Flames as atomizers

Describe a typical electrothermal atomizer for atomic absorption spectrometry. Critically compare graphite furnaces, air-acetylene flames, and nitrous oxide flames as atom cells for atomic absorption spectrometry. 

A surprising addition has recently been made to the list of elements which may be usefully determined by vapour generation techniques, namely cadmium.5 Sodium tetraethylborate was used to produce a volatile cadmium species, with citrate being used to mask interference from nickel and copper. Using an argon-diluted hydrogen diffusion flame as atomizer, the detection limit by AFS was 20 ng l-1. 

In time, the use of flames as atom reservoirs for atomic absorption spectrometry was also transformed into an analytical methodology, as a result of the work of Walsh [2], Flame atomic absorption spectrometry became a standard tool of the routine analytical laboratory. Because of the work of L vov and of Massmann, the graphite furnace became popular as an atom reservoir for atomic absorption and gave rise to the widespread use of furnace atomic absorption spectrometry, as offered by many manufacturers and used in analytical laboratories, especially for extreme trace analysis. However, in atomic absorption spectrometry, which is essentially a single-element method, developments due to the multitude of atomic reservoirs and also of primary sources available, is far from the end of its development. Lasers will be shown to give new impetus to atomic absorption work and also to make... 

The use of flames as atomizers for atomic absorption spectrometry (AAS) has also made its impact on analytical methodology, largely as the result of work by Walsh [2]. L vov and Massmann [3], [4] introduced the graphite furnace atomizer, now widely available in commercially available AAS with electrothermal atomization, especially for ultratrace analysis. 

An important question to consider when using a flame as an atomization source, is how to correct for the absorption of radiation by the flame. The products of combustion consist of molecular species that may exhibit broad-band absorption, as well as particulate material that may scatter radiation from the source. If this spectral interference is not corrected, then the intensity of the transmitted radiation decreases. The result is an apparent increase in the sam-... 

A number of chemiluminescent reactions have been studied by producing key reactants through pulsed electric discharge, by microwave dissociation, or by observing the reactions of atoms and free radicals produced in the inner cone of a laminar flame as they diffuse into the flame s cool outer cone (182,183). These are either combination reactions or atom-transfer reactions involving transfer of chlorine (184) or oxygen atoms (181,185—187), the latter giving excited oxides. 

Table 1 lists the temperatures of some commonly used flames for atomic absorption. A cool flame such as argon-hydrogen-entrained air or air-coal gas is usually not preferred because of increased danger of chemical interferences (see below). The most commonly used flame is the air-acetylene flame. 

Te. Instruments based upon the use of a chemical flame as the atom reservoir have not proved to be generally successful. The introduction of the ICP torch renewed interest in atomic fluorescence and new instruments based on the ICP torch as a source of free atoms were constructed. However, these seem to have been only slightly more satisfactory than earlier instruments and have not come into widespread use. Some detection limits are included in Table 8.6. 

For the alkaline-earth metals, as noted earlier, a simple flame of almost any type can be used to excite the metals. However, to be able to determine a wide range of metals, it is common to use either an acetylene-air or acetylene-nitrous oxide flame as the source of energy to excite the atoms. The burner is long with a slot at the top and produces a long narrow flame that is situated end-on to the optics receiving the emitted light. 

On the basis of the preceding discussion, it should be obvious that ultratrace elemental analysis can be performed without any major problems by atomic spectroscopy. A major disadvantage with elemental analysis is that it does not provide information on element speciation. Speciation has major significance since it can define whether the element can become bioavailable. For example, complexed iron will be metabolized more readily than unbound iron and the measure of total iron in the sample will not discriminate between the available and nonavailable forms. There are many other similar examples and analytical procedures that must be developed which will enable elemental speciation to be performed. Liquid chromatographic procedures (either ion-exchange, ion-pair, liquid-solid, or liquid-liquid chromatography) are the best methods to speciate samples since they can separate solutes on the basis of a number of parameters. Chromatographic separation can be used as part of the sample preparation step and the column effluent can be monitored with atomic spectroscopy. This mode of operation combines the excellent separation characteristics with the element selectivity of atomic spectroscopy. AAS with a flame as the atom reservoir or AES with an inductively coupled plasma have been used successfully to speciate various ultratrace elements. 

Flame AA utilizes a large flame as the atomizer. A photograph of this atomizer was first shown in Figure 7.20 and is reproduced here in Figure 9.2. The sample (a solution) is drawn into the flame by a vacuum mechanism that will be described. The atomization occurs immediately. The light beam for the... 

Atomization occurs in a flame as one of several processes after a solution of a metal ion is aspirated. What other processes occur and in what order ... 

Elemental sulfur is found in the flames of all the sulfur-bearing compounds discussed in the previous subsections. Generally, this sulfur appears as atoms or the dimer S2. When pure sulfur is vaporized at low temperatures, the vapor molecules are polymeric and have the formula Sg. Vapor-phase studies of pure sulfur oxidation around 100°C have shown that the oxidation reaction has the characteristics of a chain reaction. It is interesting to note that in the explosive studies the reaction must be stimulated by the introduction of O atoms (spark, ozone) in order for the explosion to proceed. 

A variety of events that will lead to smoke production can occur in the pyrotechnic flame. Incomplete burning of an organic fuel will produce a black, sooty flame (mainly atomic carbon). A highly-oxidized fuel such as a sugar is not likely to produce carbon. Materials such as naphthalene (C loH s) and anthracene ( C i H 101 - volatile solids with high carbon content - are good candidates for soot production. Several mixtures that will produce black smokes are listed in Table 8. 1. 

Chemistry (Brown et al. 1981). Direct aspiration into a flame and atomization in an electrically heated graphite furnace or carbon rod are the two variants of atomic absorption. The latter is sometimes referred to as electrothermal AAS. Typical detection limits for electrothermal AAS are <0.3 pg/L, while the limit for flame AAS and ICP-AES is 3. 0 pg/L (Stoeppler 1984). The precision of analytical techniques for elemental determinations in blood, muscles, and various biological materials has been investigated (Iyengar 1989). Good precision was obtained with flame AAS after preconcentration and separation, electrothermal AAS, and ICP-AES. 

At high flame temperatures, atoms such as K may completely lose an electron thus reducing the observed emission from the sample ... 

Product recovery. ("Caution. Finely divided metals, such as those produced in the metal atom reactions, are often pyrophoric. Therefore, at the end of a reaction the reactor is always vented with an inert gas such as N2. Residual pyrophoric metals may generally be rendered innocuous by the careful introduction of dilute HCl solution under a nitrogen purge. It is to be expected that the electrode assembly will be covered with pyrophoric metal which will glow or flame as the assembly is removed from the reactor. Care should be... 

In AAS, the flame is only required to produce ground-state atoms (cf AES, where a hot flame is preferred as atoms must also be excited). Frequently, an air-acetylene flame is sufficient to do this. For those elements which form more refractory compounds, or where interferences are encountered (see Section 2.4), a nitrous oxide-acetylene flame is preferred. In either case, a slot burner is used (100 mm for air-acetylene, 50 mm for nitrous oxide-acetylene) to increase the path length (this arises from Eqn. 2.3, Section 2.1) and to enable a specific portion of the flame to be viewed. Atoms are not uniformly distributed throughout the flame and, by... 

In a flame, as the concentration of atoms increases, the possibility increases that photons emitted by excited atoms in the hot region in the centre will collide with atoms in the cooler outer region of the flame, and thus be absorbed. This self-absorption effect contributes to the characteristic curvature of atomic emission calibration curves towards the concentration axis, as illustrated in Fig. 4.4. The inductively coupled plasma (ICP) tends to be hotter in the outer regions compared with the centre—a property known as optical thinness—so very little self-absorption occurs, even at high atom concentrations. For this reason, curvature of the calibration curve does not occur until very high atom concentrations are reached, which results in a much greater linear dynamic range. 

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. 

The 500-ml. flask is dried by warming with a soft flame as a slow stream of nitrogen is passed through the system. A solution of ethylmagnesium bromide is prepared in this flask from 12 g. (0.5 g.-atom) of magnesium turnings, 60 g. (0.55 mole) of ethyl bromide (dried over calcium chloride), and 300 ml. of tetrahydro-furan (Notes 1 and 2). 

In addition to heat transfer with the wall there may also be chemical effects such as free-radical quenching. Discuss ways in which the analysis could be further extended to include such behavior. Consider the essential role of free radicals, such as atomic H, in sustaining the flame. 

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