Excited flame measurements, species

Both atomic and molecular emission and absoiption can be measured when a sample is atomized in a flame. A typical flame-emission spectrum was shown in Figure 24-19. Atomic emissions in this spectrum are made up of narrow lines, such as that for sodium at about 330 nm, potassium at approximately 404 nm, and calcium at 423 nm. Atomic spectra are thus called line spectra. Also present are emission bands that result from excitation of molecular species such as MgOH, MgO, CaOH, and OH. Here, vibrational transitions superimposed on electronic transitions produce... 

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. 

Emissions from B02 [103], AsO, and SbO [103, 104] in an FPD flame have been used to detect organics or highly reduced species containing B, As, and Sb, respectively. These metal atoms react to form the same excited-state metal oxides discussed in their reactions with ozone above. These analytes have limits of detection measured to be approximately 50 ppbv, 10 ppbv, and 20 ppbv, respectively [93],... 

Ca brick-red, Sr carmine-red, etc). If a flame can be kept burning uniformly for an extended period of time and material fed into the flame at a const rate, the intensity of the spectral line or band will be a measure of the concn of the substance. The wave length of the emitted light will permit identification of the excited species... 

Comparison of emission spectra between 2100 A and 6500A has shown only small differences in relative concns of excited species between low-pressure diffusion flames and explns, whereas during explns peak intensities may be as much as 100 times greater. The time dependence of the free-radical emission during expln indicates the formation sequence to be OH, CH, C2, and evidence for the forbidden CO Cameron bands has been obtained. Similarly the ultraviolet absorption spectrum of the OH radical in acetylene— H2—02 detonations has been measured in conjunction with the associated rarefaction waves (Ref 7). Analysis of the absorption spectrum has indicated average rotational temps greater than 3000°K during the initial 310 microseconds... 

Experiments using the technique of laser-induced fluorescence (LIF) in flames have provided ample demonstration of its selectivity and sensitivity, and hence of its applicability as a probe for the reactive intermediates present in combustion systems. The relationship between the measured fluorescence intensity and the concentration of the molecule probed, however, must take into account the collisional quenching of the electronically excited state pumped by the laser. Because the flame contains a mixture of species, each with different quenching cross sections, it may be difficult to estimate the total quenching rate even if many of these cross sections are known. 

Laser-induced fluorescence is a sensitive, spatially resolved technique for the detection and measurement of a variety of flame radicals. In order to obtain accurate number densities from such measurements, the observed excited state population must be related to total species population therefore the population distribution produced by the exciting laser radiation must be accurately predicted. At high laser intensities, the fluorescence signal saturates (1, 2, 3 ) and the population distribution in molecules becomes independent of laser intensity and much less dependent on the quenching atmosphere (4). Even at saturation, however, the steady state distribution is dependent on the ratio of the electronic quenching to rotational relaxation rates (4, 5, 6, 7). When steady state is not established, the distribution is a complicated function of state-to-state transfer rates. 

Thus in the initial stages of the study, excitation and fluorescence spectra were measured for individual species in a cell (heated to approximately 100 C to provide sufficient vapor pressure) to determine their (near) room temperature spectra. Individual PCAH were then injected into a flame to determine the effects of flame temperatures on the spectra and to determine sensitivities. These spectra will then be used as a data base to attempt to deconvolute the complex spectra observed upon excitation of the flame itself. 

A further requirement for measurement of absolute concentrations of excited species in flames is that the volume from which emission is collected be known. The simplest experimental arrangement for flames at atmospheric pressures is to focus the radiation from the flame onto the entrance slit of a spectrograph. Reasonable assumptions can be made about the thickness of the emitting layer, and Ausloos and van Tiggelen have used the arrangement successfully in semi-quantitative determinations of excited OH, NH, NO and NH2 in flames emitting the bands of these species. 

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. 

LEI utilizes a pulsed dye laser to promote analyte atoms to a bound excited state. Laser excitation enhances the thermal (collisional) ionization rate of the analyte atom, producing a measurable current in the flame 12). The laser-related current is detected with electrodes and is a measure of the concentration of the absorbing species. LEI may proceed by photoexcitation (via one or more transitions) and thermal ionization or a combination of thermal excitation, photoexcitation, and thermal ionization. 

In the early years of flame photometry, only relatively cool flames were used. We shall see below that only a small fraction of atoms of most elements is excited by flames and that the fraction excited increases as the temperature is increased. Consequently, relatively few elements have been determined routinely by flame emission spectrometry, especMly j ew of those that emit line spectra (several can exist in flames as molecular species, particularly as oxides, which emit molecular band spectra). Only the easily excited alkali metals sodium, potassium, and lithium are routinely deterniined by flame emission spectrometry in the clinical laboratory. However, with flames such as oxyacetylene and nitrous oxide-acetylene, over 60 elements can now be determined by flame emission spectrometry. This is in spite of the fact that a small fraction of excited atoms is available for emission. Good sensitivity is achieved because, as with fluorescence (Chapter 16), we are, in principle, measuring the difference between zero and a small but finite signal, and so the sensitivity is limited by the response and stability of the detector and the stability (noise level) of the flame aspiration system. 

In flame emission methods, we measure the excited-state population and in atomic absorption methods (below), we measure the ground-state population. Because of chemical reactions that occur in the flame, differences in flame emission and atomic absorption sensitivities above 300 nm are, in practice, not as great as one would predict from the Boltzmaim distribution. For example, many elements react partially with flame gases to form metal oxide or hydroxide species, and this reaction detracts from the atomic population equally in either method and is equally temperature dependent in either. 

In Eq 1, sulfur compounds are combusted to sulfur monoxide (SO) and other products. In Eq 2, the second step of the mechanism, light energy (hv) in the blue region of the spectrum is emitted from the excited species resulting from the ozone reaction. The basic mechanism of the DP-SCD is the same as that described above, but two plasmas (flames) instead of one are provided to improve selectivity and the ability to measure lower sulfur levels without hydrocarbon interferences. A conceptual drawing of the flow dynamics used in the Dual Plasma burner is shown (Fig. 1). 

There is a voluminous hterature concerned with the study of flame spectra, but the application of spectroscopy to the study of flame kinetics followed the introduction of flame photometry as a general analytical tool. The chief interest before this was in the spectra of the flames, which could serve to demonstrate the presence of intermediates in the combustion process. These were in general detected by the emission spectra of excited species and therefore were not necessarily indicative of the concentrations of ground state species. The difficulties of constructing burners which were sufficiently large and uniform to allow the study of absorption spectra prohibited a measurement of the species in their ground states, until the development of the multiple pass technique. ... 

If the measurements of the intensity of radiation from excited species are to be related to the concentration of ground state species, it is necessary to demonstrate that the species are in thermal equilibrium so that equation (3.33) may be apphed. This may be done by three methods. In the first place, the determination of flame temperature by the line reversal technique demands that there be thermal equilibration, and the concordance of a temperature so determined with that determined by other methods, or from lines known to be equilibrated is evidence for equilibration. Secondly the comparison of measured intensities of the first and higher resonance lines should be in accord with (3.33), a comparison which may be extended to a pair of different atoms if compound formation is not important. Thirdly, and most commonly, (3.33) may be transformed into... 

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