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.     

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.     

Einsatz der flammenlosen Atom-Absorption (Graphitrohrküvette) zur Bestimmung von Kupferspuren in Stahl und anderen Metall-Legierungen nach Extraktion

The determination of small and smallest amounts of copper in steel and other metal alloys by atomic absorption spectroscopy was investigated. It has been found, that the interferences caused by accompanying elements in the determination in aqueous solution with the flame can be completely removed by extracting the copper with Pb-DDTC in chloroform and determining this extract with the heated graphite atomizer. Moreover, it was recognised, that working with the heated graphite atomizer in organic solutions shows great advantages compared with the flame, because any solvent may be used. The methods of continuous variation by Job and the molar ratio by Yoe and Jones for the determination of the complex stoichiometry were transferred to the system Cu-DDTC by working with the heated graphite atomizer.     

Mechanism of elemental spectral excitation in flame photometry

It is generally believed that emission spectra result from thermal excitation. 1t is proposed that ultraviolet light present in a flame is. also responsible for exciting metal atoms present. Experimentally it las been shown that the intensity of metal emission spectra is a function of the U.V. characteristics of the flame even at constant flame temperature. It was also demonstrated that spectral intensity can be enhanced by external radiation of U.V. liglit of tlie correct wavelength.     

spectroscopy in separated flames-VI The argon or nitrogen-sheathed nitrous oxide-acetylene flame in atomic-absorption spectroscopy

The separation of the premixed nitrous oxide-acetylene flame at a 50-mm slot burner by sheathing with argon or nitrogen is described. In comparison with the conventional flame, the interconal zone of the hot, slightly fuel-rich separated flames provides better conditions for the maintenance of free atoms of elements which form refractory oxides. Optimum conditions for the determination by atomic-absorption spectroscopy of the elements Al, Be, Ge, Mo, Si, Ti, V and Zr in both separated and conventional flames at the same burner have been established. Significant improvement in detection limits and sensitivities is obtained in the separated flames.     

Studies in atomic-fluorescence spectroscopy-VII Fluorescence and analytical characteristics of arsenic, with a microwave excited electrodeless discharge tube as source

The construction of an electrodeless arsenic discharge tube and its use for atomic-fluorescence studies is described. Cool nitrogen-hydrogen and argon-hydrogen diffusion flames as well as normal premixed flames are considered as atom reservoirs and the atomic-fluorescence emission at 15 different wavelengths is evaluated. The diffusion flames give the largest emission signals at arsenic concentrations below 200 ppm, but show a premature curvature at higher concentrations because of the presence of an abnormally high density of arsenic atoms. Above 200 ppm of arsenic, the premixed air-acetylene flame is superior. The limit of detection at 1890 A is 0.2 ppm of arsenic in the nitrogen-hydrogen diffusion flame and 1.0 ppm in the airacetylene flame. A long path-length diffusion flame is also particularly useful in atomic-absorption measurements because it absorbs very little radiation in the far ultraviolet region and gives an abundance of arsenic atoms.     

Solvent extraction of Ru(III) with 2-mercaptobenzimidazole into n-butanol

A new and rapid method has been developed for the quantitative extraction of Ru(III) with 2-mercaptobenzimidazole into n-butanol. The extraction coefficient of Ru(III) between n-butanol and an ethanolic solution of 1% 2-mercaptobenzimidazole showed a maximum value ofE=161 at a pH of 2.0. The effect of various other parameters on the extraction coefficient value of Ru(III) such as solvent effect, anions, cations, etc. have also been studied. The stoichiometry of metal to reagent has also been determined by the method of substoichiometric extraction and by the slope ratio method and was found to be 1∶3. Decontamination factor for various elements in the substoichiometric extraction of Ru(III) were also evaluated.     

Microdetermination of molybdenum in organometallic compounds by molecular emission and atomic absorption spectrometry

A direct method is described for the determination of molybdenum in mg amounts of organomolybdenum compounds by flame emission or atomic absorption spectrometry. Air/acetylene, air/hydrogen and dinitrogen oxide/acetylene flames were used. The emission of molybdenum oxide is found to be analytically useful in the hydrogen-based flames while the acetylene-based flames are better for atomic absorption. Various organomolybdenum compounds were analysed by both methods as well as by an alternative spectrophotometric method, with satisfactory agreement. The procedure involves simply dissolving the sample in a mixed solvent and aspirating the solution into the flame.     

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.     

The use of potassium hydroxide in 2-ethylhexanoic acid as ionization suppressor in the determination of calcium in lubricating oils by atomic absorption spectrometry

Calcium in lubricating oil cannot always be determined merely by diluting the sample and aspirating into the flame of an atomic absorption spectrometer. The production of free calcium atoms depends on the type of calcium additive, and is influenced by the solvent. The calcium in both the sample solution and the standard solution must be available in the same form. Application of a solution of potassium hydroxide in 2-ethyl-hexanoic acid appears to meet this requirement.     

Mechanism of inhibition of hydrogen/oxygen flames of various compositions by trimethyl phosphate

The effect of the catalytic recombination reactions of H and OH^? involving phosphorus-containing products of trimethyl phosphate (TMP) combustion on the burning velocity and the structure of H₂/O₂/N₂ flames at atmospheric pressure has been investigated. An earlier mechanism for inhibition of rich hydrogen/oxygen flames by organophosphorus compounds has been tested and modified by comparing experimental data with the results of simulation. The sensitivity analysis of the calculated flame speed to the rate constants of chain branching reactions and chain termination reactions involving phosphorus-containing compounds has revealed significant specific features of the inhibition mechanism of hydrogen flames with various stoichiometries and dilution ratios. Unlike the inhibition efficiency of hydrocarbon flames, in which the reactions of H and OH^? radicals with PO, PO₂, HOPO, and HOPO₂ play the key role, the inhibition efficiency of hydrogen flames at atmospheric pressure is determined by the interaction of hydrogen and oxygen atoms with TMP and with organophosphorus products of its decomposition in the low-temperature zone of the flame. The sensitivity analysis has demonstrated that, as the equivalence ratio (?) or the dilution ratio is increased, the ratio of the chain branching rate to the rate of chain termination via reactions involving phosphorus compounds decreases. As a consequence, the efficiency of inhibition of H₂/O₂/N₂ flames, as distinct from that of hydrocarbon flames, increases as ? is raised from 1.1 to 3.0 and as the mixture is progressively diluted with nitrogen.     

Multinuclear NMR studies of the interaction of metal ions with adenine-nucleotides

It is well-known that metal ion complexes are essential in various biological systems, including those with adenosine nucleotides which are substrates for a large number of enzymatic processes. The interactions of various metal ions with adenosine nucleotides have been intensively studied by multinuclear NMR spectroscopy. Nucleotides are polydentate ligands with various potential binding sites, including nitrogen atoms on the purine base, hydroxyl groups on the ribose sugar, and negatively charged oxygen atoms in the phosphate group. Depending on the experimental conditions (e.g. pH, concentration range, etc.) and on the size and nature of the metal ions, monodentate, or multidentate coordination to these donor atoms are possible. The review focuses on the applications of different NMR techniques in identifying the stoichiometry and the mode of metal binding in complexes formed with the most important adenosine nucleotides, like adenosine-5′-mono-, di- and triphosphates (AMP, ADP and ATP). Ligand exchange dynamics for some metal ion complexes are also presented.     

Experimental estimation of fluorescence power efficiencies of several atoms in three turbulent flames used in atomic fluorescence flame spectrometry

An experimental system for the measurement of approximate atomic fluorescence power efficiencies of atoms in turbulent flames used in atomic fluorescence flame spectrometry is described. An expression is derived for the power efficiency as a function of instrumental parameters. Experimentally measured power efficiencies for eleven elements in fuel-rich oxyhydrogen, fuel-rich oxyacetylene, and fuel-rich hydrogen/argon/entrained air flames are given. The power efficiencies vary considerably from one spectral line to another, but as a result of the entrainment of ambient air into the turbulent flames, the values obtained in different flames are approximately equal. It is also shown that fluorescence radiation should rarely produce a significant error in atomic absorption spectrometry.     

Metal ion catalysis of natural products transformations. Novel aldopentose disproportionation reactions catalyzed by rhodium(III)

The detailed stoichiometry of the reaction between the aldopentoses, ribose, arabinose, lyxose, and xylose, and the cis rhodium(III) complex of the macrocyclic tetraamine ligand rac-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane in weakly acidic aqueous solution has been studied by 13C NMR, using deuterium- and carbon-13-labeled substrates. The overall process is a catalytic disproportionation reaction, in which two aldopentose molecules are transformed into the corresponding alditol and aldonolactone, both with an unchanged configuration around carbon atoms 2, 3, and 4. The mechanism of this reaction is suggested to involve coordination of a hydrated and an unhydrated substrate molecule through their carbon-1 bound oxygen atoms followed by a hydride shift from the hydrated to the unhydrated substrate. This disproportionation process is subsequently followed by aquation of the reaction product to give the free alditol and a mixture of the aldonic acid and the corresponding aldonolactone. Concurrently with the aquation reaction, incorporation of solvent hydrogen at the carbon-1 atom of the alditol is also observed for the rhodium-coordinated alditol reaction product.     

Dissociation and ionization effects in atomic absorption spectrochemical analysis

Use of a nitrous oxide-acetylene flame in atomic absorption spectrophotometry reduces or eliminates certain chemical interferences that have been observed in cooler flames. However, ionization increases with temperature, and is significant for some elements in the nitrous oxide-acetylene flame. Ionization can be reduced by adding an easily ionized metal (e.g. alkali metal) to the solution. Elements likely to be determined using the nitrous oxide-acetylene flame which will be significantly ionized are: Al, Ba, Ti, V, Zr, Hf, Nb, Sc, Y, the lanthanides and the actinides. The ionization of an element in the nitrous oxide-acetylene flame can be readily calculated by taking absorption readings, provided that relatively sensitive atom and ion resonance lines are available. This technique possibly could be used to establish ionization potentials or partition functions of those lanthanide elements that are not now well known.     

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.     

Effect of acids on the determination of lead and cadmium by atomic absorption spectrometry

The effect of different mineral acids on the absorption signals of lead and cadmium in atomic absorption spectrometry is reported. The behaviour of the signals was studied with respect to variations in metal concentration, acid concentration, flame stoichiometry and burner height. For the determination of lead and cadmium the most suitable concentrations of hydrochloric, nitric, sulphuric and perchloric acids were 5, 5, 2.5 and 5%, respectively. Similar absorption behaviour was observed in oxidizing and reducing flames for lead and cadmium in all the acids studied.     

Comparative study on camphor enantiomers behavior under the conditions of gas-liquid chromatography and reversed-phase high-performance liquid chromatography systems modified with alpha- and beta-cyclodextrins

The dependence of retention and selectivity parameters of camphor enantiomers on the concentration of alpha- and beta-cyclodextrins were studied under conditions of GLC (matrix solvent: Glycerol, 95 degrees C) and RP-HPLC (matrix solvent: Aqueous methanolic, 20 degrees C). It has been found that beta-cyclodextrin forms complex of 1:1 stoichiometry and does not recognize enantiomers of camphor. In contrast alpha-CD forming complexes of 1:2 stoichiometry appeared to be very efficient chiral selector of (+) and (-)-camphor. Relatively considerable differences have been observed between stability constants determined by GLC and RP-HPLC, what may be explained by the various natures of the matrix solvents and the various temperatures of the measurements. On the contrary, the enantioseparation factor alpha observed at higher concentrations of alpha-cyclodextrin stabilizes on the very similar value alpha+/-(GLC) approximately = alpha-/+(HPLC) approximately = 1.6. Simple theoretical considerations focusing on the differences in the mechanisms of the studied processes have been performed. According to them the enantiomer forming the more stable complex with the cyclodextrin should be eluted from the RP-HPLC column first and GLC column last. This fact has been confirmed experimentally.     

Structural studies on the hydration of L-glutamic acid in solution

A combination of neutron diffraction augmented with isotopic substitution and computer modeling using empirical potential structure refinement has been used to extract detailed structural information for L-glutamic acid dissolved in 2 M NaOH solution. This work shows that the tetrahedral hydrogen bonding network in water is severely disrupted by the addition of glutamic acid and NaOH, with the number of water-water hydrogen bonds being reduced from 1.8 bonds per water molecule in pure water to 1.4 bonds per water molecule in the present solution. In the glutamic acid molecule, each carboxylate oxygen atom forms an average of three hydrogen bonds with the surrounding water solvent with one of these hydrogens being shared between the two oxygen atoms on each carboxylate group, while each amine hydrogen forms a single hydrogen bond with the surrounding water solvent. Additionally, the average conformation of the glutamic acid molecules in these solutions is extracted.     

Mechanism of ligand exchange studied using transition path sampling

The mechanism of intermolecular ligand exchange has been studied using transition path sampling (TPS) based molecular dynamics (MD) simulations. Specifically, the exchange of solvent molecules bound to unsaturated Cr(CO)5 in methanol solution has been investigated. The results of the TPS simulations have shown that there are multiple steps in the reaction mechanism. The first involves partial dissociation of the coordinated solvent from the Cr metal center followed by association with a new methanol molecule between the normally void first and second solvent layers. After diffusive motion of the exchanging ligands, the last step involves the originally bound methanol molecule moving into the bath continuum followed by solvation of the Cr metal fragment by the exchanging ligand. It has been found that the reaction center (defined as the organometallic fragment and two exchanging ligands only) and the solvent bath have favorable interactions. This is likely due to the adiabatic nature of the ligand exchange transition. The ability to understand the microscopic molecular dynamics of a chemical process based on a free energy analysis is also discussed.