Instrumentation for simultaneous multielement atomic absorption spectrometry with graphite furnace atomization

Graphite furnace-atomic absorption spectrometry (GF-AAS) is restricted to the determination of 4 to 6 elements simultaneously due to the limitations of hollow cathode lamps. However, a consideration of prototype continuum source instruments and recent advances in the fields of spectrometer and detector technology suggests that a multielement GF-AAS instrument, with the multielement versatility associated with atomic emission spectrometry, is possible. Such a multielement instrument would employ a continuum source and provide 1.) multielement determinations for 30 to 40 elements, 2.) wavelength and time integrated absorbance measurements which are independent of the source width, 3.) detection limits comparable to line source AAS with the potential for another order of magnitude improvement using atomization at elevated pressures, 4.) extended calibration ranges limited only by the memory of the atomizer, and 5.) high resolution inspection of the spectra surrounding the analytical wavelength. Such an instrument could provide figures of merit comparable to inductively coupled plasma-mass spectrometer with considerably less complexity.     

A thermodynamic equilibrium model for atomization in graphite furnace atomic absorption spectrometry

Summary A thermodynamic (gas-phase) equilibrium model of atomization has been developed to account for the change in appearance temperatures of some elements when they are atomized in the presence of different amounts of O₂ in argon purge gas. Experimental evidence of the effect of O₂ on the atomic absorption pulse for ten elements is presented. Of these ten elements, five elements: Ag, Mn, Ni, Mg and Cu, show no measurable change in the appearance temperature or in the peak shape of their atomic absorption pulses when the O₂ content of the purge gas is progressively increased from 0.005% to 1.0% (v/v); this phenomenon has been accounted for in terms of a lack of thermodynamic equilibrium in the gas phase. Shift of the atomic absorption pulses on the time axis (abscissa) for the other five elements: Pb, Sn, Si, Al and Ba, has been observed when the O₂ content of argon purge gas is increased. The magnitude of change in the appearance temperature has been calculated for Pb, Sn and Si based on the thermodynamic (gas-phase) equilibrium model. The calculated results agree well with the experimentally observed changes in the appearance temperatures of these three elements.

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Thermodynamisches Gleichgewichtsmodell für die Atomisierung in der Graphitofen-AAS

Zusammenfassung Es wurde ein thermodynamisches Modell für die Atomisierung in der Gasphase aufgestellt, das dem Einfluß unterschiedlicher Konzentration von Sauerstoff im Argonspülgas auf die Temperatur, bei denen die Signale einiger Elemente auftreten, Rechnung trägt. Für zehn Elemente wurde der Einfluß von Sauerstoff auf die Signale in der Atomabsorptionsspektrometrie im Graphitofen experimentell bestätigt. Für die fünf Elemente Ag, Mn, Ni, Mg und Cu ändert sich weder die Temperatur, bei der das Signal auftritt, noch die Form des Signals, wenn die Konzentration von Sauerstoff im Spülgas schrittweise von 0,005% auf 1,0% (v/v) erhöht wird. Dies wurde durch die Annahme von Abweichungen vom thermodynamischen Gleichgewicht in der Gasphase erklärt. Bei den anderen fünf Elementen wurden zeitliche Verschiebungen des Atomabsorptionssignals mit wachsendem Sauerstoffgehalt beobachtet. Die Änderung der Temperatur, bei der das Signal erscheint, wurde unter der Annahme von thermodynamischem Gleichgewicht (in der Gasphase) für Pb, Sn und Si berechnet. Die berechneten Ergebnisse stimmen gut mit den experimentell beobachteten überein.

     

Vaporization and atomization of neodymium in graphite furnace atomic absorption spectrometry

A combined theoretical and experimental approach has been used to investigate the atomization process for neodymium in graphite furnace atomic absorption spectrometry (GFAAS). Experimental results, using GFAAS and electrothermal vaporization-inductively coupled plasma-mass spectrometry, show that both oxide and carbide dissociation are responsible for Nd atom formation. A thermodynamic equilibrium model for atomization, based on the thermal dissociation of gaseous lanthanide monoxides, is presented. This model will account for the wide variation in sensitivities reported for the determination of rare earth elements by GFAAS and gives a good correlation between experimental characteristic mass values and monoxide bond dissociation energies.     

Determination of silver by graphite furnace—atomic absorption spectrometry with atomization under pressure

Summary When determined with pressurized atomization silver shows a lower peak height sensitivity than under normal conditions. Peak area is less influenced by pressure. The peak parameters are somewhat changed as compared to normal atomization. Notable is the prolongation of the mean residence time and peak time. Interferences are reduced with atomization under pressure.The linearity of the calibration curves is extended with atomization under pressure.

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Silberbestimmung im Graphittiegel. Atomabsorptions-Spektrometrie nach Zerstäubung unter Druck

Zusammenfassung Bei Zerstäubung unter Druck zeigt Silber eine geringere Empfindlichkeit der Peak-Höhe. Die Peak-Fläche wird durch Druck weniger stark beeinflußt. Im Vergleich zur Normalzerstäubung sind die Peak-Parameter etwas verändert, die Störungen sind bei Zerstäubung unter Druck geringer, die Eichkurven verlaufen über größere Strecken geradlinig.

     

Chemical reactions in the atomization of molybdenum in graphite furnace atomic absorption spectrometry

Summary Chemical reactions in the atomization of molybdenum in graphite furnace atomic absorption spectrometry have been studied using graphite platforms for atomization along with X-ray diffraction analysis. When Mo [as an aqueous solution of (NH₄)₂MoO₄] is heated in a graphite furnace, three molybdenum oxides: [MoO_2(s), MoO_3(s) and Mo₄O_11(s)], are formed at relatively low temperatures (<1,500 K). When Mo is atomized from a pyrolytic graphite surface, the charring curve of Mo shows a dip in absorbance in the temperature range 1,200–1,800 K. Hence, a charring temperature <1,200 K should be used for quantitative determination of Mo when a pyrolytically coated tube or a platform made of pyrolytic graphite is used. Mo_(s), MoC_(s) and Mo₂C_(s) have been found on both the pyrolytic and the regular graphite surface after the charring step is completed. Formation of Mo_(g) by direct sublimation of Mo_(s) and by dissociation of MoC_(g) are all thermodynamically favourable reactions at the temperature considered.

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Chemische Reaktionen bei der Atomisierung von Molybdän in der Graphitofen-AAS

Zusammenfassung Die chemischen Reaktionen bei der Atomisierung von Molybdän wurden mit Hilfe von GraphitPlattformen zusammen mit der Röntgen-Diffraktionsanalyse untersucht. Bei der Erhitzung von Molybdän [als wäßrige (NH₄)₂MoO₄-Lösung] im Graphitofen werden drei Molybdänoxide (MoO_2(s), MoO_3(s) und Mo₄O_11(s)) bei relativ niedrigen Temperaturen (<1 500 K) gebildet. Wenn Molybdän von einer pyrolytischen Graphitoberfiäche atomisiert wird, zeigt die Veraschungskurve eine Absenkung der Extinktion im Bereich von 1200 bis 1800 K. Deshalb sollte bei Verwendung eines mit pyrolytischem Graphit überzogenen Rohres oder einer entsprechenden Plattform für quantitative Molybdänbestimmungen eine Veraschungstemperatur von <1 200 K benutzt werden. Nach der Veraschungsstufe wurden sowohl auf der pyrolytischen als auch auf der normalen Graphitoberfläche Mo_(s), MoC_(s) und Mo₂C_(s) gefunden. Bildung von Mo_(g) durch direkte Sublimation von Mo_(s) und durch Dissoziation von MoC_(g) sind thermodynamisch günstige Reaktionen bei der betreffenden Temperatur.

     

石墨炉原子吸收光谱分析中的铋原子化机理研究

石墨炉原子吸收光谱法已广泛地应用于合金[1]、矿石[2]和水[3,4]等样品中铋的测定.铋是易挥发元素,为寻找有效的化学改进剂,了解其原子化机理是很有必要的.关于机理研究已有文献报道[5,6],但加入化学改进剂后在石墨管中形成的生成物的结构及其原子化机理的研究还少见报道[7].本试验以硝酸镍、氯化钯、氯化钯-硝酸镁和氯化钯-硝酸镍为化学改进剂,通过实验确定在本实验仪器条件下测定铋的最佳化学改进剂为氯化钯.进而研究铋在化学改进剂作用下的原子化机理,以达到提高原子化效率的目的.     

A Rowland Circle,multielement graphite furnace atomic absorption spectrometer

A simultaneous, multielement atomic absorption spectrometer utilizing a graphite furnace atomizer was constructed and evaluated. The optical arrangement employs a concave grating to combine the spectral output from a deuterium lamp and four hollow cathode lamps that are placed on the perimeter of a Rowland Circle. A graphite furnace atomizer is positioned on the circle at the point of convergence of the five light sources. Background correction is performed by the continuum source method. Simultaneous detection of the analyte absorption signals is accomplished with a charged-coupled device. Four test elements were used for evaluation purposes: cadmium, lead, copper and chromium. Even though the elements differ greatly in volatility, the detection limits approach the values published for single element GFAAS: 4, 12, 14 and 12 pg for Cd, Pb, Cu and Cr, respectively. The characteristic masses (integrated absorbance) for the four metals are 3, 24, 14 and 7 pg, respectively. Three drinking water reference materials are analyzed: NIST SRM #1643b (Trace Elements in Water), Fisher Scientific “Metals Drinking Water Standard,” and High Purity Standards “Drinking Water Metals Solution A and B”. The determined amounts were within 10% of the certified values for each of the four elements for all three reference materials.     

Slurry sampling for graphite furnace atomic absorption spectrometry

Summary Slurry preparations are an effective way to introduce solids into the graphite furnace. Ultrasonic agitation keeps samples mixed prior to analysis. Several aspects of the ultrasonic slurry sampling approach are discussed including contamination concerns, analyte partitioning, and the effect of particle size. In addition, sample preparation strategies for slurry preparations of non-powdered materials are reviewed. The suitability of this method for assessing homogeneity is demonstrated.     

Mechanism of atomization at constant temperature in capacitive discharge graphite furnace atomic absorption spectrometry

At constant temperature (isothermal) maintained throughout in the capacitive discharge technique, the measured absorbance at any time t due to concentration of analyte atoms can be given by: absorbance = p[A]₀{k₁/(k₁?k₂)}[exp(?k₂t)-exp(?k₁t)], where p is a function of the oscillator strength (a constant) and the efficiency with which the analyte atoms are produced, [A]₀ is the initial concentration of the analyte atoms, k₁ and k₂ are first-order rate constants for formation and decay of analyte atoms, respectively. This technique yields k₁?k₂ and k₁t?k₂t; and so the above equation reduces to: absorbance ?p[A]₀, resulting in large enhancement in sensitivity. In the case of lead, the immediate precursor of the gaseous lead monomer is the gaseous lead dimer, which is partly lost by diffusion of the lead dimer with a first-order rate constant, k₃. The kinetic parameters k₁, k₂ and k₃ have been evaluated, and the values of k₁ at different temperatures used to draw the Arrhenius plots, from which activation energies of the rate-determining steps have been determined. The activation energies have been used to elucidate atomization mechanisms by extensive correlation of the experimental energy values with the literature values.     

Effect of matrix components on chromium atomization processes in graphite furnace atomic absorption spectrometry

The effect of various organic and inorganic matrix components on chromium atomization in graphite furnace atomic absorption spectrometry is studied. The results are explained on the basis of chromium's atomization mechanisms. The two predominant mechanisms are the thermal dissociation of the oxide and of the carbide. Losses through molecular volatilization reduce the sensitivity when chromium chelate complexes are atomized. In this case, the atomization mechanism consists of the thermal dissociation of the chelate. The formation of chromium carbide from the carbon residue produced by decomposition of the organic solvents leads to a loss of sensitivity.     

Direct and simultaneous determination of copper and manganese in seawater with a multielement graphite furnace atomic absorption spectrometer

The direct and simultaneous determinations of Cu and Mn in seawater using a multielement graphite furnace atomic absorption spectrometer (Perkin-Elmer SIMAA6000) are described. Three kinds of chemical modifier (Mg(NO₃)₂, Pd(NO₃)₂ and a mixture of these) were tested. The matrix interferences were removed completely so that a simple calibration curve method could be used to determine Cu and Mn in seawater from the open ocean using Pd or a mixture of Pd and Mg as the chemical modifier. The relative standard deviations (RSDs) for the simultaneous determination of Cu and Mn in seawater from open ocean are 10% or less, and the detection limits were 0.07 μg 1⁻¹ for Cu and 0.10 μg 1⁻¹ for Mn, using Pd as the chemical modifier. The accuracy of the method is confirmed by analysis of four kinds of certified reference saline waters.     

Mechanism of boron atomization in graphite furnace atomic absorption spectroscopy

The mechanism of the atomization of boron and the enhancement of sensitivity by matrix modifier Sr(NO₃)₂ in graphite furnace AAS were discussed. X-ray diffraction and thermodynamic calculation were applied to study the mechanism of boron atomization with and without matrix modifier Sr(NO₃)₂. The formation of boron atom is due to the sublimation of solid boron which derived from the reduction of B₂O₃ by carbon. The enhancement of boron signal in the presence of Sr(NO₃)₂ is due to the formation of SrB₆ before atomization, which decreased the volatization losses of B₂O₃ and retarded the formation of B₄C.     

Laser ablation for aerosol deposition graphite furnace atomic absorption spectrometry

A commercially available pulse laser was used with a graphite furnace (GF) atomic absorption (AA) spectrometer for the trace analysis of metals in solid samples.Laser ablated solid material was deposited onto the inner surface of the GF. The optimum deposition temperature was 300 K. The deposited aerosol was atomized in a conventional GF heating regime.The analytical results in the deposition technique for Cd, Zn, Pb, Ag, Mn, Fe and Ni contained in different target materials were compared with results obtained with another laser ablation GF technique, which is characterized by the transport of the ablated material into a constant temperature GF with immediate atomization of the aerosol particles. The deposition technique improved the sensitivity and precision for the low volatile elements Cd, Zn and Pb. In contrast, the aerosol injection technique is preferable for the determination of elements that require more energy for atomization. Working with tube temperatures of up to 2800 K the authors obtained higher absorbance values (peak height) for Mn, Fe and Ni using the injection technique. The use of multiple deposition of laser ablated material inside the GF to achieve improved detection limits and higher precision for one atomization seems promising only for selected matrices.     

Surface reactions and the effects of oxygen on Pb atomization in graphite furnace atomic absorption spectrometry

Summary Previous papers have shown a pronounced shift in appearance temperatures of Pb as a result of O₂ in the system. This paper briefly summarizes the surface chemistry occurring on graphite as a result of exposure to O₂. Chemisorbed oxygen and an altered surface are suggested as the cause for the pulse shift of the Pb absorbance signal. A significant shift with an oxygenated surface but no gas phase O₂ present supports this conclusion. Time and spatially resolved absorbance traces suggest multiple release processes and readsorption of Pb on the surface. Pb/O₂ gas phase reactions do not appear to significantly alter the atomization efficiency and cannot explain the observed shifts. Similarly, O₂/Pb _x O _y(s) reactions cannot account for the first shot back and may play only a secondary role in the presence of O₂ in the gas phase. It was also observed that a significant (> 30%) amount of Pb remained on the surface after the solution was deposited and withdrawn 10 s later. This is attributed to binding of Pb to surface functionalities before the sample is desolvated.

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Oberflächenreaktionen und Wirkungen von Sauerstoff auf die Blei-Atomisierung in der Graphitofen-AAS

Zusammenfassung Vorangegangene Untersuchungen haben eine deutliche Verschiebung der Erscheinungstemperatur von Blei unter dem Einfluß von Sauerstoff im System gezeigt. In dieser Arbeit werden die Oberflächenreaktionen am Graphit unter Sauerstoffeinwirkung zusammenfassend diskutiert. Als Ursachen für die Pulsverschiebung des Bleisignals wurden der chemisorbierte Sauerstoff sowie Veränderungen der Oberfläche gefunden. Eine signifikante Verschiebung bei oxygenierter Oberfläche aber ohne gasförmigen Sauerstoff unterstützt diesen Schluß. Zeitlich und räumlich aufgelöste Absorptionsspuren deuten auf multiple Freisetzungsprozesse und Re-adsorption von Blei an der Oberfläche. Pb/O₂-Reaktionen in der Gasphase scheinen die Atomisierungswirkung nicht signifikant zu beeinflussen und können die beobachteten Verschiebungen nicht erklären. Ebenso können auch die O₂/Pb _x O _y(s)-Reaktionen nicht den first shot back erklären und spielen vielleicht nur eine sekundäre Rolle in Gegenwart von O₂ in der Gasphase. Es wurde ebenfalls beobachtet, daß ein beträchtlicher Anteil des Bleis (> 30%) an der Oberfläche zurückblieb, wenn die Lösung eingebracht und nach 10 s wieder entfernt wurde. Dies wird der Bindung von Blei an Oberflächenfunktionen vor der Desolvatation der Probe zugeschrieben.

     

Elucidating atomization mechanisms by simultaneous mass spectrometry and atomic absorption spectrometry

Summary Mechanisms responsible for atomization of selenium in graphite furnaces and for stabilization that occurs when nickel is added to selenium are investigated by mass analyzing, in real-time, the gaseous species from furnaces heated in vacuum and in atmospheric pressure environments. For the latter case, the analysis is done on a molecular beam formed from free-jet expansion of gases from the furnace. The apparatus for doing this is described. Results from the two experimental techniques, when compared, indicate that the primary loss of selenium at low temperatures is from thermal dissociation of selenium dicarbide. Secondary losses are due to vaporization of the dimer and the oxides. Nickel nitrate inhibits formation of the dicarbide, the dimer and the oxides. Mechanisms that are responsible for these findings are postulated.

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Untersuchung von Atomisierungsmechanismen durch simultane Massenspektrometrie und AAS

Zusammenfassung Die Mechanismen der Atomisierung von Selen in Graphitöfen und der Stabilisierung durch Nickelzusatz wurden durch Echtzeit-Massenanalyse der aus dem Ofen (Vakuum oder Atmosphärendruck) austretenden Gase untersucht. Im letzteren Fall wird die Analyse im austretenden Molekularstrahl durchgeführt. Das verwendete Gerät wird beschrieben. Die Ergebnisse der beiden Techniken zeigen, daß der primäre Selenverlust bei niederen Temperaturen von der thermischen Dissoziation von Selendicarbid herrührt. Sekundäre Verluste werden durch Verdampfung des Dimeren und der Oxide verursacht. Nickelnitrat verhindert die Bildung des Dicarbids, des Dimeren und der Oxide. Mechanismen für diese Vorgänge werden formuliert.

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This paper is based on work sponsored by the Division of Chemical Sciences of the Department of Energy and performed under DOE Contract No. DE-AC06-76RLO-1830     

Latest innovations in graphite furnace atomic absorption spectrometry technique

An overview of the latest innovations in the graphite furnace atomic absorption spectrometry technique is described. The use of the transverse heated graphite atomizer technology with its huge advantages, the possibilities of running powdered solids through the slurry technique, and the future possibility of using flow injection on-line with the graphite furnace are mentioned.     

Advances in ultrasonic slurry graphite furnace atomic absorption spectrometry

Summary Ultrasonic slurry graphite furnace atomic absorption spectrometry is a useful technique for automated direct analysis of solids. The effectiveness of ultrasonic agitation for mixing samples is demonstrated. This analytical approach is evaluated to identify sources of imprecision. Strategies for optimizing slurry preparations are discussed, focusing on particle size, density, analyte partitioning, and sampling limitations. Finally, a teflon bead method is presented for grinding biological and botanical samples. An optimized general approach for ultrasonic slurry sampling is presented.Presented at the 5th International Colloquium on Solid Sampling with Atomic Spectroscopy, May 18–20, 1992; Geel, Belgium. Papers edited by R. F. M. Herber, Amsterdam.     

On the uniforming of the atomization process for inorganic and organic mercury in graphite furnace atomic absorption spectrometry

The analytical performance of Pd, Au, Rh, Ir and their mixtures used as chemical modifiers has been investigated for mercury determination by graphite furnace atomic absorption spectrometry. The aim of this work was to evaluate whether chemical modification assures uniform atomization of analyte independent of its chemical form; mercury was used for this study. The investigations were performed for mercury introduced in the form of inorganic Hg(II) or organic PhHg(I). The best conditions, i.e. maximum pyrolysis temperature (450 °C), lowest temperature for atomization (1100 °C), provided almost the same sensitivity for both forms of mercury when a thermally reduced mixed modifier composed of Pd/Rh was used. The accuracy of the selected conditions was evaluated by a recovery test for various natural waters.     

Peak profile characteristics and atomization mechanisms for selenium in the presence of mercury for graphite furnace atomic absorption spectrometry

Summary Time resolved absorbance profiles, as well as the effects of inorganic acids and mercury salts, have been studied for selenium graphite furnace atomic absorbance signals. Conventional furnace operating procedures and wall atomization were used. Absorbance profiles were found to be sensitive to the mass of the counter ion and mercury present. Negative shifts in the appearance temperature were noted for low levels of mercury salts, whilst the higher levels of mercury caused only slight increasing shifts in the appearance temperature. Peak-area absorbance increases with mercury mass, but a steady-state absorbance was reached above a certain mercury chloride concentration. If mercury is present, a thermal dissociation of a mercury selenium compound is the suggested Se atomization mechanism.