Spectroscopy in separated flames-III Use of the separated nitrous oxide-acetylene flame in thermal emission spectroscopy

The separation of a premixed nitrous oxide-acetylene flame at a modified commercial burner is described. The reducing interconal zone of the fuel-rich separated flame exhibits low radiative background. The reducing atmosphere and high temperature of this flame result in an effective medium for the excitation of the atomic line spectra of the refractory elements. The use of the fuel-rich flame in the flame photometry of these elements has been investigated.     

Atomic fluorescence spectroscopy of beryllium

The atomic fluorescence of beryllium has been observed. A high-intensity beryllium hollow-cathode lamp was used as the source. Oxy-acetylene and nitrous oxide-acetylene flames were studied. A newly designed burner assembly for nitrous oxide-acetylene flames used for atomic fluorescence studies is described. The sensitivity for beryllium at 2349 Å was 10 p.p.m. in the oxy-acetylene flame and 0.5 p.p.m. in the nitrous oxide-acetylene flame. The analytical calibration curves for both flames are presented. No significant interference was found from the cations studied. Some anionic interferences were removed by EDTA. The effects of some organic solvents were investigated.     

Absolute analysis by flame atomic absorption spectroscopy: Present status and some problems

Recent progress and main problems encountered in the theoretical interpretation of some stages in the spectrochemical analysis by flame AAS with a slot burner and a sharp-line source are reviewed. The effect of “narrowing” of the aerosol stream as compared with the gas stream above the flame front was established theoretically and confirmed experimentally. The resulting theory describing the analyte distribution across the flame permitted to explain many features of these flames, in particular, the effect of sensitivity enhancement in the presence of an excess of matrix. A simple method is proposed for the determination of atomic diffusion coefficients.The results of calculations of the composition and temperatures of flames employed in analytical practice, obtained for a wide range of the fuel—oxidant ratio, were used to determine the capabilities of these flames as to the dissociation of monoxides. Practically total dissociation of almost all elements of the Periodic Table was proved theoretically and confirmed experimentally to occur in the nitrous oxide—acetylene flame. The formation of low-volatile lithium and tin carbides in the presence of carbon was established. This effect accounts for “anomalies” in the behavior of these elements in low-temperature flames.The line shifts Δν_s in flames were measured by interferometric scans of line profiles from a hollow-cathode lamp and flame. The existence of a theoretical relationship between Δν_s and the Lorentz line width Δν_L was confirmed. Calculations of line absorption were generalized to take into account the shift and hyperfine structure of the lines. Systematic errors in these calculations do not exceed 10%.A discussion is given of the major difficulties facing absolute measurements based on this analytical technique.     

Laser absorption spectroscopy diagnostics of nitrogen-containing radicals in low-pressure hydrocarbon flames doped with nitrogen oxides

Absolute concentration profiles of NH2 and HNO have been measured in low-pressure methane/air flat flames doped with small amounts of NO and N2O. Addition of a small amount of nitrogen oxides does not alter significantly the flame speeds, temperature profiles and other parameters of the relatively well-understood methane/air flames. Intracavity laser absorption spectroscopy (ICLAS) and cavity ring-down spectroscopy (CRDS) are high-sensitivity techniques used to measure absolute concentrations of minor species in flames. In this work ICLAS is used to monitor NH2 and HNO, whereas CRDS is used for temperature measurements using OH spectra in the UV range. The (090)-(000) and (080)-(000) bands of the A2A1-X2B1 electronic transition of NH2 and (100)-(000) and (011)-(000) bands of the A1A"-X1A' transition of HNO are used. Methane flames of different equivalence ratios are used. NH2 and HNO are observed in the flame as well as in the zone surrounding the flame, closer to the walls of the low-pressure chamber where the burner is located. An absorption originating from the species in this zone can affect substantially the results of line-of-sight experiments. A slow flow of nitrogen through the optical window holders was added in order to separate the spectra of HNO originating from the central flame zone. Calculations based on the commonly used GRI-Mech chemical mechanism predict two maxima in the HNO concentration profile in the NO doped flames. The first is located in the vicinity of the burner, and the second is closer to the luminescence flame zone. We were able to observe the first maximum, and its measured location agrees well with prediction. On the other hand, GRI-Mech strongly underpredicts the observed absolute concentration of HNO in this maximum. The measured absolute concentrations of NH2 are in reasonable agreement with the GRI-Mech predictions.     

Spectroscopy in separated flames--VII: Determination of bismuth by atomic-fluorescence spectroscopy in a separated air-acetylene flame with electronically modulated electrodeless discharge tube sources

The application of electronically modulated and unmodulated bismuth and iodine electrodeless discharge lamps as sources for the excitation of bismuth atomic fluorescence in conventional and nitrogen-separated air-acetylene flames has been investigated. Separation of the flame results in improved detection limits for bismuth even when a modulated source is employed. The effect of 500-fold weight excesses of foreign ions on the determination of bismuth at 302.46 nm with a modulated iodine source and separated flame has been studied; only calcium and zirconium are found to cause significant interference. The determination of bismuth in aluminium alloy samples is reported.     

Absorption profiles of flames used in atomic-absorption spectroscopy

A study has been made of the distributions of metallic atoms in their ground electronic state when an aqueous or an organic solvent containing the metal present in the solution in various combined forms is sprayed into various flames used in atomic-absorption spectroscopy. It has been found that with almost all elements studied, rich flames give greater peak absorbances than lean flames, and the flame stoichiometry determines the number of free atoms in a flame. It has also been observed that the spatial distribution of free atoms in a flame depends not only on the flame stoichiometry but also on the species which the desired metal forms in the solution and the flame, on the other anions, complexing agents, and cations present in the solution, on chemical and physical properties of the solvent, on the type of atomizer-burner and on the flame temperature.     

An experimental and theoretical evaluation of the nitrous oxide-acetylene flame as an atomization cell for flame spectroscopy

The formation of free atoms from aerosols of metal-containing solutions introduced into nitrous oxide-acetylene flames is examined by: (a) inference from well identified reactions and equilibria prevailing in cooler flames; (b) calculations employing a thermodynamic flame model; and (c) experimental observation of relative free-atom number densities in the flames as a function of stoichiometry. The calculated partial pressures of the major natural flame species and some of the spectroscopically observed minor species are presented as a function of the flow ratio of nitrous oxide to acetylene (p). Predicted relative number densities of Na, Mg, Cu, Fe, Li, Be, Al, W, Ti and Si as a function of p are compared with measured free-atom absorbances in an argon-shielded flame. These comparisons were completed for various heights above the burner tip. The data reported show that: (a) the degree of metal atomization in the nitrous oxide-acetylene flame can be adequately described by the equilibrium state; (b) in general, when solute vaporization is complete, there exists a value of ρ at which atomization is complete for metals that form monoxides with dissociation energies less than ~ 6.5 eV; and (c) certain metals may form carbon-containing compounds in the interconal zone.     

Polymer flammability. I

The transport of oxygen by diffusion from the environment into a gas stream was investigated as a model for the analogous process in a diffusion flame. The amount transported at steady-state conditions depended on the flow rate, diameter, and spatial orientation of the gas stream. A change of the same extrinsic parameters in a diffusion flame caused changes of burner surface temperature, maximum flame temperature, and flame height. These responses were correlated and yielded an overall activation energy of the rate-controlling reaction step in the combustion process equal to 49 kcal/mole. This value was the same for several types of diffusion flames examined and appeared to be associated with the CO/CO₂ conversion process at the high-temperature flame boundary. Flame quenching was demonstrated to occur at a minimum fuel flow rate and minimum environmental oxygen concentration which were characteristic for a given fuel. Quenching conditions were related to the diffusion rate of oxygen into the product effluent stream. Quenching of a polymer flame by depletion of environmental oxygen was governed by the same processes. The effect of extrinsic parameters on polymer flames is discussed in Part II.     

Emission spectra of nitrous oxide supported acetylene flames at atmospheric pressure

The emission spectra of a premixed flame of acetylene supported by nitrous oxide have been recorded under different fuel-gas mixture conditions. The emission spectra in these flames of a series of metals, for which it is difficult to obtain a significant population of ground state atoms for atomic absorption spectroscopy in more conventional flames, have also been studied. The red secondary zone which is present in the fuel-rich flames shows emission attributable to long-lived CN and NH species which form a strongly reducing atmosphere to inhibit refractory oxide formation from elements such as molybdenum, titanium and aluminium introduced into the flame. An attempt has also been made to explain some of the reactions which may occur between the flame species above the primary reaction zone.     

在掌上实验室探究酒精灯火焰温度——得出不同的结论

本文介绍在掌上实验室进行的酒精灯3层火焰温度的探究实验,探讨酒精灯3层火焰温度的大小及其关系。     

Electrochemical diffusion potential in the gas phase

Potentiometric based electrochemical measurement of diffusion potential at a junction between two flowing flame plasma gases is described. A flame electrochemical cell was constructed using a specially designed burner, which supports two individual flames, each fed by separate premixed methane/oxygen/nitrogen streams. The two flames were in intimate contact, creating a flowing fluid gaseous junction. By aspirating metal salt solutions into these premixed feed gases, the concentration gradient at the interface between the two flames may be controlled. A measurable electrochemical diffusion potential was formed at this junction, the magnitude of which was dependent on the concentration ratio of charged species with different mobilities. In our flame electrolyte, the dominant charged species were atomic or molecular cations and electrons, which have a difference in mobilities of approximately three orders of magnitude. A two-electrode system, in conjunction with a high impedance electrometer was used to measure the potential difference across the flame electrochemical cell. The measured potential difference was analysed using theory developed for the liquid junction potentials by the Henderson equation.     

Spectroscopy in separated flames-V The argon- or nitrogen-sheathed nitrous oxide-acetylene flame in flame emission spectroscopy

The separation of the premixed nitrous oxide-acetylene flame by sheathing with argon or nitrogen is described. The interconal zone of the hot, slightly fuel-rich flame exhibits low background and noise levels and an extended reducing atmosphere, providing better conditions for the excitation of atoms of elements which form refractory oxides. The limits of detection found for nine such elements are greatly superior to those obtainable in the conventional unsheathed flame under similar conditions.     

The application of a piezoelectric scanning Fabry-Perot interferometer to the study of atomic line sources—II: Line-widths for calcium in air-acetylene and nitrous oxide-acetylene flames

The variation in observed half-width of the calcium 422.7 nm line with calcium concentration has been studied interferometrically in air-acetylene and nitrous oxide-acetylene flames supported as cylindrical flames, with and without flame shielding and inert gas separation, and at a long path burner. Extrapolation of the curves obtained to zero added calcium concentration is shown to permit correction for self-absorption and calculation of interaction broadening half-width and damping constants (a-parameters).     

Application of an oxygen-shielded air-acetylene flame to atomic spectroscopy

A burner has been designed which provides an oxygen-shielded air-acetylene flame for atomic-absorption work. The chemical reducing properties of the oxygen-shielded flame operated under fuel-rich conditions are enhanced by the higher C: O ratio obtainable in the flame and by the higher flame temperature just above the reaction zone. The flame is inherently essentially free from the risk of flashback, and is offered as an alternative to the nitrous oxide-acetylene flame for use with certain types of equipment and for particular applications.     

Comparison of atomic fluorescence power efficiencies for the helium-oxygen-acetylene and air-acetylene flames

Power efficiencies for five elements have been measured for the helium-oxygen-acetylene and air-acetylene flames. The increased power efficiencies found in this study for the helium-diluted flame, coupled with its enhanced atom-formation capabilities, suggest that lower atomic fluorescence detection limits should exist. However, in a comparison study with an air-acetylene flame using identical experimental conditions, a decreased atomic fluorescence signal-to-noise ratio was found for most elements in the helium-diluted flame. This decrease is ascribed to greater background emission noise in the hotter helium-diluted flame and decreased nebulization efficiency caused by the low density of the helium-containing nebulizer gas. A comparison of flame emission detection limits for the two flames confirms the increased sensitivity of the hotter helium-oxygen-acetylene flame, despite its lower nebulization efficiency.     

The atomic absorption spectroscopy of selenium

Selenium can be determined in aqueous solution by atomic absorption spectroscopy at 1960, 2040,2063 or 2075 Å; the sensitivities for these lines with a Techtron 10-cm air-acetylene burner are in the ratio of 1:9:60:93. When a Beckman tripleburner (air-hydrogen) and a triple-pass optical system are used, the most sensitive1960 Å line provides a sensitivity of 0.5 p.p.m. and a detection limit of 1.0 p.p.m. The performence in air-hydrogen and air-acetylene flames is described,and optimum experimental conditions determined.With the Se 1960 Å line and a Techtron 10-cm air-acetylene burner, selenium extracted into methyl isobutyl ketone as its diethyldithiocarbamate complex gives a sensitivity of 0.30 p.p.m.,which is a 2.4-fold increase over that found in aqueous solution.     

The determination of aluminum by atomic absorption spectroscopy

A study of the determination of aluminum by atomic absorption spectroscopy has been made using a solution of aluminum cupferrate in 4-methyl-2-pentanone to feed oxy-acetylene or oxy-hydrogen flames. Investigations were made on the effect of the variables, viz., slit-width, flow rates, flow ratios and flame positions, on the intensity of aluminum absorption. The data were studied to determine the optimum conditions. These investigations brought out the comparative merits of the two flames for the determination of aluminum, and also elucidated the mechanism of aluminum absorption in an oxy-acetylene flame.     

Some observations on the chemiluminescence of atoms in acetylene flames supported by air and argon-oxygen mixtures

A study has been made of the chemiluminescent emission of As, Bi, Cd, Ge, Hg, I, Mo, Pb, Sb, Se, Sn, Te, V and Zn in the primary combustion zones of air-acetylene and argon-oxygen-acetylene flames, supported at an open burner port during the aspiration of aqueous solutions of their salts. In general, elements having excitation, potentials greater than 4 eV show considerably greater atomic chemiluminescence in the primary zone than “thermal” atomic emission in the interconal region. Various mechanisms are suggested for the energy-transfer reactions between metal atoms and excited flame species, particularly carbon monoxide.     

Atomic fluorescence and atomic emission measurements with an image dissector echelle spectrometer

This paper deals with the investigation of an image dissector echelle spectrometer as an analytical instrument for flame atomic fluorescence spectrometry and for flame atomic emission spectroscopy. The fluorescence was induced by high-pressure xenon arc lamps, which emitted continuum spectra and had higher power ratings, i.e. 1.6 and 2.5 kW, than those normally used for the same purpose. The experimental set-up included two different types of premix burners and one type of total consumption burner. A spherical reflector was applied to improve the utilization of the fluorescence radiation. Two different coatings were tested. None gave the expected enhancement.Detection limits and growth curves were measured for 8 different elements (Ca, Co, Cu, Fe, K, Mg, Na and Ni) in a non-separated air/acetylene flame. The attained detection limits were found to be equally good or somewhat better in flame atomic fluorescence excited with continuum sources than previously reported in the literature, i.e. using similar flames. In flame atomic emission spectroscopy better detection limits have been reported before.     

Mechanical feed burner with total consumption for flame photometry and atomic absorption spectroscopy

A mechanical feed burner has been developed for use in flame photometry and atomic absorption spectroscopy. The optimum flame conditions for cinisaion and absorption are very different. These conditions are also modified when organic instead of aqueous solvents are used. When different organic solvents are used, the interference is eliminated with atomic absorption but not emission. Flame profiles of a.tomic absorption and emission signals indicate that the processes are independent; the best signal for each is obtained at different parts of the flame. With emission, it appears that line spectra and background emission originate from the same process e.g. chemiluminescence