Oxidant-fuel flame

Mode X (nm) Oxidant + fuel Flame buffer concentration... 

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. 

Direct-Flame Incinerators. In direct-flame incineration, the waste gases are heated in a fuel-fired refractory-lined chamber to the autoignition temperature where oxidation occurs with or without a visible flame. A fuel flame aids mixing and ignition. Excess oxygen is required, because incomplete oxidation produces aldehydes, organic acids, carbon monoxide, carbon soot, and other undesirable materials. 

CHO+, which reacts with water formed in the flame to form H30+, allowing a measurable electrical current to flow across an electrode gap. A schematic representation of an FID is shown in Fig. 14.7, showing the fuel and oxidant flows, flame tip, location of the flame and collector electrode. Like the carrier gas, the fuel (hydrogen) and oxidant (air) gases must be highly pure and carefully flow controlled. For each GC, the manufacturer provides recommendations. 

Table 24.1, records the temperatures of commonly used fuels and oxidants in flames in emission spectroscopy. 

Table 24.1 Temperatures of Commonly Used Fuels and Oxidants in Flames...

Ammonium perchlorate (NH ClO ) This is a good oxidizer, and can be used to make excellent propellants and colored flames. However, it is a self-contained oxidizer-fuel system (much like ammonium nitrate). The mixing of NH f (fuel) and ClOa (oxidizer) occurs at the ionic level. The potential for an explosion cannot be ignored. Conclusion if this material is used, it must be treated with respect and minimum quantities of bulk powder should be prepared. 

A variety of events that will lead to smoke production can occur in the pyrotechnic flame. Incomplete burning of an organic fuel will produce a black, sooty flame (mainly atomic carbon). A highly-oxidized fuel such as a sugar is not likely to produce carbon. Materials such as naphthalene (C loH s) and anthracene ( C i H 101 - volatile solids with high carbon content - are good candidates for soot production. Several mixtures that will produce black smokes are listed in Table 8. 1. 

The nitrous oxide-acetylene flame is both hot and reducing. A characteristic red, interconal zone is obtained under slightly fuel-rich conditions. This red feather is due to emission by the cyanogen radical. This radical is a very efficient scavenger for oxygen, thus pulling equilibria such as... 

Compared with the AP decomposition flame thickness, the fuel-oxidant redox flame extends a much greater distance from the propellant surface and depends on the rate of both chemical reaction and diffusional mixing. 

Almost any flammable mixture will, under favorable conditions of confinement, support an explosive flame propagation or even a detonation. When a fuel-oxidant mixture of a composition favorable for high-speed combustion is weakened by dilution with an oxidant, fuel, or an inert substance, it will first lose its capacity to detonate. Further dilution will then cause it to lose its capacity to burn explosively. Eventually, the lower or upper flammability limits will be reached and the mixture will not maintain its combustion temperature and will automatically extinguish itself. These principles apply to the combustible cryogens hydrogen and methane. The flammability and detonability... 

Figure 5 Effect of operating at optimal (top) and sub-optimal (bottom) fuel flow when repeatedly nebulizing 10 mg l 1 aluminium into a nitrous oxide-acetylene flame...

In some respects, optimization procedures in flame AES are simpler than those in either AAS or AFS. This is so because of the absence of a light source (apart from the flame atomizer). Moreover, there is little incentive to employ hydrogen as a fuel in flame AES, because the lower flame temperatures (compared to the corresponding acetylene flames) do not favour intense thermally excited atomic emission. Air-acetylene and nitrous oxide-acetylene flames are most widely... 

Because of the great strength of the B—O bond, even in a fuel-rich nitrous oxide-acetylene flame, the atomization efficiency of boron is so low that its determination in almost all environmental materials is not possible by flame spectrometric methods. The author favours a solution spectrofluorimetric... 

Tin is best determined by AAS at 224.6 or 235.5 nm in a reducing nitrous oxide-acetylene flame. Hydrogen as a fuel may give improved sensitivity, but only at the expense of serious interference problems. 

Being an element that is apparently not particularly toxic, not particularly soluble, and not essential to biota, titanium has not been determined often in environmental samples. It forms a very stable bond to oxygen, so a fuel-rich nitrous oxide-acetylene flame is essential for its determination. By AAS the detection limit at 364.3 nm is generally around 100 ng ml-1, or slightly better... 

The extent of chemical interferences in flame spectrometry varies with flame conditions and analyte concentration. Thus it is most unlikely that the same wrong answer will be obtained at two different heights in the flame or at two different fuel-to-oxidant ratios. Indeed it has been suggested that the former of these two options may provide automated detection of chemical interferences.2 The burner was moved up and down using a microprocessor-controlled stepper motor. Alternatively, results in air-acetylene and nitrous oxide-acetylene flames may be compared. Similarly, it is unlikely that the same wrong answer will be obtained at two different dilutions. Thus if it is thought that there might be a risk of chemical interference, the determinations on a selection of samples should be repeated under diverse conditions, either on the same or different instruments. 

Extraction hoods serve a number of useful purposes. These include removal of carbon dioxide and steam (the commonest combustion products) and of other, less desirable combustion products, such as sulfur dioxide and other acidic gases from some sample solutions, and of whatever elements were present in the samples. They also remove soot from excessively fuel-rich flames, which otherwise can make a real mess in the laboratory. It is unwise to run instruments for extended periods without fume extraction, even if there is nothing obviously toxic in the sample solutions. This is especially true for the nitrous oxide-acetylene flame. It is well worth considering interfacing the fume extraction switch with the instrument power supply, because it is easy to forget to turn on the extractor when everything else is automated. 

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. 

Beer, J. M., A. F. Sarofim, L, D, Timothy, S. P. Hanson, A. Gupta, and J. M. Levy, "Two phase processes involved in the control of nitrogen oxide formation in fossil fuel flames," Proceedings of the Joint Symposium on Stationary Combustion NO Control, U.S. Environmental Protection Agency and Electee Power Research Institute, IERL-RTP-1086, Vol. 4, pp. 43-83, October 1980. 

Tin is of interest as it is released from cans into food. The maximum EU limit for Sn in canned food is currently 200 pg g-1, these levels being well suited for FAAS. A reducing (fuel-rich) nitrous oxide-acetylene flame is required to properly atomize Sn. As this type of flame is more unstable than air-acetylene, it may result in somewhat poorer repeatability. The quantification of naturally occurring levels of Sn in foodstuffs is quite different and would require ET-AAS. 

Aspirate solutions into a fuel-lean air—acetylene flame to measure magnesium at 285.21 nm and into a fuel-rich nitrous oxide—acetylene flame to determine calcium at 422.67 nm. 

The doublet at 318.34/318.40 nm can be resolved from the adjacent 318.54 nm line with a 0.03 nm spectral bandpass. As all three lines have similar absorption sensitivity, transmission of the three lines using a 0.2nm spectral bandpass is recommended for increased detectivity and precision. Optimum sensitivity is obtained with a slightly fuel-rich nitrous oxide-acetylene flame ionization should be suppressed with 1000 jug K ml-1. Variable enhancement of absorption by Al, Fe, Cr and other elements is removed by addition of 2000 jug Alml-1. 

---