Acetylene flame

Thermal energy in flame atomization is provided by the combustion of a fuel-oxidant mixture. Common fuels and oxidants and their normal temperature ranges are listed in Table 10.9. Of these, the air-acetylene and nitrous oxide-acetylene flames are used most frequently. Normally, the fuel and oxidant are mixed in an approximately stoichiometric ratio however, a fuel-rich mixture may be desirable for atoms that are easily oxidized. The most common design for the burner is the slot burner shown in Figure 10.38. This burner provides a long path length for monitoring absorbance and a stable flame. 

Description of Method. Copper and zinc are isolated by digesting tissue samples after extracting any fatty tissue. The concentration of copper and zinc in the supernatant are determined by atomic absorption using an air-acetylene flame. 

M HNO3. The concentration of Cu and Zn in the diluted supernatant is determined by atomic absorption spectroscopy using an air-acetylene flame and external standards. Copper is analyzed at a wavelength of 324.8 nm with a slit width of 0.5 nm, and zinc is analyzed at 213.9 nm with a slit width of 1.0 nm. Background correction is used for zinc. Results are reported as micrograms of Cu or Zn per gram of FFDT. 

Quantitative aluminum deterrninations in aluminum and aluminum base alloys is rarely done. The aluminum content is generally inferred as the balance after determining alloying additions and tramp elements. When aluminum is present as an alloying component in alternative alloy systems it is commonly deterrnined by some form of spectroscopy (qv) spark source emission, x-ray fluorescence, plasma emission (both inductively coupled and d-c plasmas), or atomic absorption using a nitrous oxide acetylene flame. 

Hot RF and - DC plasma, are discharge, plasma jets Oxy-acetylene flames Low pressure microwave plasma, holt filament. Low pressure DC or RF glow discharge Thermal decomposition... 

Atomic absorption spectroscopy is more suited to samples where the number of metals is small, because it is essentially a single-element technique. The conventional air—acetylene flame is used for most metals however, elements that form refractory compounds, eg, Al, Si, V, etc, require the hotter nitrous oxide—acetylene flame. The use of a graphite furnace provides detection limits much lower than either of the flames. A cold-vapor-generation technique combined with atomic absorption is considered the most suitable method for mercury analysis (34). 

Atomic absorption spectroscopy is an alternative to the colorimetric method. Arsine is stiU generated but is purged into a heated open-end tube furnace or an argon—hydrogen flame for atomi2ation of the arsenic and measurement. Arsenic can also be measured by direct sample injection into the graphite furnace. The detection limit with the air—acetylene flame is too high to be useful for most water analysis. 

For high pressure (15-400 psig), two types of acetylene flame arresters were developed by Linde and are still available from Praxair on special order. The first type is available in 1.5-inch diameter by 3- to 4-foot-long cylinders packed with sintered metal, fine wire wool, or finely divided alumina. The second type is available in 6-inch diameter by 15-inch-long cylinders packed with round nickel shot. 

Protego offers a crimped metal ribbon flame arrester approved in Germany for acetylene service. It is similar in design to their flame arresters for hydrogen service bnt the hydranlic diameter of the flame arrester apertures for qnenching acetylene flames is 0.15 mm rather than 0.20 mm for hydrogen. 

Although APDC complexes are soluble in many organic solvents, it is found that 4-methylpent-2-one (isobutyl methyl ketone) and heptan-2-one (n-pentyl methyl ketone) are, in general, the most satisfactory for direct nebulisation into the air/acetylene flame used in atomic absorption spectroscopy. 

Solutions in organic solvents may, with certain reservations, be used directly, provided that the viscosity of the solution is not very different from that of an aqueous solution. The important consideration is that the solvent should not lead to any disturbance of the flame an extreme example of this is carbon tetrachloride, which may extinguish an air-acetylene flame. In many cases, suitable organic solvents [e.g. 4-methylpentan-2-one (methyl isobutyl ketone) and the hydrocarbon mixture sold as white spirit ] give enhanced production of ground-state gaseous atoms and lead to about three times the sensitivity... 

Lamprecht, A., Atakan, B., and Kohse-Hoinghaus, K., Fuel-rich propene and acetylene flames A comparison of their flame chemistries. Combust. Flame, 122, 483, 2000. 

Frenklach, M. and Warnatz, J., Detailed modeling of PAH profiles in a sooting low-pressure acetylene flame. Combust. Sci. Tech., 51,265,1987. 

Eraslan, A. N., Chemiionization and ion-molecule reactions in fuel-rich acetylene flame, Combust. Flame, 74,19, 1988. 

Pd content was determined by atomic absorption spectroscopy at 247.6 nm with an air-acetylene flame. 

Spencer and Sachs [29] determined particulate aluminium in seawater by atomic absorption spectrometry. The suspended matter was collected from seawater (at least 2 litres) on a 0.45 tm membrane filter, the filter was ashed, and the residue was heated to fumes with 2 ml concentrated hydrofluoric acid and one drop of concentrated sulfuric acid. This residue was dissolved in 2 ml 2 M hydrochloric acid and the solution was diluted to give an aluminium concentration in the range 5-50 pg/1. Atomic absorption determination was carried out with a nitrous oxide acetylene flame. The effects of calcium, iron, sodium, and sulfate alone and in combination on the aluminium absorption were studied. 

Atomic absorption spectrometry coupled with solvent extraction of iron complexes has been used to determine down to 0.5 pg/1 iron in seawater [354, 355]. Hiire [354] extracted iron as its 8-hydroxyquinoline complex. The sample is buffered to pH 3-6 and extracted with a 0.1 % methyl isobutyl ketone solution of 8-hydroxyquinoline. The extraction is aspirated into an air-acetylene flame and evaluated at 248.3 nm. 

Carr [562] has studied the effects of salinity on the determination of strontium in seawater by atomic absorption spectrometry using an air-acetylene flame. Using solutions containing 7.5 mg/1 strontium and between 5 and 14% sodium chloride, he demonstrated a decrease in absorption with increasing sodium chloride concentration. To overcome this effect a standard additions procedure is recommended. 

The ratio, Nj/N0, can therefore be calculated. For the relatively easily excited alkali metal sodium, it is 9.9 x 10 6 at 2000 °K and 5.9 x 10 4 at 3000 °K this latter temperature is about the highest commonly obtained with flames used for atomic absorption or emission work. Hence, only about 1(T3 % of the sodium atoms are excited at 2000 ° and 6 x 1(F2 % at 3000°. For an element such as zinc,Nf/N0 is 5.4 x 10"10 at 3000 and so only 5 x 10"8% is excited. In spite of the small fraction excited, good sensitivities can be obtained for many elements by flame photometry if a high temperature flame is used, because the difference between zero and a small but finite number is measured. For example, seventy elements can be determined by flame photometry using the nitrous oxide-acetylene flame 1H. 

Table 1 lists the temperatures of some commonly used flames for atomic absorption. A cool flame such as argon-hydrogen-entrained air or air-coal gas is usually not preferred because of increased danger of chemical interferences (see below). The most commonly used flame is the air-acetylene flame. 

Refractory compounds can be determined using a nitrous oxide-acetylene flame. The formation of refractory oxides with gases in flames might not be considered an interference, since it is constant under a given set of conditions but it does decrease the sensitivity markedly so that measurement of the element may not be possible. 

Ionization interference is particularly a problem in the high temperature nitrous oxide-acetylene flame, where even elements such as manganese can be appreciably... 

Boron in blood and tissue has recently been determined by Bader and Branden-berger 11 ) by dry ashing, and then aspirating the acidified solution into a nitrous oxide-acetylene flame. A limit of detection of 15 ppm in the solution was reported. 

Devoto 115)has described an indirect procedure for the determination of 0.1 ppm arsenic in urine. The arsenomolybdic acid complex is formed and extracted from 1 ml of urine at pH 2 into 10 ml of cyclohexanone. The molybdenum in the complex is then measured. Before extracting the arsenic, phosphate in the urine is separated by extracting the phosphomolybdic acid complex at pH 1 into isobutyl acetate. The direct determination of arsenic in biological material and blood and urine is best done using a nitrous oxide-acetylene flame 116>. The background absorption by this flame is low at 1937 A, and interferences are minimized due to the high temperature of the flame. 

Atomic absorption also gave better results for aluminum using a nitrous oxide-acetylene flame. In general, optical emission was more rapid. 

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