Acetylene based flame

Temperatures can vary slightly from the values given depending on the fuel oxidant ratio used. Acetylene-based flames are the most commonly used for analytical spectroscopy. 

Flame AAS may be used to determine the concentration of most metals by using an acetylene-based flame. An instrumental arrangement for a flame atomic absorption spectrometer is presented in Fig. 11. There are virtually no spectral interferences that affect AAS. Nonetheless, the main limit to analysis by AAS is due to physico-chemical interferences. The proliferation of ICP instruments has significantly reduced the overall application of flame AAS in most industrial applications. More than likely, the use of flame atomic absorption will dwindle to only a few specialized applications (similar to flame emission instruments). 

The air-hydrogen flame is more transparent than hydrocarbon-based flames, and sometimes offers advantages in atomic absorption when working at short wavelengths. This flame has been demonstrated superior to the air-acetylene and nitrous oxide-acetylene flames for the determination of low concentrations of tin, for instance. 

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]. 

Acetylene is condensed with carbonyl compounds to give a wide variety of products, some of which are the substrates for the preparation of families of derivatives. The most commercially significant reaction is the condensation of acetylene with formaldehyde. The reaction does not proceed well with base catalysis which works well with other carbonyl compounds and it was discovered by Reppe (33) that acetylene under pressure (304 kPa (3 atm), or above) reacts smoothly with formaldehyde at 100°C in the presence of a copper acetyUde complex catalyst. The reaction can be controlled to give either propargyl alcohol or butynediol (see Acetylene-DERIVED chemicals). 2-Butyne-l,4-diol, its hydroxyethyl ethers, and propargyl alcohol are used as corrosion inhibitors. 2,3-Dibromo-2-butene-l,4-diol is used as a flame retardant in polyurethane and other polymer systems (see Bromine compounds Elame retardants). 

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. 

An atomic absorption method was published by AOAC Int. (2000) for determination of the anti-foaming agent polydimethylsiloxane in pineapple juice, that is based on extraction with 4-methyl-2-pentanone and aspiration into a nitrous oxide/acetylene flame. A silicone lamp was used for detection. 

It should be noted that not all flames have the behaviors discussed above. For example, the equilibrium species distribution in some H2-N20-Ar flames has essentially the same mole number as the reactants. As a result the adiabatic flame temperature is achieved directly in the flame front with no long recombination tail. Ammonia-oxygen flames exhibit a slow approach to chemical equilibrium, albeit with a long dissociation, not recombination, tail [279], Here the temperature in the flame front overshoots the adiabatic flame temperature, with the equilibrium temperature being approached from above as the dissociation reactions proceed. In certain highly strained, rich, hydrocarbon flames (e.g., C2H2-H2-O2), such as those used for flame-based diamond growth, the temperature can also overshoot the adiabatic flame temperature in the flame front. Here the overshoot is caused by the relatively slow dissociation of the excess acetylene [270]. 

In 1892, Lewes (13, 14, 66) first proposed that acetylene is an important intermediate in combustion processes. He suggested a theory of luminosity of flames based upon its formation and subsequent incandescent decomposition. This did not prove successful as a general explanation of luminosity, but in recent years a modified version of this concept has been advocated by Porter (58, 59) and others. They suggest that acetylene is the starting point for the major reactions of carbon formation in flames and combustion. 

Junction beads can be made for platinum-based thermocouples, such as types R, S, or B, by welding with a high-temperature flame (e.g. oxy-acetylene). Using a flame for junction formation in alloy-based (e.g. K- or 2 -type) thermocouples does not work well since the wires tend to oxidize rather than fuse. Beads are more effectively made by electric arc for these thermocouple types. 

Molybdenum. Molybdenum can be analyzed by P CAM 173 for total Mo, by S-193 (12) for soluble Mo, or by S-376 for insoluble Mo. The standard nitric wet ashing used in P CAM 173 does not distinghish between soluble and insoluble Mo which have OSHA standards of 5 mg/cu m and 15 mg/cu m. Nitric acid digestion may not dissolve some insoluble Mo that require nitric/perchloric acid or base/nitric acid depending on the solubility properties. Soluble Mo compounds are hot water leached from the cellulose membrane filter used in all three methods. A fuel-rich air/acetylene flame used in P CAM 173 is replaced by an oxidizing nitrous oxide/acetylene flame to achieve total atomization of Mo as detected at 313.3 nm. Aluminum and traces of acid enhance the Mo flame response therefore, 400 ppm A1 is added to the final solution of both S-193 and S-376 and 0.1 N nitric acid is added to the water leach-soluble Mo final solution, S-193. 

The most commonly used atomiser is the chemical flame, based upon the combination of a fuel gas (e.g., acetylene) with an oxidant (e.g., air or nitrous oxide). The sample solution is introduced into the flame using a nebuliser in which the passage of the oxidant creates a partial vacuum by the venturi effect and thus the sample solution is drawn up through a capillary. Thus, an aerosol is produced having a wide variety of droplet sizes. This process is shown in Fig. 2. 

The importance of catalysts in our energy and pollution conscious age is growing. Many catalysts depend for their activity on low levels of rather exotic metals, while even trace surface levels of elements such as lead may impair their activity. Thus there is plenty of scope for atomic absorption spectrometry. Many homogeneous catalysts are deposited on an alumina base, thus obtaining dissolution is not always easy and some interference in the air/acetylene flame may be encountered. A leaching procedure (e.g. with nitric acid) to dissolve adsorbed trace metals may be used to circumvent these problems. 

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