Atomization in flames

Precision For absorbances greater than 0.1-0.2, the relative standard deviation for atomic absorption is 0.3-1% for flame atomization, and 1-5% for electrothermal atomization. The principal limitation is the variation in the concentration of free-analyte atoms resulting from a nonuniform rate of aspiration, nebulization, and atomization in flame atomizers, and the consistency with which the sample is heated during electrothermal atomization. 

Spectral interferences in AAS arise mainly from overlap between the frequencies of a selected resonance line with lines emitted by some other element this arises because in practice a chosen line has in fact a finite bandwidth . Since in fact the line width of an absorption line is about 0.005 nm, only a few cases of spectral overlap between the emitted lines of a hollow cathode lamp and the absorption lines of metal atoms in flames have been reported. Table 21.3 includes some typical examples of spectral interferences which have been observed.47-50 However, most of these data relate to relatively minor resonance lines and the only interferences which occur with preferred resonance lines are with copper where europium at a concentration of about 150mgL 1 would interfere, and mercury where concentrations of cobalt higher than 200 mg L 1 would cause interference. 

C. Pyroluminescence Emission from metal atoms in flames Produced from structural rearrangements in solids... 

In general, color emitters used as components of pyrolants are metallic compounds rather than metal particles. Metal particles agglomerate to form liquid metal droplets and liberation of metal atoms in flames occurs only at the surface of the droplets. On the other hand, metallic compounds decompose at relahvely low temperatures compared with metal particles and liberate dispersed metal atoms. Table 12.5 shows typical salts used to obtain emissions of the requisite colors. 

Two-Photon Ionization Spectrometry of Alkali Atoms in Flames 49... 

What Really Does Happen to Electronically Excited Atoms in Flames ... 

Fig. 32. Computed fluxes of hydrogen atoms in flame of Fig. 25. (a) Convective flux, Afiun (see eqn. (64)) (b) ordinary diffusions flux, j (c) thermal diffusional flux,yjf (d) overall flux, MGyf.

Smyth, K. C. et al. What really does happen to electronically excited atoms in flames in Laser Probes for Combustion Chemistry (ed. Crosley, D. R.), ACS Symp. Ser. 134, p. 175, Washington, D.C., Amer. Chem. Soc. 1980... 

AAS is split into two types according to the method by which the sample is atomized. In flame atomic absorption spectrometry (FAAS) the sample is aspirated into a flame which is placed in the path ofthe light. In electrothermal atomic absorption (EAAS) the sample is placed in a graphite tube and heated in a brief pulse by passing an electric current through the tube. Generally, EAAS is more sensitive, giving better detection limits, but suffers from more matrix interference effects than FAAS. 

Prior steps to the stage of atomization in flame are the treatment of the sample, dissolving it in a convenient matrix, and the stage of pneumatic nebulisation. In FAAS the stage of atomization is performed in flame. The temperature of the flame is determined by the fuel/oxidant coefficient. The optimum temperatures depend on the excitation and ionization potentials of the analyte. 

For many elements the atomization efficiency, defined as the ratio of the number of atoms to the total number of analyte species, atoms, ions, and molecules in the flame, is 1, although for other elements (e.g., the lanthanoids) it is less than 1, even in a nitrous oxide-acetylene flame. However, the formation of atoms is not the end of the story since once formed they may be lost through compound formation or ionization. Ionization increases exponentially with temperature and is a particular problem for the elements on the left of the periodic table, i.e., the alkali and the alkaline-earth elements. It is also a problem with Al, Ga, In, Sc, Ti, and T1 in the nitrous oxide-acetylene flame. A summary of atomization in flames is presented in Figure 2. 

R.P. Lucht, J.P. Salmon, G.B. King, D.W. Sweeney, N.M. Laurendeau Two-photon-excited fluorescence measurement of hydrogen atoms in flames. Opt. Lett. 8, 365 (1983)... 

Atom production in flames is an extremely complex process consisting of many stages and involving numerous sides processes. A quantitative theory of analyte atomization in flames is absent. Qualitatively, the process can be described as follows the solution droplets sprayed into the flame are first dried, the resulting solid microparticles become molten and vaporize or thermally decompose to produce gaseous molecules that are finally dissociated into free atoms. The atoms may further be excited and ionized and form new compounds. These processes are dependent upon the temperature and the reducing power of the flame and occur within a few milliseconds - the time required by the sample to pass through the... 

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