Flame absorption profiles

FIGURE 9-4 Flame absorption profiles for three elements. 

Flame Absorption Profiles. Figure 9-4 shows typical absorption profiles for three elements. Magnesium exhibits a maximum in absorbance at about the mid-... 

The best sensitivity is obtained in this method when the source line is narrower than the absorption profile of the atoms in the flame. Obviously, the other situation tends towards Fig. 2.1. 

Gaussian Laser Profile-Voigt Atom Profile. This case turns out to be a better approximation of our experimental situation, i.e., the laser FWHM is fairly broad compared to the absorption line width and the absorption profile of atoms in an atmospheric combustion flame is described by a Voigt profile. Here the laser is assumed to have a Gaussian spectral profile as well as a Gaussian atomic absorption profile. In this case, convolution of two Gaussian functions is still a Gaussian function. Evaluation of the ratio n2/nT, and the fluorescence radiance. Bp, allows determination of the half width of the fluorescence excitation profile, 6X... 

Measure radiant furnace heat profile This will enable the comparison of the heat absorption profiles of SRC and coal flame and will also provide the necessary information to make judgements on the adequacy of furnace cooling surfaces. 

Cases have been observed where the isotopic line absorption profiles completely overlap, e.g. boron-10 and -11 in a krypton-filled lamp at 249.7 nm [244]. Hannaford and Lowe [245] later showed that this was caused by an unusually large Doppler half-width induced by the fill-gas, and, if neon is used, the 208.9 and 209.0 nm lines can allow the determination of boron-10 and boron-11 isotope ratios. The 208.89/208.96 nm doublet was found to be more useful than the 249.68/249.77 nm doublet. Enriched isotope hollow-cathode lamps were used as sources. A sputtering cell was preferred to a nitrous oxide/acetylene flame as the atom reservoir, as it could be water-cooled to reduce broadening and solid samples could be used, thus avoiding the slow dissolution in nitric acid of samples of boron-10 used as a neutron absorber in reactor technology. 

Fig. 2. Vertical absorption profile of 10-cm air-acetylene flame. Lower curve obtained with aqueous calcium solution, upper curve with isopropanol (from reference (S5) by courtesy of Perkin-Elmer Corporation).

A large number of experimental techniques have been used in studying flame chemistry, and all of these have been employed in flame ionization studies e.g., flame temperature profiles have been obtained with thermocouple probes excited species concentrations by emission spectroscopy ground-state free-radical or atomic concentrations by ESR and absorption spectroscopy and stable species profiles by means of small gas-sampling probes feeding directly to an analytical mass spectrometer. 

Parameters discussed previously for line broadening in emission spectrometry are also applicable for fluorescence spectrometry, especially since many of the same atom producers (flames, inductively coupled plasmas, etc.) are used for both techniques. In fluo rescence, the width of the absorption profile can be... 

Absorbance profile for Ag and Cr in flame atomic absorption spectroscopy. 

It would appear that measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysis. In practice, however, the absolute measurement of the absorption coefficients of atomic spectral lines is extremely difficult. The natural line width of an atomic spectral line is about 10 5 nm, but owing to the influence of Doppler and pressure effects, the line is broadened to about 0.002 nm at flame temperatures of2000-3000 K. To measure the absorption coefficient of a line thus broadened would require a spectrometer with a resolving power of 500000. This difficulty was overcome by Walsh,41 who used a source of sharp emission lines with a much smaller half width than the absorption line, and the radiation frequency of which is centred on the absorption frequency. In this way, the absorption coefficient at the centre of the line, Kmax, may be measured. If the profile of the absorption line is assumed to be due only to Doppler broadening, then there is a relationship between Kmax and N0. Thus the only requirement of the spectrometer is that it shall be capable of isolating the required resonance line from all other lines emitted by the source. 

For resonance lines, self-absorption broadening may be very important, because it is applied to the sum of all the factors described above. As the maximum absorption occurs at the centre of the line, proportionally more intensity is lost on self-absorption here than at the wings. Thus, as the concentration of atoms in the atom cell increases, not only the intensity of the line but also its profile changes (Fig. 4.2b) High levels of self-absorption can actually result in self-reversal, i.e. a minimum at the centre of the line. This can be very significant for emission lines in flames but is far less pronounced in sources such as the inductively coupled plasma, which is a major advantage of this source. 

The present work involves measurement of k in a 0.1 atmosphere, stoichiometric CH -Air flame. All experiments were conducted using 3 inch diameter water-cooled sintered copper burners. Data obtained in our study include (a) temperature profiles obtained by coated miniature thermocouples calibrated by sodium line reversal, (b) NO and composition profiles obtained using molecular beam sampling mass spectrometry and microprobe sampling with chemiluminescent analysis and (c) OH profiles obtained by absorption spectroscopy using an OH resonance lamp. Several flame studies (4) have demonstrated the applicability of partial equilibrium in the post reaction zone of low pressure flames and therefore the (OH) profile can be used to obtain the (0) profile with high accuracy. 

In Chapter 1, section 7, it was explained that very precise overlap of atomic absorption and emission profiles is required to obtain sensitive absorbance measurements. Absorption spectra of atoms at flame temperatures are much simpler than the emission spectra emitted by hollow cathode lamps. The possible transitions corresponding to electronic excitation of an atom may be shown as vertical lines on an energy level diagram, in which the vertical displacement... 

Not all transitions which are observed in the emission spectrum have the unexcited state (the ground state) as their lower energy level. In other words they require partial excitation before atomic absorption can occur. However, in flames, most atoms exist only in the ground state, and only transitions with the ground state as their lower energy state exhibit sensitive absorption.13,14 Because the number of such transitions is small, the probability of overlap of the atomic absorption line profile of one element with the emission line profile of another element is extremely small. The spectral selectivity of AAS is therefore excellent in this respect. 

The absorption line profile in the atomizer (e.g. in the flame) will still peak at the initial emission line peak. Absorbance will be reduced as the emission line becomes broader, and even more dramatically when the emission line shows reversal. Thus atomic absorption signal decreases with increasing lamp current (Figure 2). As might be expected, the drop off in signal is greater for more volatile elements such as cadmium and zinc. 

For the purposes of this discussion, deposition temperature is defined as the gas temperature close to the substrate. Due to the exhaust flow in the deposition hoods and the natural rising of hot air, the deposition temperature profile is skewed above the flame s longitudinal axis. The actual temperature of the substrate in the deposition zone depends upon the substrate material — size, absorption and emissivity characteristics — as well as the dwell time of the flame on one area of the substrate and whether or not any cooling is being applied to the substrate. 

SAFETY PROFILE A poison by intravenous route. Moderately toxic by ingestion and skin absorption. A severe skin and eye irritant. Mutation data reported. Combustible when exposed to heat or flame. To fight fire, use CO2, dry chemical, fog, mist. When heated to decomposition it emits acrid and irritating fumes. 

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