Flame air / acetylene

Amongst other devices used to produce the required atoms in the vapour state are the Delves cup which enables the determination of lead in blood samples the sample is placed in a small nickel cup which is inserted directly into an acetylene-air flame. The tantalum boat is a similar device to the Delves cup in this case the sample is placed into a small tantalum dish which is then inserted into an acetylene-air flame. The use of these devices, especially for small sample volumes, has now been largely superseded by the graphite furnace. 

Increase in flame temperature often leads to the formation of free gaseous atoms, and for example aluminium oxide is more readily dissociated in an acetylene-nitrous oxide flame than it is in an acetylene-air flame. A calcium-aluminium interference arising from the formation of calcium aluminate can also be overcome by working at the higher temperature of an acetylene-nitrous oxide flame. 

The determination of magnesium in potable water is very straightforward very few interferences are encountered when using an acetylene-air flame. The determination of calcium is however more complicated many chemical interferences are encountered in the acetylene-air flame and the use of releasing agents such as strontium chloride, lanthanum chloride, or EDTA is necessary. Using the hotter acetylene-nitrous oxide flame the only significant interference arises from the ionisation of calcium, and under these conditions an ionisation buffer such as potassium chloride is added to the test solutions. 

Connect the appropriate gas supplies to the burner following the instructions detailed for the instrument, and adjust the operating conditions to give a fuel-lean acetylene-air flame. 

Set up a double-beam atomic absorption spectrophotometer with a lead hollow cathode lamp and isolate the resonance line at 283.3 nm adjust the gas controls to give a fuel-lean acetylene-air flame in accordance with the operating manual supplied with the instrument. 

The atomic absorption characteristics of technetium have been investigated with a technetium hollow-cathode lamp as a spectral line source. The sensitivity for technetium in aqueous solution is 3.0 /ig/ml in a fuel-rich acetylene-air flame for the unresolved 2614.23-2615.87 A doublet under the optimum operating conditions. Only calcium, strontium, and barium cause severe technetium absorption suppression. Cationic interferences are eliminated by adding aluminum to the test solutions. The atomic absorption spectroscopy can be applied to the determination of technetium in uranium and its alloys and also successfully to the analysis of multicomponent samples. 

The degeneracy of the excited state is 2, whereas that of the ground state is 1. The fraction of Na in the excited state in an acetylene-air flame at 2 600 K is, from Equation 21-2,... 

Oxyacetylene and acetylene-air flames have been widely used and studied and the results show that acetylene must disappear very rapidly in any oxygenated system at... 

Determination with atomic absorption spectroscopy with the use of an acetylene-air flame and hollow-cathode lamp, e.g. 

Figure 19. The laser-induced fluorescence excitation spectrum of the Ct swan band system in an acetylene-air flame (21)...

Many of the interelement interferences result from the formation of refractory compounds such as the interference of phosphorous, sulfate, and aluminum with the determination of calcium and the interference of silicon with the determination of aluminum, calcium, and many other elements. Usually these interferences can be overcome by using an acetylene-nitrous oxide flame rather than an acetylene-air flame, although silicon still interferes with the determination of aluminum. Since the use of the nitrous oxide flame usually results in lower sensitivity, releasing agents such as lanthanum and strontium and complexing agents such as EDTA are used frequently to overcome many of the interferences of this type. Details may be found in the manuals and standard reference works on AAS. Since silicon is one of the worst offenders, the use of an HF procedure is preferable when at all possible. 

The LEI signal produced by amplitude-modulated continuous wave (CW) dye laser excitation has been shown to be less concomitant-dependent than signals obtained with pulsed excitation38. CW excitation is almost completely immune to interferences from low ionization potential sample matrices at virtually any position in an acetylene-air flame whereas pulsed excitation produces the maximum signal recovery only near the cathode surface. CW is more tolerant in this regard because convection or diffusion will move the analyte ion into the nonzero field near the cathode surface during the synchronization window for chopping rates less than 500 Hz. 

Fig. 12. Oscilloscope trace for irradiation of an acetylene-air flame containing Na and U atoms with a single laser pulse S2K At the excitation wavelength, 539.9 nm, there is a discrete two-photon sodium transition and a broad band uranium transition...

The hydrogen-air flame is of little use. For the determination of Sn, however, its sensitivity is higher than the acetylene-air flame. Because of its higher transmission at short UV wavelengths the hydrogen-air flame is also used for the determination of As and Se with the 193.7 nm and the 196.0 nm lines, respectively. 

By use of releasing agents Considering the reaction M-X-i-R = R- Xh-M, it becomes evident that an excess of the releasing agent (R) will lead to an enhanced concentration of the required gaseous metal atoms (M) which will be of special significance if the product R-X is a stable compormd. Hence in the determination of calcium in presence of phosphate the addition of excess of strontium chloride to the test solution will lead to the formation of strontium phosphate and the calcium can then be determined in an acetylene-air flame without any interference due to phosphate. Also addition of EDTA to a calcium solution before analysis may increase the sensitivity of the subsequent flame spectrophotometric determination which may be due to the formation of an EDTA complex of calcium which is readily dissociated in the flame. 

For thallium, determinations at the 276.78 nm line with an acetylene/air flame are used throughout. Matrix problems are very low, but the sensitivity with regard to the low level of occurrence is poor. The sensitivity can be increased by mounting a slotted quartz tube on the burner head STAT = "slotted tube atom trap") (Milner, 1983), which leads to a detection limit of about 20 mg/kg in the solid sample, which is insufficient for the analysis of biological matrices. In MIBK extracts, determination of thallium is much more sensitive in flame AAS than in aqueous solutions (till about 7-fold). This can be used for solvent extraction of thallium from 0.1M HBr (Hubert and Chao, 1985), as xanthate at pH 8 (Aihara and Kiboku, 1980), or as iodide with tri-n-octylphosphinoxide into MIBK, and direct aspiration of the organic phase into the flame. 

For multi-element analysis, the significantly more energetic plasma sources are therefore by far superior to flames in most regards. Today, flames are only used for the determination of alkali metals, as these can be excited at low temperatures and give simple spectra free of interferences. The determination of these metals is especially important for the analysis of biological fluids, so that emission in acetylene-air flames is still routinely used in highly automated, and simplified systems based on single or multiple interference filters and photomultiplier detection (see Fig. 12.23). The systems often also include automatic addition of an internal standard and dilution [39]. 

An acetylene/air flame is to be recommended in the case of pre-chamber atomization and a hydrogen/oxygen flame in the case of direct atomization. ... 

The classic source is a chemical combustion flame such as an acetylene-air flame. Table 2 shows flame temperatures for various fuels and oxidants. In today s instruments, acetylene-air and acetylene-nitrous oxide are the most common fuel-oxidant mixtures. In general, flames are considered relatively low-temperature sources (2000-3000°C) and have their greatest utility in determination of alkali metals and alkaline earth metals. (The cyanogen/oxygen flame shown in the table is considered an exotic flame and is rarely used in analysis both because of the toxicity of the fuel as well as the high production rate of CO as a combustion byproduct.)... 

Many of the problems connected with the study of flame structure stem from the narrowness of the spatial region to be studied, for example the stoichiometric acetylene-air flame at atmospheric pressure has a primary reaction zone smaller than one-tenth of a millimeter (0.004 inch). It is not possible with present-day techniques to measure the properties of such a flame with sufficient spatial resolution to obtain meaningful second derivatives. The thickness of the reaction zone, however, depends inversely on both pressure and burning velocity. This behavior is... 

E. N. Taran and V. I. Tverdokhlebov, Some electrical properties of a rarefied acetylene air flame with an admixture of alkali metal salts, High Temperature 4, 160-165 (1966). 

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