Measurable flame emission

Although flame emission measurements can be made by using an atomic absorption spectrometer in the emission mode, the following account refers to the use of a simple flame photometer (the Coming Model 410 flame photometer). Before attempting to use the instrument read the instruction manual supplied by the manufacturers. 

Principles and Characteristics Flame emission instruments are similar to flame absorption instruments, except that the flame is the excitation source. Many modem instruments are adaptable for either emission or absorption measurements. Graphite furnaces are in use as excitation sources for AES, giving rise to a technique called electrothermal atomisation atomic emission spectrometry (ETA AES) or graphite furnace atomic emission spectrometry (GFAES). In flame emission spectrometry, the same kind of interferences are encountered as in atomic absorption methods. As flame emission spectra are simple, interferences between overlapping lines occur only occasionally. 

Finally, and apart from the importance of micelles in the solubilization of chemical species, mention should also be made of their intervention in the displacement of equilibria and in the modification of kinetics of reactions, as well as in the alteration of physicochemical parameters of certain ions and molecules that affect electrochemical measurements, processes of visible-ultraviolet radiation, fluorescence and phosphorescence emission, flame emission, and plasma spectroscopy, or in processes of extraction, thin-layer chromatography, or high-performance liquid chromatography [2-4, 29-33],... 

For flame emission measurements, burners of the Meker type with a circular orifice covered by a grille are used whereas in atomic absorption spectrometry, a slit burner is preferred. In both cases, the flame consists of two principal zones or cones (Figure 8.21(b)). The inner cone or primary... 

Table 8.7). Thus, intensity and concentration are directly proportional. However, the intensity of a spectral line is very sensitive to changes in flame temperature because such changes can have a pronounced effect on the small proportion of atoms occupying excited levels compared to those in the ground state (p. 274). Quantitative measurements are made by reference to a previously prepared calibration curve or by the method of standard addition. In either case, the conditions for measurement must be carefully optimized with reference to the choice of emission line, flame temperature, concentration range of samples and linearity of response. Relative precision is of the order of 1-4%. Flame emission measurements are susceptible to interferences from numerous sources which may enhance or depress line intensities. 

In principle, atomic fluorescence is a simpler and more versatile technique than atomic absorption, but suffers from a susceptibility to quenching effects and to background noise arising from the scattering of radiation by particles in the flame. The latter is particularly serious for refractory materials and in high-temperature flames. Detection limits for some elements are lower than by atomic absorption or flame emission measurements, e.g. elements with resonance lines around 200 nm or below, such as As, Se,... 

Besides flame AA and graphite furnace AA, there is a third atomic spectroscopic technique that enjoys widespread use. It is called inductively coupled plasma spectroscopy. Unlike flame AA and graphite furnace AA, the ICP technique measures the emissions from an atomization/ionization/excitation source rather than the absorption of a light beam passing through an atomizer. 

Recalling Figure 9.4, we know that thermal energy sources, such as a flame, atomize metal ions. But we also know that that these atoms experience resonance between the excited state and ground state such that the emissions that occur when the atoms drop from the excited state back to the ground state can be measured. While there are several techniques that measure such emissions, including flame emissions... 

Step-V The thermal excitation of some atoms into their respective higher energy levels will lead ultimately to a condition whereby they radiate energy (flame emission) measured by Flame Emission Spectroscopy (FES), and... 

The technique of flame emission spectroscopy is used to determine the concentration of Ba, K, and Na ions by measuring the intensity of emission at a specific wavelength by the atomic vapour of the element generated from calcium acetate i.e., by introducing its solution into a flame. 

Temperatures estimated from the measured intensity distributions at each port location during an 02/Ar oxidizer run in the 24-inch long combustor are plotted in Fig. 8.4, along with the measured hemispherical emissive power in the wavelength range from 425 to 800 nm. (The hemispherical emissive power, E, is related to the radiant intensity, /, by = ttI. Radiant intensity is also referred to as radiance.) The stoichiometry, 0/F)/ 0/F)st, for this run was about f.fO. The measured combustion temperature was about 2900 K, as compared to an adiabatic flame temperature of about 3650 K. The intensity measurements indicate that ignition occurs about 12 in. downstream from the injector. The intensity is near its peak at the most downstream port location, which indicates that combustion is still underway at that location. 

One often unsuspected source of error can arise from interference by the substances originating in the sample which are present in addition to the analyte, and which are collectively termed the matrix. The matrix components could enhance, diminish or have no effect on the measured reading, when present within the normal range of concentrations. Atomic absorption spectrophotometry is particularly susceptible to this type of interference, especially with electrothermal atomization. Flame AAS may also be affected by the flame emission or absorption spectrum, even using ac modulated hollow cathode lamp emission and detection (Faithfull, 1971b, 1975). 

Potassium at trace concentrations in aqueous samples can be measured by a flame photometer at a wavelength of 766.5 nm. Either a flame photometer or an atomic absorption spectrometer operating in flame emission mode can be used for such analysis. 

All sodium compounds impart a golden yellow color to flame. Sodium can be identified spectroscopically by characteristic line spectra. Trace sodium may be measured quantitatively by flame atomic absorption or flame emission photometric method. The element may be measured at 589 nm using an air-acetylene flame. If using an ICP-atomic emission spectrophotometer, sodium may be measured at 589.00 or 589.59nm. Metallic sodium may be analyzed quantitatively by treating with ethanol and measuring the volume of hydrogen liberated. 

Various approaches to measuring flame temperature are well described in Gaydon s book on flames (see Appendix C). The best methods are spectroscopic rather than those which use thermocouples. The sodium line reversal method is perhaps the easiest. Sodium is added to the flame and the sodium D lines viewed against a bright continuum source (e g. a hot carbon tube). When the flame is cooler than the source the lines appear dark because of absorption. When the flame is hotter than the tube, the bright lines stand out in emission. The current to the tube, which will have been precalibrated for temperature readings by viewing the tube with an optical pyrometer, is adjusted until the lines cannot be seen. At this reversal point, the flame and tube temperature should be equal. 

Atomic absorption and flame emission spectroscopy, also called flame photometry, are two methods of quantitative analysis that can be used to measure approximately 70 elements (metals and non-metals). Many models of these instruments allow measurements to be conducted by these two techniques, which rely on different principles. Their applications are numerous, as concentrations in the mg/l (ppm) region or lower can be accessed. 

In flame emission spectroscopy (FES), the radiation intensity emitted by a small fraction of the atoms that have passed into the excited state by the elevated temperature is measured. 

It would appear in all cases that measurements should be based on atomic absorption instead of flame emission. Absorption spectra are simpler than emission spectra. In reality, however, the absorption measurements can be complicated by the presence of interferences, chemical interaction, instability of the energy level, and other phenomena that occur at elevated temperatures (Fig. 14.3). 

Previous experience indicates that flame emission is preferable for five or six of the elements. With present detectors, reliable measurements can be obtained as long as the ratio Ae/A-0 is superior to 10-7. Therefore, elements such as the alkaline earths that give coloured flames are easily measured by emission (see Table 14.1). 

Analysis by atomic (or optical) emission spectroscopy is based on the study of radiation emitted by atoms in their excited state, ionised by the effect of high temperature. All elements can be measured by this technique, in contrast to conventional flames that only allow the analysis of a limited number of elements. Emission spectra, which are obtained in an electron rich environment, are more complex than in flame emission. Therefore, the optical part of the spectrometer has to be of very high quality to resolve interferences and matrix effects.-... 

A flame photometric detector measures optical emission from phosphorus, sulfur, lead, tin, or other selected elements. When eluate passes through a Hrair flame, as in the flame ionization detector, excited atoms emit characteristic light. Phosphorus emission at 536 nm or sulfur emission at 394 nm can be isolated by a narrow-band interference filter and detected with a photomultiplier tube. 

Infrared emission spectroscopy can be used for the laboratory study of heated samples as one would encounter in pyrot reactions or in the detonation of primary expls. One difficulty associated with the measurement of emission spectra of condensed phase samples is that the temp of the sample has to be uniform, or else radiation emitted from elements situated below the surface will be absorbed by the cooler particles near the surface. Emission spectrometry finds application in the study of flames and smoke... 

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