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.     

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 method for removal of chloride interference in determination of aluminium by atomic-absorption spectrometry with a graphite furnace

Methods for removal of the chloride interferences in determination of aluminium by atomic-absorption spectrometry with a graphite furnace have been investigated. Two mechanisms of chloride interference have been established. The first arises from co-ordination of the chloride to aluminium. This interference can be removed by preventing the co-ordination. The other is due to co-existing chloride salts remaining until the atomization step. This interference can be removed by volatilizing the chloride or by converting it and/or aluminium chloride into another substance such as the oxides before the atomization step. The tetra-ammonium salt of EDTA is very suitable as an additive to overcome chloride interference because of its ability to co-ordinate aluminium and other cations, and also its effect when heated.     

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.     

Use of an electrostatic preconcentration system to identify interferences in atomic-absorption spectrometry

Interference effects can be identified by deviations from ideality of the slopes of the concentration curves generated with an electrostatic trapping system. The technique is employed to detect spectral interferences in atomic-absorption measurements, due to the presence of sodium chloride, and caused by imperfect background correction. The electrostatic trap is found to be a particularly convenient means of obtaining the required concentration curves because the device gives a ready control of the concentration factor, reliable performance for a range of different solutions, and ease of use in dealing with the necessary number of samples.     

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.

     

Background correction in combination with spectrometry of optical forward scattering and graphite furnace atomization

Summary Coherent Optical Forward Scattering in combination with graphite furnace atomization is an advantageous technique for multi-element trace determination. Using only one continuum light source it is already characterized by good detection limits for many elements measured simultaneously in one detected spectrum. A single- or multi-element line source extremly decreases these detection limits. The property of a zero method includes inherent noise suppression with respect to the zero-intensity-noise from the light source. That is why the remaining option of modulation can be applied for the determination of matrix dependent interferences. It is shown that unspecific transmission losses due to background absorption can be detected by modulation of the degree of polarization. Background from unspecific emission can be determined from the baseline intensity of the spectrum or by additional modulation techniques. Time-resolved spectra of Cd in human blood are presented as well as time-resolved and background-corrected line intensities.

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Untergrundkorrektur bei der Spektrometrie der optischen Vorwärtsstreuung mit Graphitofenatomisierung

Zusammenfassung Die kohärente optische Vorwärtsstreuung mit Graphitrohratomisierung ist ein vorteilhaftes Verfahren zur Multi-Element-Spurenanalyse. Mit nur einer einzigen Lampe mit kontinuierlichem Spektrum besitzt das Verfahren bereits für viele in einem Spektrum simultan erfaßbare Elemente gutes Nachweisvermögen. Durch Verwendung von Ein-oder Viel-Elementlampen läßt sich dieses Nachweisvermögen erheblich steigern. Die Eigenschaft der Nullmethode begründet inhärente Unterdrückung des Rauschens der Null-Intensität. Deshalb können verbleibende Modulationsmöglichkeiten zur Erfassung matrix-abhängiger Interferenzen eingesetzt werden. Es wird gezeigt, daß unspezifische Transmissionsverluste durch Untergrundabsorption mit Hilfe einer Modulation des Polarisationsgrades erfaßt werden können, während der Untergrund infolge unspezifischer Emissionen aus der Basislinie des Spektrums oder mit Hilfe zusätzlicher Modulationstechniken bestimmt werden kann. Zeitaufgelöste Spektren von Cd, das menschlichem Blut zugesetzt wurde, sowie zeitaufgelöste Linienintensitäten und ihre Untergrundkorrekturen werden wiedergegeben.

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Dedicated to Prof. Dr. Dr. Ing. E.h. W. Hanle on the occasion of his 85th birthday

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Part of thesis     

The reduction and elimination of matrix interferences in graphite furnace atomic absorption spectrometry

The approaches to reduction or elimination of matrix interferences encountered in graphite furnance atomic absorption spectrometry is reviewed. These techniques include matrix modification, application of active gas, and coating tubes with metallic compounds. The research work carried out in the author's laboratory is emphasized. A more universal matrix modifier, palladium, is proposed for the determination of mercury, lead, tellurium, bismuth, arsenic, thallium and indium in environmental samples.     

A sensitive atomic-absorption spectrometric method for the determination of tin with atomization from impregnated graphite surfaces

The atomization of Sn from graphite surfaces is potentially hindered by reactions with the surface. The impregnation of graphite tubes with other carbide-forming elements (W, Zr, Ta, Mo) favourably alters the surface characteristics of the graphite furnace for the atomization of Sn. At the acid concentrations needed to prevent the hydrolysis of Sn, these surfaces are considerably more stable (even after more than 100 atomization cycles) than those of pyrolytic graphite. Two graphite furnaces of different design, the HGA 72 and the HGA 76, were tested. With impregnated graphite tubes the determination of Sn is possible in the HGA 72 with a detection limit of approximately 15 pg. In the HGA 76 the tin determination is vastly improved with respect to prolonged lifetime of the furnaces and stable signals over much longer periods of time. Detailed interference studies reveal that the use of the "gas stop" mode minimizes the influence of many ions that are frequently either introduced by the decomposition reagents or present in the sample itself. The practical potential of this method is demonstrated for the determination of Sn in a slag material and in copper- and aluminium-based alloys.     

Trace metal analyses in octocorals by microwave acid digestion and graphite furnace atomic-absorption spectrometry

Eight octocoral species from two reefs off the Venezuelan coast were analyzed to determine their Cd, Cu, Ni, Pb and Zn content. A microwave sample work-up procedure, including drying and acid digestion, was developed and found to be faster and more convenient than conventional methods. Accurate and precise results were obtained for the determination of the metals of interest, using graphite furnace atomic-absorption spectrometry (GFAAS) with L'vov platforms. The accuracy of the results was checked against synthetic standards prepared to simulate the matrix of the analyzed octocoral species. Standard deviation values for synthetically prepared standards were less than 1% in all cases. On the other hand, standard deviations for sample results were considerably higher due to heterogeneity of the samples and environmental conditions at the sampling sites.     

Indirect method for the determination of aluminium by atomic-absorption spectrometry using an air-acetylene flame

The enhancement of the atomic-absorption signals of iron, cobalt, nickel and chromium in a fuel-rich air-acetylene flame by small amounts of aluminium makes possible the indirect determination of aluminium in the concentration range 0.01-10 ppm. The optimization of working conditions and the occurrence of interferences are reported.     

Halide interferences in an electrothermal graphite furnace atomic absorption spectrometry with group IIIB elements as studied by atomic and molecular absorption signal profiles

Molecular absorptions of monohalides (MX; X = F, Cl and Br) of alkali metals, alkaline-earth metals and Group IIIB elements produced in a conventional electrothermal graphite furnace atomizer have been observed by a rapid measurement system. Their characteristic data are summarized as appearance and peak temperatures and the analytical sensitivities. Furthermore, molecular absorptions ofAlF and InF are utilized to study the interference of fluoride with aluminum and indium atomic absorptions. Although the formations of AlF and InF suppress the atomic absorptions ofAl and In, respectively, the time-overlap in signal appearance of atomic absorptions of these two elements and molecular absorptions of their monofluorides is not observed. This is contradictory to pure “vapor phase mechanism”, and indicates that processes occurring on the graphite surface are also important. Usefulness of strontium nitrate as a matrix modifier for the determination of indium was also demonstrated.     

Enhancement of sensitivity in atomic-absorption spectrometry by addition of a graphite lid to a cup furnace

By using a cup furnace covered with a graphite lid, it is possible to enhance the atomic-absorption signal for Cd, Zn, Sb, Pb, In, Cu and Fe in solution and for Pb in tin metal (directly atomized). When the analyte is atomized in the cup furnace, part of it condenses on the lid, from which it can be re-evaporated and atomized to give a second absorption signal and hence greater sensitivity. When the lid is small, so that the temperature lag is short, the initial atomic-absorption intensity is also enhanced. The enhancement is due to an increased residence time of the atomized analyte in the cup.     

Determination of chromium in steel by atomic-absorption spectrometry with an air-acetylene flame

A new atomic-absorption procedure is described for the determination of chromium, at levels up to 1%, in steel. The method involves the use of the air-acetylene flame and incorporates 8-hydroxyquinoline as a releasing agent to suppress metallic interferences. Chemical operations have been reduced to a minimum in order to provide a simple, rapid and accurate procedure.     

Development of a slurry atomization method for the determination of cadmium in food samples by electrothermal atomization atomic-absorption spectrometry

Matrix modifiers have been compared for the determination of cadmium in foodstuffs by ETA-AAS with the sample injected in the form of a slurry. Addition of 800 mug/ml Pd stabilized cadmium to a similar extent as did ammonium dihydrogen phosphate, but avoided the increase in background signal associated with the latter. An analytical procedure was developed, based on palladium matrix modification, platform atomization with a pre-atomization cooling step and integrated absorbance measurements. The method allowed the analysis of milk, liver and olive leaf slurries at concentrations up to at least 50 mg/ml by direct calibration with aqueous standards. The accuracy of the analytical results was within 15% and the detection limit for cadmium in analysis of a 50 mg/ml slurry was 10 ng/g.     

Feasibility of eliminating interferences in graphite furnace atomic absorption spectrometry using analyte transfer to the permanently modified graphite tube surface

A procedure is proposed to avoid spectral and/or non-spectral interferences in graphite furnace atomic absorption spectrometry (GF AAS) by transferring the analyte during the pyrolysis stage from a solid sampling platform to the graphite tube wall that has been coated with a permanent modifier, e.g. by electrodeposition of a platinum-group metal. The direct determination of mercury in solid coal samples was chosen as a model to investigate the feasibility of this idea. The graphite tube surface was coated with palladium and the analyte was transferred from the solid sampling platform to the tube wall at a temperature of 500±50 °C. A characteristic mass of m₀=64 pg Hg was obtained for an atomization temperature of 1300 °C, proposing a quantitative transfer of the analyte to the tube wall. Calibration against aqueous mercury standards was not feasible as this element was lost in part already during the drying stage and could not be trapped quantitatively on the modified graphite tube surface. However, the results for all except one of the coal reference materials were within the 95% confidence interval of the certificate when the slope of a correlation curve between the integrated absorbance, normalized for 1 mg of sample, and the certified value for mercury was used for calibration. A detection limit of 0.025–0.05 μg g⁻¹ Hg in coal, calculated from three times the standard deviation of the investigated coal samples, could be obtained with the proposed method. The spectral interference due to excessive background absorption in the direct determination of mercury in coal could be eliminated completely. It is expected that this analyte transfer can be used in a similar way to eliminate other spectral and/or non-spectral interferences in the GF AAS determination of other volatile analytes.     

Determination of mercury in environmental samples by cold vapour generation and atomic-absorption spectrometry with a gold-coated graphite furnace

Mercury is determined at below the pg/ml level by a combination of cold vapour generation, trapping in a gold-coated graphite furnace and atomic-absorption detection. The mercury is reduced to the element by stannous chloride, stripped from solution by a stream of nitrogen and collected on a gold-coated porous graphite disk in a graphite furnace. It is then atomized by increasing the graphite furnace temperature and detected by an atomic-absorption spectrophotometer. The absolute detection limit and the characteristic mass were found to be 5 and 20 pg for 0.0044 absorbance, respectively. The concentration limit of detection was 0.1 pg/ml for a 50-ml sample, and the linear dynamic range covered three orders of magnitude. The precisions of the measurements were 2.7% for 0.1 ng and 2.6% for 2 ng of mercury. Analyses of NBS and NIES reference materials showed quantitative recovery. Analytical results obtained by the technique are presented for natural waters, marine biota and sediments.     

Arsenic as internal standard to correct for interferences in the determination of antimony by hydride generation in situ trapping graphite furnace atomic absorption spectrometry

The feasibility of using internal standardization (IS) to correct for interferences in hydride generation with in situ trapping in graphite furnace was evaluated. Arsenic was chosen as internal standard for Sb determination and Ir was used as permanent modifier. Fluctuations in the main parameters that affect the analytical results were minimized by IS and an effective contribution was verified in the studies of liquid phase interferences. Cobalt and Ni²⁺ were selected to illustrate the potential use of IS on the correction of interference by transition metals. The application of IS allows the Sb determination in samples containing up to 20-fold higher concentration of the Co²⁺ and Ni²⁺ when compared to the procedure without IS. The relative standard deviation of measurements varied from 0.3% to 0.7% and from 1.1% to 3.2% with and without IS, respectively. Recoveries within 92% and 107% of spiked aqueous solution containing Sb(III) and Sb(V) were found.     

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.