Plasma excitation sources

Atomization and Excitation Atomic emission requires a means for converting an analyte in solid, liquid, or solution form to a free gaseous atom. The same source of thermal energy usually serves as the excitation source. The most common methods are flames and plasmas, both of which are useful for liquid or solution samples. Solid samples may be analyzed by dissolving in solution and using a flame or plasma atomizer. 

Choice of Atomization and Excitation Source Except for the alkali metals, detection limits when using an ICP are significantly better than those obtained with flame emission (Table 10.14). Plasmas also are subject to fewer spectral and chemical interferences. For these reasons a plasma emission source is usually the better choice. 

Sensitivity Sensitivity in flame atomic emission is strongly influenced by the temperature of the excitation source and the composition of the sample matrix. Normally, sensitivity is optimized by aspirating a standard solution and adjusting the flame s composition and the height from which emission is monitored until the emission intensity is maximized. Chemical interferences, when present, decrease the sensitivity of the analysis. With plasma emission, sensitivity is less influenced by the sample matrix. In some cases, for example, a plasma calibration curve prepared using standards in a matrix of distilled water can be used for samples with more complex matrices. 

New developments are, however, needed to make a major step forward in the field of speciation analysis. The first part, isolation and separation of species, may be the easiest one to tackle. For the second part, the measurement of the trace element, a major improvement in sensitivity is needed. As the concentration of the different species lies far below that of the total concentration (species often occur at a mere ng/1 level and below), it looks like existing methods will never be able to cope with the new demands. A new physical principle will have to be explored, away from absorption spectrometry, emission spectrometry, mass spectrometry, and/or more powerful excitation sources than flame, arc or plasma will have to be developed. The goal is to develop routine analytical set-ups with sensitivities that are three to six orders of magnitude lower than achieved hitherto. 

The DC plasma was introduced as an excitation source for atomic emission spectrometry by Margoshes and Scribner [721] and Korolev and Vainshtein [722], Modified designs have been characterised by a number of other authors [614,719-729]. Commercial equipment is now available from several manufacturers. The principle of the plasma torch arrangement used in these instruments is illustrated in Fig. 5.21 [730]. 

The outline of the construction of a typical plasma emission spectrometer is to be seen in Figure 8.10. The example shown has an inductively coupled plasma, excitation source, but the outline would be similar were a dc source to be fitted. Different combinations of prisms and diffraction gratings may be used in the dispersion of the emitted radiation, and in the presentation of the analytical signal. Instruments are computerized in operation and make use of automatic sample handling. Sophisticated data handling packages are employed routinely to deal with interferences, and to provide for clarity in data output. 

Besides flame AA and graphite furnace AA, there is a third atomic spectroscopic technique that enjoys widespread use. It is called inductively coupled plasma spectroscopy. Unlike flame AA and graphite furnace AA, the ICP technique measures the emissions from an atomization/ionization/excitation source rather than the absorption of a light beam passing through an atomizer. 

In the past, flames used for atomic absorption spectrometry have also been used for atomic emission spectrometry, and these are described in some detail in Chapter 2. However, the advent of plasma excitation sources has resulted in the demise of flame atomic emission spectrometry, for the reasons discussed in Section 4.2.3. 

In the past, much atomic emission work has been performed on atomic absorption instruments which use a flame as the excitation source. However, these have been surpassed by instruments which utilise a high-temperature plasma as the excitation source, owing to their high sensitivity and increased linear dynamic range. 

Based on the configurations in Figure 1.5, many analytical techniques have been developed employing different atomisation/excitation sources. For example, two powerful AAS techniques are widespread one uses the flame as atomiser (FAAS) whereas the other is based on electrothermal atomisation (ETAAS) in a graphite furnace. Although the flame has limited application in OES, many other analytical emission techniques have evolved in recent decades based on dilTerent atomisation/excitation plasma sources. 

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). 

Ionization Methods/Processes. The recent development of several new ionization methods in mass spectrometry has significantly improved the capability for the analysis of nonvolatile and thermally labile molecules [18-23]. Several of these methods (e.g., field desorption (FD), Californiun-252 plasma desorption (PD), fast heavy ion induced desorption (FHIID), laser-desorption (LD), SIMS, and fast atom bombardment (FAB) or liquid SIMS) desorb and ionize molecules directly from the solid state, thereby reducing the chance of thermal degradation. Although these methods employ fundamentally different excitation sources, similarities in their mass spectra, such as, the appearance of protonated, deprotonated, and/or cationized molecular ions, suggest a related ionization process. 

After GC-AAS the GC coupled with microwave-induced plasma (GC-MIP) spectrometer is probably the most widely investigated hybrid system for speciation. The MIP is a low-power excitation source for emission spectrometry. In this... 

In AAS, the excitation source inert gas emission offers a potential background spectral interference. The most common inert gases used in hollow cathode lamps are Ne and Ar. The data taken for this table and the other tables in this book on lamp spectra are from HCLs however, electrodeless discharge lamps emit very similar spectra. The emission spectra for Ne and Ar HCLs and close lines that must be resolved for accurate analytical results are provided in the following four tables. This information was obtained for HCLs and flame atom cells and should not be considered with respect to plasma sources. In the Type column, I indicates that the transition originates from an atomic species and II indicates a singly ionized species. 

The emission spectra are similar but often not identical to those excited by UV (18). The energy source is the recombination of free radicals that occurs in the flame and thus flame-excited luminescence is the same as the radical recombination luminescence observed when free neutral radicals from plasmas are used as an excitation source. A simple hydrogen diffusion flame is the simplest source for demonstrating the phenomenon. 

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. 

When the image of elongated excitation sources, e.g., arcs, flames, plasmas, etc., is focused along the slit height of a polychromator, the spatial intensity information (vertical axis) is accurately relayed to the exit focal plane, concurrently with the horizontal spectral dispersion. Thus, by (electronically) dividing the target into a few tens of tracks, the entire spectral profile of these sources can be simultaneously observed and quantitatively studied. 

Plasma source. The excitation source used in this work is an argon supported, dc plasma source (Spectrametrics, Inc.,... 

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