Slurry sampling graphite furnace atomic absorption spectrometry: determination of trace metals in mineral coal

A procedure for lead, cadmium and copper determination in coal samples based on slurry sampling using an atomic absorption spectrometer equipped with a transversely heated graphite tube atomizer is proposed. The slurries were prepared by weighing the samples directly into autosampler cups (5-30 mg) and adding a 1.5 ml aliquot of a diluent mixture of 5% v/v HNO(3), 0.05% Triton X-100 and 10% ethanol. The slurry was homogenized by manual stirring before measurement. Slurry homogenization using ultrasonic agitation was also investigated for comparison. The effect of particle size and the use of different diluent compositions on the slurry preparation were investigated. The temperature programmes were optimized on the basis of pyrolysis and atomization curves. Absorbance characteristics with and without the addition of a palladium-magnesium modifier were compared. The use of 0.05% m/v Pd and 0.03% m/v Mg was found satisfactory for stabilizing Cd and Pb. The calibration was performed with aqueous standards. In addition, a conventional acid digestion procedure was applied to verify the efficiency of the slurry sampling. Better recoveries of the analytes were obtained when the particle size was reduced to <37 mum. Several certified coal reference materials (BCR Nos. 40, 180, and 181) were analyzed, and good agreement was obtained between the results from the proposed slurry sampling method and the certificate values.     

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.     

Surface and subsurface examination of graphite tubes after electrodeposition of noble metals for electrothermal atomic absorption spectrometry

The modification phenomena of noble metals (Pd, Ir, Rh) electrodeposited onto the inner surface of pyrolytic graphite (PG) coated furnaces were investigated mainly by electron microprobe analysis with energy dispersive X-ray emission detection. The conditions of electrodeposition were optimized in order to achieve the best analytical performance of atomic absorption measurements. Investigations concerning the distribution of noble metals on the tube surface and in-depth were performed at different stages of the tube history. It was found that the noble metals used for the modification do not form a compact layer on the surface but penetrate into the pyrolytic graphite structure already at the deposition step. When two metals were deposited together, both penetrated into the graphite structure. The degree of penetration of the pyrolytic graphite at high temperature differs for various metals. It was also demonstrated that electrodeposited noble metals remain in sub-surface domains of the graphite for hundreds of atomization cycles, which means that they can be used as permanent modifiers.     

Determination of heavy metals by electrothermal atomic absorption spectrometry after electrodeposition on a graphite probe

Traces of heavy metals were separated and preconcentrated electrochemically at a controlled potential on the graphite ridge probe. After the electrolysis, the electrode-probe was inserted in the graphite furnace for atomization of metal deposit by an automatic system. Conditions for the electrodeposition, such as pH of solutions, the deposition potential and concentration of electrolyte, were optimized. Detection limits improved with increased time of electrodeposition and were 16 ng l⁻¹ Cu, 1.0 ng l⁻¹ Cd, 6.0 ng l⁻¹ Pb, 64 ng l⁻¹ Ni, 14 ng l⁻¹ Cr (III) and 17 ng l⁻¹ Cr (VI) for a 10-min deposition. This method was applied for the determination of copper, cadmium, lead, nickel and of chromium species in seawater.     

On-line coupling of ion chromatography and atomic spectrometry and use of direct graphite furnace atomic absorption spectrometry as new tools for ultra trace determinations in refractory metals

On-line coupling of inductively coupled plasma (ICP) techniques such as ICP-AES and ICP-MS with ion chromatography (IC) offers unique features for ultra-trace analysis. An on-line preconcentration procedure based on cation exchange enables sub-ng/g analysis in complex matrices like molybdenum and tungsten. The best dissolution reagent for these matrices is hydrogen peroxide, which can be cleaned to ultra high purity with the same metal free chromatography equipment used for the preconcentration. Preconcentration is possible for elements that show cationic reactions within acidic peroxide containing solutions. In this study 28 elements detrimental for microelectronics applications are observed. A comparison of the combinations IC-ICP-AES and IC-ICP-MS with glow discharge mass spectrometry (GDMS) for the analysis of today's purest tungsten samples shows the analytical power and accuracy of the coupled devices. Graphite furnace atomic absorption spectrometry (GFAAS) as an extremely sensitive analytical technique is applied with and without the same sample pretreatment as used for the on-line coupling. Direct GFAAS measurements of alkali metals are complementary to IC-ICP techniques. The data evaluated with these wet chemical techniques are compared to the usual manufacturers characterisation technique GDMS. With respect to the low concentrations present in these high purity materials (ng/g level in the solid) the discrepancies between all methods are acceptable. The sensitivity of IC-ICP-MS is in most cases far superior to IC-ICP-AES and for some elements also to GDMS. Furthermore the specific advantages of on-line coupling such as the elimination of isobaric interferences in ICP-MS or spectral interferences in ICP-AES are shown for ICP-AES and ICP-MS determinations.     

微波消解-GFAAS测定浅水湖泊底泥中重金属元素

探讨了采用微波消解作为底泥样品的前处理方法,运用石墨炉原子吸收法测定浅水湖泊底泥中Cu,Pb,Zn,Cd,Cr含量的实验条件.方法的RSD为2.0％～4.1％,平均回收率为97.4％～101.5％,Cu,Pb,Zn,Cd,Cr的检出限分别为0.4,5,2.8,0.25,2.5ng.该法适合于浅水湖泊底泥中重金属含量的测定.     

Background overcorrection problems for lead in the presence of phosphate with various metals in Zeeman graphite furnace atomic absorption spectrometry

Phosphate in various chemical forms has been widely used as a chemical modifier for the determination of Pb by graphite furnace AAS. When Pb was determined in bone digestate, with NH₄H₂PO₄ modifier and a transversely-heated furnace and a longitudinal Zeeman background correction system, low recoveries of Pb were found. This was found to be caused by a background overcorrection problem associated with the matrix of phosphate plus Ca and/or Mg. The overcorrection problem caused erroneously low Pb concentrations to be predicted when using aqueous Pb standards for calibration. The overcorrection problem was not reproduced with a transverse Zeeman correction system. While the magnitude and shape of the background signal changed with different phosphate compounds, the important features remained constant. We suggest that the background overcorrection problem was caused by molecular absorption of PO formed during atomization in the presence of any of several metals, most specifically the alkaline earth elements, since the PO molecular absorption bands appear to be subject to Zeeman-effect splitting.     

Automatic microemulsion preparation for metals determination in fuel samples using a flow-batch analyzer and graphite furnace atomic absorption spectrometry

The principal thermodynamic advantages of using microemulsions over standard emulsions for flow metal analysis are the greatly increased analyte stability and emulsive homogeneity that improve both the ease of sample preparation, and the analytical result. In this study a piston propelled flow-batch analyzer (PFBA) for the determination of Cu, Cr and Pb in gasoline and naphtha by graphite furnace atomic absorption spectrometry (GF AAS) was explored. Investigative phase modeling for low dilution was conducted both for gasoline and naphtha microemulsions. Rheological considerations were also explored including a mathematical flow derivation to fine tune the system's operational parameters, and the GF AAS coupling. Both manual and automated procedures for microemulsion preparation were compared. The results of the paired t test at a 95% confidence level showed no significant differences between them. Further recovery test results confirmed a negligible matrix effect of the sample on the analyte absorption signals and an efficient stabilization of the samples (with metals) submitted to microemulsion treatment. The accuracy of the developed procedure was attested by good recovery percentages in the ranges of 100.0 ± 3.5% for Pb in the naphtha samples, and 100.2 ± 3.4% and 100.7 ± 4.6% for Cu and Cr, respectively in gasoline samples.     

Experimental and theoretical intensities for graphite furnace atomic emission spectrometry

A constant-temperature atomizer in which tube dimensions and heating modes could be varied was used to experimentally measure atomic emission intensities for 12 elements. These results were compared with intensities calculated from theoretical expressions describing peak height and peak area emission signals. The best agreement between experimental and calculated intensities was obtained for a 1.8 cm long atomizer configuration providing both spatial and temporal isothermality, and using peak area signal evaluation.     

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.     

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.     

Preconcentration of lead in biological matrices for graphite furnace atomic absorption spectrometry

Lead was transferred by a two-step extraction procedure from complicated biological matrices into an aqueous solution. Thereafter, lead could be accurately determined in the aqueous solution without interference by graphite furnace atomic absorption spectrometry (GFAAS). Biological samples were digested sequentially by a mixture of sulphuric acid and nitric acids and a strong oxidant, potassium peroxodisulphate. Lead was extracted by dithiocarbamate in chloroform from the digest solution and subsequently back-extracted into a mercury(II) solution. Matrix materials such as chloride, phosphate and sulphate, which were known to cause serious interference in the determination of lead by GFAAS, could be eliminated in the preconcentration procedure. Close to 90% recovery of lead was achieved. The proposed procedure was applied for the analysis of several NBS biological standard reference materials containing lead in the mg kg^?1 range and satisfactory results were obtained.     

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.     

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.     

In situ preconcentration of trace metals in high purity water onto graphite tube by multiple injections followed by graphite furnace atomic absorption spectrometry

Direct determination of trace metals at extremely low levels in high purity water is described. The method incorporates an in situ preconcentration technique in the graphite furnace of the atomic absorption spectrometer. The in situ preconcentration process involves multiple injections and evaporation in apyrocoated graphite tube. The feasibility of the proposed method of multiple injections (up to 9×90 l) prior to analysis has been investigated. A SLRS-2 certified water sample has been used for quality control, and the results are in good agreement with the certified value and with determination by standard addition method. Typically, up to 0.8 ml of sample could be analyzed. Due to the relatively large sample volume, the detection limits for the elements of interest are in the range of 3–9 ng/l.     

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.

     

Determination of silicon using electrothermal Zeeman atomic absorption spectrometry in presence of some transition metals as modifiers

The transition metals ions Cr(III), Mn(II), Fe(III), Co(II), Ni(II), and Zn(II) in addition to Ca(II) have been used as modifiers in the determination of silicon using electrothermal Zeeman atomic absorption spectrometry. Co(II) proved to be the best. Graphite tubes treated with zirconium, in the presence of Co(II) as a modifier, exhibit higher sensitivity by a factor of five than untreated tubes. The modifier concentration and ashing and atomization temperatures have been optimized. Also interference of different inorganic cations and anions was studied.