A Thermo-Chemical Reactor for analytical atomic spectrometry

A novel atomization/vaporization system for analytical atomic spectrometry is developed. It consists of two electrically and thermally separated parts that can be heated separately. Unlike conventional electrothermal atomizers in which atomization occurs immediately above the vaporization site and at the same instant of time, the proposed system allows analyte atomization via an intermediate stage of fractional condensation as a two stage process: Vaporization → Condensation → Atomization. The condensation step is selective since vaporized matrix constituents are mainly non-condensable gases and leave the system by diffusion while analyte species are trapped on the cold surface of a condenser. This kind of sample distillation keeps all the advantages of traditional electrothermal atomization and allows significant reduction of matrix interferences. Integration into one design a vaporizer, condenser and atomizer gives much more flexibility for in situ sample treatment and thus the system is called a Thermo-Chemical Reactor (TCR).     

Influence of pressure and atomizer length on absorption curves in ETA-AAS measurements for standardless analysis

A new method for standardless analysis in electrothermal atomic absorption spectroscopy has been recently proposed whose implementation requires the use of special atomizers and power supplies not available in the market. Up to now, with the proposed method, only volatile elements have been determined with good results because it can be applied only if all atoms injected are simultaneously present in the atomizer for a time interval sufficiently long that a good value of absorbance can be measured. This can be obtained if the heating rate is sufficiently high and the removal of atoms from the atomizer slow. From a theoretical model based only on diffusion, it can be deduced that the possibility of standardless analysis in the mode described above can be enhanced if the length of the atomizer is increased and the diffusion coefficient of the analyte is decreased. From the results obtained by comparing data from atomizers of 36 and 50 mm in length and at different pressures, it can be inferred that the removal mechanism is basically controlled by diffusion but that other mechanisms, like convection, are also present. Received: 2 May 1997 / Revised: 15 December 1997 / Accepted: 15 January 1998     

Influence of pressure and atomizer length on absorption curves in ETA-AAS measurements for standardless analysis

A new method for standardless analysis in electrothermal atomic absorption spectroscopy has been recently proposed whose implementation requires the use of special atomizers and power supplies not available in the market. Up to now, with the proposed method, only volatile elements have been determined with good results because it can be applied only if all atoms injected are simultaneously present in the atomizer for a time interval sufficiently long that a good value of absorbance can be measured. This can be obtained if the heating rate is sufficiently high and the removal of atoms from the atomizer slow. From a theoretical model based only on diffusion, it can be deduced that the possibility of standardless analysis in the mode described above can be enhanced if the length of the atomizer is increased and the diffusion coefficient of the analyte is decreased. From the results obtained by comparing data from atomizers of 36 and 50 mm in length and at different pressures, it can be inferred that the removal mechanism is basically controlled by diffusion but that other mechanisms, like convection, are also present. Received: 2 May 1997 / Revised: 15 December 1997 / Accepted: 15 January 1998     

Tungsten devices in analytical atomic spectrometry

Tungsten devices have been employed in analytical atomic spectrometry for approximately 30 years. Most of these atomizers can be electrically heated up to 3000 °C at very high heating rates, with a simple power supply. Usually, a tungsten device is employed in one of two modes: as an electrothermal atomizer with which the sample vapor is probed directly, or as an electrothermal vaporizer, which produces a sample aerosol that is then carried to a separate atomizer for analysis. Tungsten devices may take various physical shapes: tubes, cups, boats, ribbons, wires, filaments, coils and loops. Most of these orientations have been applied to many analytical techniques, such as atomic absorption spectrometry, atomic emission spectrometry, atomic fluorescence spectrometry, laser excited atomic fluorescence spectrometry, metastable transfer emission spectroscopy, inductively coupled plasma optical emission spectrometry, inductively coupled plasma mass spectrometry and microwave plasma atomic spectrometry. The analytical figures of merit and the practical applications reported for these techniques are reviewed. Atomization mechanisms reported for tungsten atomizers are also briefly summarized. In addition, less common applications of tungsten devices are discussed, including analyte preconcentration by adsorption or electrodeposition and electrothermal separation of analytes prior to analysis. Tungsten atomization devices continue to provide simple, versatile alternatives for analytical atomic spectrometry.     

Electrothermal atomic-absorption and atomic-fluorescence spectrometry with a tungsten-coil atomizer

A pulsed electrothermal atomizer of the tungsten-coil type and apparatus for its application in atomic-absorption and atomic-fluorescence spectrometry are described. A tungsten-coil atomizer is shown to be just as good as commercial electrothermal atomizers with regard to sensitivity and reproducibility, but to have better operating characteristics. A theoretical model for formation of the atom cloud is given. Mechanisms for atomization of different groups of element in an atmosphere of pure argon and in the presence of reductants (hydrogen and carbon) are proposed.     

Study of the electrothermal atomization of selenium (IV) in transversely-heated graphite atomizers in the presence of nickel compounds as chemical modifiers

The electrothermal atomization of selenium, in transversely-heated graphite atomizers, was studied with regard to the effect of the addition of nickel nitrate and nickel chloride as chemical modifiers. Particular importance was given to the behavior of the analyte in aqueous standards and in sodium chloride containing solutions, after thermal reduction of the modifiers prior to the injection of the analyte solution into the atomizer. Thermal reduction of either nickel compound brings about an enhancement in the sensitivity of selenium measurements, as compared to those obtained by the injection of both the analyte and the modifier together into the atomizer. Additionally, thermal reduction of the modifiers permits the presence of chloride, as sodium chloride, to be tolerated in amounts as high as 500 μg Cl⁻ with sensitivity losses lower than 10%.     

Shadow spectral imaging of absorbing layers in a transversely heated graphite atomizer: Part 1. Analyte atoms

The technique of shadow spectral imaging was used to investigate dynamics of formation and dissipation of Ag, In, Ga, Bi, Mn, Cu and Tl atomic layers in a transversely heated graphite tube atomizer (THGA) with and without integrated platform under gas-stop and gas-flow conditions. It is shown that non-uniform heating of the tube walls and platform surface in the radial cross section is the main reason for analyte transfer from atomizer bottom to less heated sides of the tube and platform before atomization temperature is reached. This transfer in the atomizer transverse cross section can be an additional factor that reduces matrix interferences in the THGA. In all the investigated cases, the atomic absorbing layers are not spatially uniform. Absorbance gradients grow up to 0.2 mm^− 1 even in the case of chemically inert silver atomization. Inverse atomization of In, Bi, Ga and Tl when atoms first appear in the atomizer's upper part was detected in THGA with platform. The effect of the internal gas flow on the spatial structure of analyte atoms is less pronounced in the transversely heated atomizer as compared to the end-heated furnaces.     

Simulation of atomic absorption signals: A kinetic model with two independent sources

A kinetic model of atomization processes based on the solution of one-dimensional diffusion equation with two independent sources is proposed. One of the sources describes the atomization of atoms from the graphite furnace surface, while another one describes the atom formation inside the walls of the furnace and their subsequent outflow into the analytical zone. This mechanism is used to describe electrothermal atomization of Cu and Ag. The simulations show that the form of atomic absorption signal of Cu is determined to the great extent by the processes of desorption from the graphite surface and diffusion inside the graphite. The tailing of the back edge of absorption profile can be explained by the rather slow diffusion process of copper atoms in the graphite. At the same time, for the atomization of Ag, the process of separation of single atoms from clusters is the limiting process.A new interpretation for the shift of absorbance maximum as the initial mass of Ag increases is proposed.     

Computer simulation of two-step atomization in graphite furnaces for analytical atomic spectrometry

The processes of sample fractionation by two-step atomization with the intermediate condensation of the analyte on a cold surface in graphite furnaces were theoretically studied. The transfer equation was solved for the atoms, molecules, and condensed particles of the sample from a flow of argon directed along this surface. The spatial distributions of vapor and the condensate formed were calculated depending on the composition and flow rate. It was found that a cold surface section with a length of 6 mm is sufficient for the complete trapping of atomic analyte vapor from an argon layer having a velocity of about 1 m/sec and a thickness of 5 mm. In this case, the molecules and clusters condensation coefficients smaller than unity were deposited insignificantly; that is, they were fractionally separated. The results of the shadow spectral visualization of the process of sample fractionation on a cold probe surface of in commercial HGA and THGA atomizers were interpreted. The advantages of analytical signals upon the evaporation of a sample condensate from the probe in these atomizers and inductively coupled plasma were demonstrated.     

The relation of double peaks, observed in quartz hydride atomizers, to the fate of free analyte atoms in the determination of arsenic and selenium by atomic absorption spectrometry

The mechanism at the origin of double peaks formation in quartz hydride atomizers were investigated by continuous flow hydride generation atomic absorption spectrometry. Arsenic and selenium were used as model analytes. The effect of atomization mode (flame-in-gas-shield (FIGS), miniature diffusion flame and double flame (DF)) and some experimental parameters as oxygen supply rate for microflame and the distance from atomization to free atoms detection point, were investigated on the shape of both analytical signals and calibration graphs. Rollover of calibration graphs and double peak formation are strictly related each to the other and could be observed only in FIGS atomizer mode under some particular conditions. A mechanism based on incomplete atomization of hydrides cannot explain the collected experimental evidences because the microflame of FIGS is able to produce quantitative atomization of large amount of hydrides even at supply rate of oxygen close to extinction threshold of microflame. The heterogeneous gas–solid reactions between finely dispersed particles, formed by free atom recombination, and the free atoms in the gaseous phase are at the origin of double peak formation.     

Some observations on the mechanisms of atomization in atomic absorption spectrometry with atom-trapping and electrothermal techniques

The mechanisms of collection and release of sixteen elements in atom-trapping atomic absorption spectrometry with a water-cooled silica trap in an air-acetylene flame are examined. Ag, Au, Cd, Co, Cu, Fe, Ni, Pb, Se and Zn appear to accumulate as metals whilst K, Li, Na, Cr, Mg and Mn are trapped as silicates or oxides. Al and V are also trapped as oxides, but were not studied further. No evidence could be found that the surface temperature of the trap exceeds 1700 K during the release cycle. Plots of appearance time of atoms vs. m.p. suggest that while direct evaporation can play a part in atomization, sputtering by energetic species in the scouring flame gases may explain the appearance of gaseous atoms at the relatively low temperatures involved. The atomization phenomena are related to those observed with electrothermal atomizers based on carbon and tantalum. It is suggested that sputtering processes may also be involved in such atomizers.     

Untersuchungen zur bestimmung seltener erden durch atomabsorption mit elektrothermischer atomisierung

The determination of rare earths by atomic absorption spectrometry with electrothermal atomizationThe electrothermal atomization of traces of rare earths has been investigated with different atomizers (carbon rod, graphite furnace, tantalum ribbon). The best analytical results are obtained with a modified tantalum thermal atomizer, because the formation of rare earth carbides is then impossible. Mixed argon—hydrogen atmospheres improve the concentration of atoms in the plasma, because hydrogen reduces the rare earth oxide radicals. The optimal analytical conditions are described. The detection limits are: 25 pg Yb, 22 pg Eu, 62 pg Tm, 2000 pg Sm, 300 pg Ho, 300 pg Dy, 1300 pg Er.     

Selenium hydride atomization,fate of free atoms and spectroscopic temperature in miniature diffusion flame atomizer studied by atomic absorption spectrometry

The mechanism of hydride atomization and the fate of free atoms was investigated in the miniature diffusion flame. Selenium hydride was used as a model for the other hydrides. Mercury vapor was employed as an analyte to study physical processes, such as macroscopic movements and free atom diffusion, controlling the distribution of free analyte atoms in the observation volume, separately from chemical reactions of the free atoms. Free atoms were detected by atomic absorption spectrometry. Spectroscopic temperature measurements based on atomic absorption at 196.1 and 204.0 nm Se lines were used to determine the temperature distribution. The spatial temperature distribution was highly inhomogeneous ranging from 150°C to 1300°C. The whole flame volume is actually a cloud of hydrogen radicals maintaining analyte in the free atom state since hydrogen radicals formed in outer zone of the flame diffuse to its cooler inner parts.     

Monte Carlo optimization of signal-to-noise ratio for electrothermal atomizers

This work uses Monte Carlo simulation models for Cu and Ag to study the change in the signal-to-noise ratio (S/N) with variations on the electrothermal atomizer length and diameter. A 5 × 5 grid corresponding to different lengths (2.0–4.0 cm) and diameters (0.30–0.70 cm) is filled with S/N data from the simulations. It is assumed that the S/N is shot-noise limited. The dosing hole diameter is held at 1.5 mm in all cases. A double-desorption type mechanism for Ag atomization was tested against experimental profiles and good agreement was achieved. Spherical microdroplets release Ag(g) with a fractional order (2/3) and an activation energy of 238 kJ mol⁻¹. Ag(g) readsorbs as dispersed atoms on the wall upon collision. The secondary desorption is first order and has a lower activation barrier (117 kJ mol⁻¹). The S/N predicted by the Monte Carlo simulation does not strongly depend on the heating rate or the nature of the analyte being determined. The optimum with respect to geometry is broad. The optimum furnace diameter is near 0.50 cm, while the optimum furnace length is at the 4.0-cm limit used in these studies. Most commercial atomizers have diameters close to the optimum but shorter lengths, probably reflecting the pragmatic consideration that the longer tubes require larger power supplies and prolonged heating times which may substantially reduce the useful life of the furnace. Further increases in length may also accentuate atomizer nonisothermality which deters analytical improvement.     

Recent developments in atomizers for electrothermal atomic absorption spectrometry

This review first describes general requirements to be met for suitable base materials used to produce electrothermal atomizers (ETAs). In this connection the physical and chemical properties of adequate types of graphite and metals are discussed. Further, various atomizer designs, their temperature dynamics during atomization and general performance characteristics are critically reviewed. For end-heated Massmann-type atomizers, discussions are focused on recent developments of, e.g., contoured tubes to achieve improved temperature homogeneity over the tube length, second surface atomizers to realize temporally isothermal atomization and tubes with graphite filters to reduce interference effects. The state-of-the-art of platform equipped, side-heated atomizers with integrated contacting bridges are characterized mainly with respect to heating dynamics, as well as susceptibility to interference- and memory effects. In contrast to end-heated ETAs, the tube ends of side-heated ETAs are freely located in the furnace compartment and, as a consequence of this configuration, convective gas flows can easily appear. The magnitude and effect of these flows on analytical performance are discussed and measures are suggested, permitting operation under diffusion controlled conditions. A critical comparison of classical constant temperature atomizers with state-of-the-art platform equipped ETAs is made and from this it is concluded that future ETA developments are likely to involve only minor modifications aiming at, e.g., the reduction of cycling times or the improvement of tube surface properties.     

Tungsten atomizer--theory of atomization mechanism of some volatile analytes

Several types of chemical reactions may participate in the evolution of free atoms in a tungsten furnace. Reactions may take place either in the homogeneous or heterogeneous phase. The assumed reactions may be classified into four types according to the phases in which they take place. Reactions occurring in the gaseous phase, i.e. in the inner volume of the furnace, are kinetically more significant. However, for atomization of easily volatile analytes heterogeneous reaction between gaseous compounds and between condensed salts of analytes and the solid surface of the furnace become significant. With regards to the reaction mechanisms during drying, pyrolysis and atomization of nitrates of volatile analytes, three basic types of chemical reactions may be assumed. Free atoms of analytes arise by evaporation of the elementary form of analytes at atomization temperature, where the particular analyte in its elementary form is produced by direct reduction of analyte nitrate by tungsten or by hydrogen at higher temperatures. Precursory reactions of atom formation are reduction reactions which occur between analyte nitrates and tungsten, between analyte nitrates and hydrogen, as well as reactions of thermal dissociation of relevant nitrates. The importance of particular types of precursory reactions for formation of metallic analytes or their oxides is documented by dependence of Gibbs energy values of particular reactions on the temperature.     

Recent developments in atomizers for electrothermal atomic absorption spectrometry

This review first describes general requirements to be met for suitable base materials used to produce electrothermal atomizers (ETAs). In this connection the physical and chemical properties of adequate types of graphite and metals are discussed. Further, various atomizer designs, their temperature dynamics during atomization and general performance characteristics are critically reviewed. For end-heated Massmann-type atomizers, discussions are focused on recent developments of, e.g., contoured tubes to achieve improved temperature homogeneity over the tube length, second surface atomizers to realize temporally isothermal atomization and tubes with graphite filters to reduce interference effects. The state-of-the-art of platform equipped, side-heated atomizers with integrated contacting bridges are characterized mainly with respect to heating dynamics, as well as susceptibility to interference- and memory effects. In contrast to end-heated ETAs, the tube ends of side-heated ETAs are freely located in the furnace compartment and, as a consequence of this configuration, convective gas flows can easily appear. The magnitude and effect of these flows on analytical performance are discussed and measures are suggested, permitting operation under diffusion controlled conditions. A critical comparison of classical constant temperature atomizers with state-of-the-art platform equipped ETAs is made and from this it is concluded that future ETA developments are likely to involve only minor modifications aiming at, e.g., the reduction of cycling times or the improvement of tube surface properties.     

Tungsten atomizer – theory of atomization mechanism of some volatile analytes

Several types of chemical reactions may participate in the evolution of free atoms in a tungsten furnace. Reactions may take place either in the homogeneous or heterogeneous phase. The assumed reactions may be classified into four types according to the phases in which they take place. Reactions occurring in the gaseous phase, i.e. in the inner volume of the furnace, are kinetically more significant. However, for atomization of easily volatile analytes heterogeneous reaction between gaseous compounds and between condensed salts of analytes and the solid surface of the furnace become significant. With regards to the reaction mechanisms during drying, pyrolysis and atomization of nitrates of volatile analytes, three basic types of chemical reactions may be assumed. Free atoms of analytes arise by evaporation of the elementary form of analytes at atomization temperature, where the particular analyte in its elementary form is produced by direct reduction of analyte nitrate by tungsten or by hydrogen at higher temperatures. Precursory reactions of atom formation are reduction reactions which occur between analyte nitrates and tungsten, between analyte nitrates and hydrogen, as well as reactions of thermal dissociation of relevant nitrates. The importance of particular types of precursory reactions for formation of metallic analytes or their oxides is documented by dependence of Gibbs energy values of particular reactions on the temperature.     

Use of modifiers with metal atomizers in electrothermal AAS: a short review

The evolution of chemical modifiers for Mo and W atomizers is slowly progressing, based mainly on trial and error experimentation. Despite repeated use of some chemical compounds, such as thiourea for Mo atomizers, at this point there is no panacea similar to the Pd+Mg mixture used widely in graphite furnace atomic absorption spectrometry. Clearly, the chemical processes involved during atomization from a metal atomizer differ significantly from those occurring in a graphite tube, so successful graphite modifiers may not be readily adapted to metal atomizers. As a result, the analyst must begin anew with the evaluation of many potential modifiers in hopes of finally arriving at some universal solution. The purpose of this review, with 62 references, is to describe that journey to date, and to point out some promising paths that may lead to future success: such as the development of permanent chemical modifiers for W and Mo electrothermal atomizers.