sample introduction or on the volatilization, ionization and excitation in the radiation source differ widely from one source to another, most sources emit line-rich spectra and thus the risks for spectral interferences in AES are high. In the wavelength range 200-400 nm, as an example, only for arc and spark sources have more than 200 000 spectral lines yet been identified with respect to wavelength and element in the classical MIT Tables. Consequently, spectral interferences are much more severe than in AAS or AFS work. 

For compact solids arc and spark ablation are a viable approach for metals [100, 214]. Aerosols with particle sizes at the pm level [216] and detection limits at the pg/g level are obtained [213]. Owing to the separate ablation and excitation stages, matrix influences are particularly low, as shown for aluminum [100] and for steel samples [213], In the first case, only for supereutectic silicon concentrations were matrix effects obtained (Fig. 100). For low-alloyed samples straight calibration curves are obtained and in the case of high-alloyed steels, even samples with widely different Cr or Ni contents are on the same calibration curves, which, are in fact slightly curved. 

Not only is there a need for the characterization of raw bulk materials but also the requirement for process controled industrial production introduced new demands. This was particularly the case in the metals industry, where production of steel became dependent on the speed with which the composition of the molten steel during converter processes could be controlled. After World War 11 this task was efficiently dealt with by atomic spectrometry, where the development and knowledge gained about suitable electrical discharges for this task fostered the growth of atomic spectrometry. Indeed, arcs and sparks were soon shown to be of use for analyte ablation and excitation of solid materials. The arc thus became a standard tool for the semi-quantitative analysis of powdered samples whereas spark emission spectrometry became a decisive technique for the direct analysis of metal samples. Other reduced pressure discharges, as known from atomic physics, had been shown to be powerful radiation sources and the same developments could be observed as reliable laser sources become available. Both were found to offer special advantages particularly for materials characterization. 

Traditional excitation sources included combustion flames, arcs, and sparks. Flames are limited by relatively low temperatures so that it is difficult to analyze refractory elements or elements with high excitation energies, particularly at low concentrations. In addition, combustion products and flame gases cause both chemical and spectral interferences. Arcs and sparks are capable of higher temperatures, but are strongly affected by the nature of the sample. Minor variations in sample composition can cause variation in the excitation conditions, requiring a close matching of samples and standards or the use of an internal standard. 

---