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.     

Some observations on the atomic absorption spectrometry of gallium with electrothermal atomizers

Signal strength can be doubled in normal graphite furnaces compared to pyrolytically coated ones, thus proving the essential role of carbon in the atomization of gallium. Platform vaporization enhances both peak height and area signals. The use of ascorbic acid as matrix modifier improves sensitivity considerably, with enhancements being more pronounced in pyrolytically coated furnaces.     

Graphite atomizers modified with high-melting carbides for electrothermal atomic absorption spectrometry. II. Practical aspects

Data relating to practical applications of carbide-modified graphite atomizers (CMGAs) published mainly since 1985 are critically discussed. All elements being determined by means of electrothermal atomic absorption spectrometry are divided into ten groups according to their characteristics in graphite atomizers. CMGAs are the most effective for the determination of the elements forming rather stable oxides, such as Ge, Sn, Ga, In, as well as B and Si. The application of CMGAs to the determination of carbide-forming elements (Cr, Mo, V, Ti, etc.) may cause a significant decrease in their sensitivity. Concerning the high-volatility analytes (semi-metals and boron), CMGAs are usually effective only in the presence of regular chemical modifiers, primarily the salts of Pd and Ni. CMGAs may be successfully used for the determination of some organoelement (with Sn, Pb, Se, As) compounds, as well as for the trapping of volatile hydrides. CMGAs seem to be especially promising for analysis of biological samples, organic extracts, solid samples and samples containing high concentrations of mineral acids. High-melting carbides are prospective permanent modifiers.     

On-line electrothermal atomic absorption spectrometry configurations: recent developments and trends

In the present mini-review, an account of the actual state-of-the-art and future possibilities offered by on-line ET-AAS is presented. Topics such as: (1) on-line analyte preconcentration (by means of precipitation, sorption, solvent extraction, and solid phase extraction); (2) analyte separation by means of chromatography, and electrochemical, microdialysis and chemical vapor generation processes; and (3) sample treatment (by microwave sample digestion, sample emulsification and dilution processes) are used to illustrate the versatility of flow injection, sequential injection analysis, stop flow and continuous flow, when coupled to a graphite furnace. The use of some of the on-line systems for speciation and the simultaneous determination of different analytes is underlined.     

New developments in flow injection separation and preconcentration techniques for electrothermal atomic absorption spectrometry

Flow injection (FI) on-line separation and preconcentration systems for electrothermal atomic absorption spectrometry (ETAAS) are reviewed, highlighting the main developments in the field since 1990 and work achieved in the authors laboratory. These include developments in on-line preconcentration systems based on column techniques, solvent extraction, coprecipitation and hydride sequestration. Advantages, limitations and potentials of the FI-ETAAS combination are discussed.     

New developments in flow injection separation and preconcentration techniques for electrothermal atomic absorption spectrometry

Flow injection (FI) on-line separation and preconcentration systems for electrothermal atomic absorption spectrometry (ETAAS) are reviewed, highlighting the main developments in the field since 1990 and work achieved in the authors' laboratory. These include developments in on-line preconcentration systems based on column techniques, solvent extraction, coprecipitation and hydride sequestration. Advantages, limitations and potentials of the FI-ETAAS combination are discussed.     

Modular L-design of hydride atomizers for atomic absorption spectrometry

A novel modular L-shaped design of hydride atomizer for atomic absorption spectrometry is described. It makes it possible to replace the optical tube of the atomizer and, mainly, to employ optical tubes made also from other materials than fused quartz. The design is useful mainly for further improvement of hydride atomizers based on the multiatomizer concept. Employing selenium hydride as the analyte and arsine as the interferent, a preliminary evaluation of performance of three types of L-shaped multiatomizers based on various optical tubes in terms of sensitivity, linearity of calibration graph and resistance to atomization interferences is made. The “classical” T-shaped multiatomizer was employed as a reference.The L-shaped multiatomizer with the optical tube analogous to that employed in the “classical” T-shaped multiatomizer offers virtually the same performance as the reference multiatomizer. Optical tube made of fused quartz with holes with smaller diameters does not offer significantly better performance compared to the reference T-shaped multiatomizer. However, the L-shaped multiatomizer with optical tube fabricated from porous quartz glass overpowers all the other multiatomizers substantially in terms of the resistance against interferences: even the maximum As interferent concentration of 5 µg ml^− 1 does not significantly influence the observed signal. This should be compared with multiatomizers based on plain fused quartz tubes with holes: tolerance limit around 0.5 µg ml^− 1; interferent concentration of 1 µg ml^− 1 causing 20% signal depression.     

Tube-probe atomisation in electrothermal atomic absorption spectrometry

A method is described in which liquid samples are deposited by a conventional autosampler on to a tubular graphite probe positioned inside a 7.5 mm i.d. Pye Unicam SP-9 atomiser cuvette. The tube-probe has certain advantages in comparison with the flat probe systems described previously. In particular, the precision of determinations in acid media is improved since the sample solution is better confined within the atomiser and hence the effects of droplet spreading are curtailed. Also, the tube-probe is shown to reduce diffusional loss effects at high atomiser temperatures, in comparison with flat probe operation. The characteristic mass values obtained for volatile and medium volatile elements are similar for tube and flat probes. Since a greater sample volume (up to 40 μl) can be deposited and dried in the tube-probe, improved detection limits are anticipated for these elements. For more involatile elements, the greater mass of the tube-probe results in poorer sensitivity by a factor of × 3 for V and × 4 for Cr.     

Glassy carbon tubes for electrothermal atomic absorption spectrometry

Summary The use of glassy carbon as a tube material in electrothermal atomic absorption spectrometry requires modifications to the power supply if temperatures and heating rates comparable to those for graphite tubes are to be obtained. Glassy carbon tubes frequently have a longer lifetime than pyrolytic graphite coated tubes made of polycrystalline electrographite. Peak height sensitivity for glassy carbon is better by a factor of two for some volatile elements, but up to a factor of five inferior for less volatile elements than that for pyrolytic graphite coated tubes. Peak area sensitivity is generally inferior by about a factor of two. Sample volume is limited to 5–10 l because of the smooth surface.From the signal shape it can be deduced that adsorption of analyte atoms at the tube wall plays an important role in glassy carbon, and is responsible at least in part for the lower sensitivity. Non-spectral interferences can be less pronounced in glassy carbon tubes for those interferents which interact with graphite tube surfaces. Glassy carbon is, however, no alternative to pyrolytic graphite coated tubes.

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Glasartiger Kohlenstoff als Rohrmaterial für elektrothermische Atomabsorptionsspektrometrie

     

Optimization of temperature in carbon-furnace atomic emission spectrometry with commercially available electrothermal atomizers

For instruments without automatic background emission correction, optimization of the applied temperature may be necessary to achieve the lowest analyte detection limits. Optimum temperatures of less than the maximum available are reported for most elements with commercially available instruments in which the heating rate and the final temperature are subject to independent control. Temperature optimization, however, has little effect on furnaces in which the final temperature and the rate of heating are controlled solely by the same applied voltage. Improved emission detection limits are reported for many elements as a result of temperature optimization.     

Plasma afterglow atomization in electrothermal atomic absorption spectrometry

Potentialities of an Ar/H₂ microwave induced plasma afterglow at 8.2 mbar as an atomization source in electrothermal atomic absorption spectrometry have been examined. More specifically the atomization efficiency, as shown from appearance temperatures, and the reaction mechanisms of the atomization of the oxides and chlorides of alkaline earth and transition metals have been investigated and compared with conventional electrothermal atomization. For all the investigated metal chlorides and alkaline earth oxides, a considerable decrease in appearance temperature (some 500 K), is observed in the plasma afterglow. Such enhanced atomization is believed to be linked to reactions with H atoms. No plasma enhancement, however, is measured for the atomization of the transition metal oxides. All metal oxides are effectively reduced to free metal in the solid state by the Ar/H₂ afterglow, and as a consequence the supply rate is governed by the metal sublimation for these compounds. For metal chlorides, however, strong evidence is found for the atomization process to proceed via gas phase reactions.     

Flow injection on-line electrothermal atomic absorption spectrometry

Today the greatest hindrances to couple the continuous FI system to discrete ETAAS operations have been overcome as demonstrated by the great number of papers published in the last few years. This paper reviews 109 references to the development and expansion of the FI-ETAAS methodology. The selected FI-ETAAS systems, namely: on-line column preconcentration and separation systems; on-line knotted reactor preconcentration systems; on-line aerosol deposition systems; flow-injection for in situ trapping of volatile compounds and miscellaneous on-line systems.     

Improved cesium sensitivity in electrothermal atomic absorption spectrometry

The sensitivity for cesium determination by electrothermal atomic absorption spectrometry is improved four-fold by the addition of a large excess of potassium nitrate. Zeeman background corrections is used to compensate for the large non-specific absorption signal resulting from the potassium. The characteristic concentration and detection limit are 0.44 and 2 μg l^?1, respectively, and the coefficient of variation is 2% at the 50 μg l^?1 level. The procedure is suitable for the rapid determination of cesium in leach solutions from nuclear waste fixation experiments.     

High current power supply for electrothermal atomic absorption spectrometry

A new power supply for electrothermal atomization — atomic absorption spectrometry (ETA-AAS) with associated equipment for temperature control is presented. The system is composed of two high current commercial batteries and a MOSFET switch which controls the temperature of the atomizer by switching the current in an on-off way through a feedback loop connected to a personal computer and suitably programmed peripherals. The delivered power can be changed from 0 to 100% in 1% steps which gives the possibility of roughly controlling both the temperature at low power when the temperature sensor is not working and the heating rate. With this system, curves for which the simultaneous presence of all atoms injected in the atomizer is claimed, have been extended to Pb, Ag, Tl and Zn. With a more conventional power supply previously used, such a claim could be made only for Cd and Hg.     

The tube-in-tube technique in electrothermal atomic absorption spectrometry

Summary A new tube-in-tube is presented. It consists of a small graphite tube which is inserted into a usual Perkin-Elmer graphite oven. Three bars, 1×1 mm over the whole length of the tube, prevent direct contact with the outer one and smoothe the temperature profile within the tube. It causes a thermal delay in the atomization of the atomic cloud. Thus, expulsion of the atomic cloud by the thermally expanding inert gas grows less important compared to its growth. A significant increase of sensitivity is the result.

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Die Rohr-in-Rohr-Technik bei der elektrothermalen Atomabsorptions-SpektrometrieII. Das Dreisteg-Rohr

Zusammenfassung Ein neuer Rohr-in-Rohr-Atomisator wird vorgestellt. Er besteht aus einem kleinen Graphitrohr, welches in ein übliches Perkin-Elmer-Graphitrohr hineingebracht wird. Drei Stege (1×1 mm) über die gesamte Rohrlänge verhindern den direkten Kontakt mit dem Außenrohr und glätten das Temperaturprofil im Innenrohr. Die Atomisierung erfolgt thermisch verzögert. Deshalb verliert das Austreiben der Atomwolke durch das sich thermisch ausdehnende Intergas im Verhältnis zu deren Anwachsen an Bedeutung. Ein deutlicher Anstieg der Meßempfindlichkeit ist das Ergebnis.

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Herrn Prof. Dr. W. Fresenius zum 70. Geburtstag gewidmet

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Part I: Atomspektrometrische Spurenanalytik, Bd. 2 (B. Welz, Ed.). Verlag Chemie Weinheim — in print     

Transverse heated filter atomizer for electrothermal atomic absorption spectrometry

The filter furnace atomization concept was applied for the transverse heated atomizer. A graphite filter with graphite fiber reeled onto it was inserted into the tube of the standard transverse heated graphite atomizer (THGA) in the place of the platform. Automatic plugging of the sampling hole was applied during the atomization stage. The performance of the filter atomizer (THFA), compared with the THGA, was tested for the determination of Ag, As, Au, Bi, Cd, Cu, Ga, In, Mn, Pb, Sb, Se and Tl. The analytical performances of the THFA displayed some advantages in comparison with the THGA. The sampling volume varied in the range of 5–90 μl, while drying time for any volume was less than half of that used for the THGA. Owing to the reduced diameter of the analytical zone (2 mm) along the filter axis, a sensitivity improvement was observed for all elements, 1.3–2.8-fold without plugging and 4.3–4.8-fold for Bi, Cd, Pb and Tl with plugging of the dosing hole. An increased peak width (by two to five times for the elements tested) limited the determination of less-volatile metals. The intensity of light decreased by 20–30% in comparison with the THGA. Taking into account the sensitivity, sampling volume, light loss and signal width, the calculated gain in relative detection limit is substantial (about 2.5–7 times) only for volatile elements when the plugging is applied. The pyrolysis temperatures for Ag, As, Au, Cd, Cu and Se in the THFA without addition of modifier were by 200–600°C higher than in the THGA using Pd/Mg modifier. The lifetime of THFA tubes was similar to that of THGA tubes.     

Non-atomic absorption from matrix salts volatilized from graphite atomizers in atomic absorption spectrometry

The non-atomic absorption signals obtained from alkali halides atomized from three types of graphite atomizer are examined. The wavelength dependence of the signals identifies the absorption as that of charge-transfer transitions of the alkali halide molecules. The contribution of light scattering to the total non-atomic absorption signal is shown to be of small significance; the observed light-scattering is not Rayleigh scattering. The temperature dependence of the loss of sodium chloride from a rod atomizer is studied experimentally and compared with calculated vaporization rates based on the kinetic theory of gases.     

Time resolution of interferences in electrothermal atomic absorption spectrometry

A fast detector-amplifier-readout system is used for studying interferences in electrothermal graphite atomizers. The effects of different matrix components (K, B, Ca, Mg, and Cl), and graphite tube surfaces significantly alter the atomization processes of lead.