Flame emission excitation process

Even in these cases, over 90% of such atoms are likely to remain in the ground state if cooler flames, e.g. air-propane, are used (Table 8.7). The situation should be contrasted with that encountered in flame photometry which depends on the emission of radiation by the comparatively few excited atoms present in the flame. However, because of fundamental differences between absorption and emission processes it does not follow that atomic absorption is necessarily a more sensitive technique than flame emission. 

The nature of the radiation processes is not fully understood. Ball (10,11), with the aid of a stroboscopic shutter, visually observed cool flames as actual flame fronts moving across the combustion chamber of a motored engine. This was later confirmed by Getz (53). The source of cool flame emission in tube experiments has been attributed to excited formaldehyde by Emeleus (51) and Gaydon (52). Cool flame spectra in engines obtained by Levedahl and Broida (70) and Downs, Street, and Wheeler (35) were reported to be due to excited formaldehyde. The nature of the blue flame spectra has not been fully explored, although some evidence points to carbon monoxide emission (35). 

Atomic absorption spectroscopy (AAS) and flame emission spectroscopy (FES), also called flame photometry, are two analytical measurement methods relying on the spectroscopic processes of excitation and emission. Methods of quantitative analysis only, they are used to measure of around seventy elements (metal or non-metal). Many models of these instruments allow measurements to be conducted by these two techniques although their functioning principles are different. There exists a broad range of applications, as concentrations to the gtg/L (ppb) level can be accessed for certain elements. 

Regarding historical insight and descriptions of principles and fundamentals of flame atomic emission spectrometry, a chapter on flame photometry appeared in the first edition of Treatise on Analytical Chemistry (Vallee and Thiers 1965) covering the flame and burner, photometer/spec-trometer, fundamental discussion of excitation and processes within the flame, cation and anion interferences and handling of analytical samples. In an analogous, expanded, detailed and excellent treatment of EAES in the second edition of the Treatise on Analytical Chemistry, Syty (1981) discusses types of flames used for excitation, processes within flames, spectral, chemical and physical interferences and remedies. 

The flame excitation source of a flame emission spectrometer must fulfill several requirements if it is to be satisfactory. These include the ability to (1) evaporate a liquid droplet sample, (2) vaporize the sample, (3) decompose the compound(s) in the evaporated sample, and (4) spectrally excite the ground state atoms. These processes must occur at a steady rate to achieve a steady emission signal. 

The magnitude of the atomic absorption signal is directly related to the number of ground state atoms in the optical path of the spectrometer. Ground state atoms are produced from the sample material, usually by evaporation of solvent and vaporization of the solid particle followed by decomposition of the molecular species into neutral atoms. Normally these steps are carried out using an aspirator and flame. These are the same processes that are involved in flame emission spectroscopy as described in Chapter 9. When ground state atoms are produced, some excited state atoms also occur and, for easily ionizable elements, some ions and electrons are produced. 

Millikan20 has described an ingenious fluorescence technique for measuring relaxation rates of the CO(t> = 1) molecule. In a flow tube at 5-20cm.sec-1, CO is excited to (o = 1) at the inlet with infrared emission from the CO fundamental (2143 cm-1), a suitably intense source being a CH4(rich)/02 flame. There are two competing processes by which the vibrational excitation can decay... 

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. 

Palmer and his co-workers have observed more complex C2 emission from diffusion flames of alkali metals (Na or K) in haloforms and carbon tetra-halides [160-165], The Swan bands show a much broader v distribution than those emitted from other systems, and although the v = 6 level is sometimes excited preferentially, it is clear that more than one process excites the A 3I1B state in the diffusion flame reactions. Detailed interpretation is difficult and is hampered by a lack of precise thermochemical data for some of the species involved. Tewanson, Naegeli, and Palmer [165] have suggested that three quite distinct mechanisms may cause excitation (i) the association of C... 

Addition of H atoms to the O + Q02 flame gives rise to the CH, C2, and OH emission [176] that is associated with the O + C2H2 reaction. A number of processes could excite CH A A the v = 0 level, which is predominantly filled, lies 66.0 kcal/mole (2.86 eV) above the X 2I1 ground state. The most likely [170, 178, 181] excitation mechanism is the reaction... 

Radiation absorbed by atoms under conditions used in atomic absorption spectrometry may be re-emitted as fluorescence. The fluorescent radiation is characteristic of the atoms which have absorbed the primary radiation and is emitted 1n all directions. It may be monitored in any direction other than in a direct line with radiation from the hollow-cathode lamp which ensures that tha detector will not respond to the primury absorption process nor to unabsorbed radiation from the lamp. The intensity of fluorescent emission is directly proportional to the concentration of the absorbing atoms but it is diminished by collisions between excited atoms and other species within the flame, a process known as quenching. Nitrogen and hydrocarbons enhance quenching, and flames incorporating either should be avoided or their effect modified by dilution with argon. 

Studies of low-pressure flames offer several advantages. In particular, the flame can be maintained flat, and the light from different parts of the reaction zone studied separately the reaction volume from which light is collected is determined with much greater accuracy for such flames. At low pressures, chemiluminescent processes are more important than thermal excitation, collisional quenching of excited species is reduced, and self-absorption is diminished. A typical investigation of the low pressure flame is that of Gaydon and Wolfhard quantitative measurements of the C2 emission were made. 

So far, we have been concerned mainly with emission of radiation from electronically excited states. Emission may also arise from vibrational transitions in various reaction systems. The species HO2 has long been postulated as an important chain carrier in combustion reactions, although emission from electronically excited HO2 has yet to be demonstrated unequivocally. However, Tagirov has observed radiation in flames at a frequency of 1305 cm which he ascribes to transitions from vibrationally excited HO2. Investigations of vibrational quenching processes are of great interest, and if the vibrationally excited species emit infrared radiation, then emission spectrometry may be the most satisfactory way of following the reaction. Davidson et describe a shock-tube study of the relaxation of... 

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