Atomization efficiencies in halocarbon-loaded acetylene-air flames—II. Formation of dihalides and combined effects on vaporization and gas phase atomization processes

Halocarbon vapours (CCl₄ and in some cases CF₂Cl₂) were introduced into an acetylene-air flame at increasing rates while nebulizing individual solutions of alkaline earth elements, iron, manganese, chromium, molybdenum, tin and also magnesium in the presence of aluminium matrix. Following the theory developed by Sugden and Bulewicz dealing with the signal depression caused by the presence of halocarbons, the formation of dichloride species for the alkaline earth metals and also for iron, manganese and chromium could be elucidated. From the anomalous shapes of the Sugden-Bulewicz plots, incomplete solute vaporization is inferred for magnesium in the presence of aluminium. For the latter binary system, an almost complete evaporation of magnesium could be attained at 1.2% volume concentration of CCl₄ vapour in the flame.     

Some observations on the interdependence of cyanide concentration and atomization efficiency in the nitrous oxide-acetylene flame

A relationship is derived between CN concentration and atomization efficiency in the C₂H₂-N₂O flame. The relationship is examined under various experimental conditions and β values are estimated for a number of elements. The possibility of complex vapour phase oxide formation by U is considered.     

Halogenation with CCl4 and CF2Cl2 vapours in arc emission spectrometry

Halocarbon vapour diluted with air or nitrogen was applied during the arc excitation of carbide forming elements in solution form, of metal samples (copper, aluminium) and powder samples (alumina, glass, RU-powder) on graphite supporting electrodes. Means were developed for the carrier vaporization of CCl₄ and for the introduction of the halocarbon vapour into the arc discharge. The gaseous agent was supplied continuously during excitation. This possibility was also subjected to some theoretical predictions. On applying CCl₄ with samples introduced in solution form, the volatilization rates of the most refractory elements (e.g. W) were found to increase at least 50 times on the basis of line intensities. Fractional distillation could be attained on constituents and matrices similar to those reported with solid agents, but the gaseous agents could be applied more easily and without contamination problems. The overall effect of halogenation on excitation processes was evaluated from line intensities integrated over the total evaporation time of a complex powder sample, with and without graphite powder dilution. A high intensification (a factor of 3–12) was obtained for the u.v. lines of the refractory constituents with halogenation, which was attributed to the increased efficiency of these elements in entering the excitation zone. Decrease in the intensities of barium atom and ion lines in the VIS range and a decrease of self-absorption were found as a consequence of halide formation in the arc fringe.     

Investigation of the mechanism of formation of chlorine atoms in chlorine-hydrogen plasmas

To explain the trend of an experimental dependence for concentration of chlorine atoms in a discharge, the principal processes of the formation and decay in the plasma of a chlorine mixture with hydrogen were considered. The rate coefficients and rates of processes involving electrons were calculated by mathematical modeling in order to reveal the contribution of each process to the observed concentration of chlorine atoms. The greatest contribution is made by direct electron-impact dissociation. The experimental concentration of chlorine atoms at a high percentage of hydrogen is explained by an increase in electron concentration and, hence, in the rate of the direct dissociation process. It is suggested that chain reactions can make a substantial contribution to the formation of atoms at low hydrogen concentrations.     

Optimization of the electrothermal vaporization conditions for inductively coupled plasma excitation spectrometry: Selective volatilization versus covolatilization approaches

A new version of the outlet port of a graphite furnace electrothermal vaporizer (upward streaming system) is described, in which the hot sample vapour is mixed with an auxiliary carrier argon stream of ambient temperature. The operation procedures using carrier volatilization of organic liquids as gas phase additives are also outlined. The selective volatilization and transport efficiency for As, Cd, Hg, Pb, Sb, Se and Zn could be increased by applying sodium thiosulfate as chemical modifier to solution samples with controlled nitric acid content. On the other hand, a near simultaneous vaporization of 16 elements using chlorination with CCl(4) vapour at 2100 degrees C could be performed for a multielement analysis. By wetting the auxiliary carrier argon stream, the linearity of the analytical curves was improved (except for chromium), when applying multielement standards. Linear analytical curves could also be obtained in the presence of alkali and alkaline earth metal matrices in multielement standards using halocarbon assisted electrothermal vaporization sample introduction.     

Optimization of the electrothermal vaporization conditions for inductively coupled plasma excitation spectrometry: Selective volatilization versus covolatilization approaches

A new version of the outlet port of a graphite furnace electrothermal vaporizer (upward streaming system) is described, in which the hot sample vapour is mixed with an auxiliary carrier argon stream of ambient temperature. The operation procedures using carrier volatilization of organic liquids as gas phase additives are also outlined. The selective volatilization and transport efficiency for As, Cd, Hg, Pb, Sb, Se and Zn could be increased by applying sodium thiosulfate as chemical modifier to solution samples with controlled nitric acid content. On the other hand, a near simultaneous vaporization of 16 elements using chlorination with CCl₄ vapour at 2100° C could be performed for a multielement analysis. By wetting the auxiliary carrier argon stream, the linearity of the analytical curves was improved (except for chromium), when applying multielement standards. Linear analytical curves could also be obtained in the presence of alkali and alkaline earth metal matrices in multielement standards using halocarbon assisted electrothermal vaporization sample introduction.     

Saturation of energy levels in analytical atomic fluorescence spectrometry—II. Experimental

In Part I of this investigation, a theoretical model was proposed to describe the saturation of atomic energy levels under conditions of intense but brief irradiation by a suitable excitation source. The experimental verification of that model is presented herein. In this study, the effects of dye laser-induced saturation of analyte concentration, flame composition and atomic properties of the elements were all examined and quantitated in terms of a measurable parameter, the saturation spectral power density (SSPD). The results of those studies reveal that SSPD is relatively independent of analyte concentration and flame composition but is a strong function of the nature of each particular atomic transition employed. Moreover, because of strong quenching in most analytical flames, a simple steady-state model for saturation applies even for brief (5.6 ns) pulses from a nitrogen-laser pumped dye laser. Most importantly, it is shown that reliable values for the SSPD can be obtained only through careful experimental design; considerations important in such measurements are therefore carefully detailed.     

Radiolytic generation and photochemistry of 9-xanthenyl radicals in halogenated solvents

The photochemistry of 9-xanthenyl radicals produced by pulse radiolysis of xanthene in halocarbon was studied by means of a successive laser flash photolysis in the presence and absence of oxygen. In deaerated solutions rapid (during 6 ns laser pulse) and permanent photobleaching due to chlorine atom transfer from solvents to the excited 9-xanthenyl radical was observed with quantum yields of 0.04 and 0.26 in 1,2-dichloroethane and CCl₄, respectively. In the solutions containing oxygen, equilibrium between 9-xanthenyl radicals and peroxyl radicals was established and recovery of the photobleached 9-xanthenyl radicals was observed, which was accounted for by dissociation of peroxyl radicals. The whole reaction scheme of formation and decay of 9-xanthenyl radicals in CCl₄ is discussed based on the kinetic simulations.     

Photooxidation of tetrathiatetracene and tetraselenatetracene in halocarbon solutions: formation of new radical-cation salts

The photooxidation of tetrathiatetracene TTT and its selenium analogue TSeT to halocarbon solutions by visible and UV irradiation have been studied. This reaction leads to the formation of new chlorine, bromine, and iodine radical-cation salts which have been characterized.     

Efficiency of sample introduction into inductively coupled plasma by graphite furnace electrothermal vaporization

A laboratory constructed graphite furnace electrothermal vaporizer (GF-ETV) was used for studying transport efficiencies. This device enables collection of the vaporization products that exit the central sampling hole of the horizontal graphite tube. For determination of the transport efficiency between the GF-ETV and the ICP-torch three methods were tested: (1) deposition of the aerosol particles and the vapour of certain elements by mixing the vaporization product with supersaturated steam and subsequent condensation (direct method); (2) dissolution of the deposited sample fraction from the interface components (indirect method); and (3) calculation from line intensities when applying GF-ETV and pneumatic nebulization sample introduction methods using mercury as a reference element. The latter, `mercury reference method' required 100% transport efficiency for mercury with the ETV, which could be approximated with the use of argon as carrier gas (without halocarbon addition). With a 200 cm³/min flow rate of internal argon in the graphite tube, the transport efficiency was between 67 and 76% for medium volatility elements (Cu, Mn and Mg) and between 32 and 38% for volatile elements (Cd and Zn). By adding carbon tetrachloride vapour to the internal argon flow, the transport efficiency increased to 67–73% for the five elements studied.     

Investigation of the functionality of different burner types used for the Wickbold decomposition method

The functionality of different burner types used for the Wickbold decomposition method was investigated to understand the low recovery rate problem. The elements chlorine and sulphur were the subject of recovery experiments. The influence of the amount of sample transported into the oxyhydrogen flame on the efficiency of combustion is presented. The difference between the manner of transportation into the flame, direct nebulisation of liquids or previous evaporation are investigated. A new nebuliser for introducing liquids into the oxyhydrogen flame with an evaporation step was developed to reduce decomposition problems. Received: 6 November 1996 / Revised: 16 December 1996 / Accepted: 3 January 1997     

A possible steady state kinetic model for the atomization process during flame atomic spectrometry: Application to atomization interference effects of aluminum on Group II elements

The atomization interference effects of AlCl₃ on MgCl₂ and CaCl₂ during flame atomic absorption spectrometry were studied in terms of a steady state kinetic model which takes into account relative rates of thermal dissociation of analyte and interferant metal salts, recombination of counter atom and analyte and interferant atoms, charge transfer between analyte and interferant species, and ion/electron collisional de-ionization within the plasma. Data are presented showing that the interference of AlCl₃ on the atomization of MgCl₂ and CaCl₂ in an air-acetylene flame is due (a) to interference effects due to AlCl₃ which occur prior to complete atomization of the Group II metal chlorides, and (b) to the recombination of the Group II metal and the chlorine atom which is enhanced by the increased density of the chlorine atom as a result of the dissociation of the AlCl₃ interferant.     

Pyrene measurements in sooting low pressure methane flames by jet-cooled laser-induced fluorescence

This paper presents in detail the study we carried out concerning the pyrene measurement by jet-cooled laser-induced fluorescence (JCLIF) in different sooting low pressure methane flames. The aim of this paper is both to demonstrate the potentialities of this technique for the measurement of such moderately sized polycyclic aromatic hydrocarbons under sooting flame conditions and to provide new experimental data for the understanding and the development of chemical models of the soot formation processes. Several concentration profiles of pyrene measured in different sooting flame (various pressure and equivalence ratio) are presented. The validation of the JCLIF method for pyrene measurements is explained in detail as well as the calibration procedure, based on the standard addition method, which has been implemented for the quantification of the concentration profiles. Sensitivity lower than 1 ppb was obtained for the measurement of this species under sooting flame conditions.     

A possible steady state kinetic model for the atomization process during flame atomic spectrometry: Application to atomization interference effects of aluminum on Group II elements

The atomization interference effects of AlCl₃ on MgCl₂ and CaCl₂ during flame atomic absorption spectrometry were studied in terms of a steady state kinetic model which takes into account relative rates of thermal dissociation of analyte and interferant metal salts, recombination of counter atom and analyte and interferant atoms, charge transfer between analyte and interferant species, and ion/electron collisional de-ionization within the plasma. Data are presented showing that the interference of AlCl₃ on the atomization of MgCl₂ and CaCl₂ in an air-acetylene flame is due (a) to interference effects due to AlCl₃ which occur prior to complete atomization of the Group II metal chlorides, and (b) to the recombination of the Group II metal and the chlorine atom which is enhanced by the increased density of the chlorine atom as a result of the dissociation of the AlCl₃ interferant. Received: 23 February 1998 / Revised: 18 December 1998 / Accepted: 25 December 1998     

Stocastic Measurement of Signals in Flame Atomic Absorption Spectrophotometry

Abstract

In flame atomic absorption spectrophotometry (US) there is usually more than one signal frequency for a given concentration when measured over a time interval. Certain elements have greater variability of signals than others. This study shows there is not much difference in correlation coefficients of a concentration and the mode and concentration and the arithmetic mean of the signals.     

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.     

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.     

Heterogeneous reactivity of chlorine atoms with ammonium sulfate and ammonium nitrate particles

In this laboratory study, model particles of ammonium sulfate (AS) and ammonium nitrate (AN) were exposed to chlorine atoms and uptake experiments were performed in a coated wall flow tube reactor coupled to a molecular beam mass spectrometer. The reactive surfaces were prepared by coating the inner surface of the reactor using two different methods: either by depositing size-selected particles on the halocarbon wax or by spray depositing thin films using a constant output atomizer. The observed uptake coefficients vary for (NH(4))(2)SO(4), ranging from γ(Cl)(AS)≈ 1 × 10(-3) for size-selected particles to γ(Cl)(AS)≈ 6 × 10(-2) for thin films prepared by spray. An uptake coefficient of γ(Cl)(AN)≈ 2.5 × 10(-3) of Cl˙ on size-selected NH(4)NO(3) particles was measured. A heterogeneous recombination of Cl atoms to from Cl(2) molecules was observed for the two surfaces. Furthermore, an ageing process was observed for AS particles, this phenomenon leading to the formation of new chlorine species on the solid substrate.     

Graphite furnace and flame atomic-absorption technique for thermoanalytical investigations

Aerosol particles formed from the vapour of electrothermally heated substances were introduced into an acetylene-air flame for atomization and detection. Thus individual observations could be made on condensed phase processes taking place in the furnace. Curves of absorbance vs. furnace temperature for several zinc compounds were recorded and compared with the corresponding thermoanalytical DTG-curves.     

Spectroscopic flame temperature measurements and their physical significance-III. Existence of isothermal zones in some laboratory flames

For the accurate measurement of certain atomic and molecular properties, such as absolute or relative transition probabilities, dissociation energies and optical cross sections, it is desirable to form and observe atoms and molecules in an isothermal environment whose temperature can be measured precisely. In this paper it is shown that a premixed N₂O-C₂H₂ flame burning on a long path, slot burner can provide not only a relatively high temperature (~3000 K) but also an isothermal environment as well. In this flame, well developed atomic emission spectra of all the metallic elements may be readily observed. This flame and burner system should therefore be useful for the determination of more accurate relative transition probabilities for many elements.