Spectroscopic determination of the degree of atomization in an electrothermal atomizer

The degree of atomization, β, is measured for an electrothermal atomizer by comparing the emission lines of copper atoms and CuCl molecules. Hydrogen increases the β value. The determined β value of CuCl is used as a standard for the determination of β values of other elements (nickel, chromium, tin and lead).     

Laser-excited atomic-fluorescence spectrometry in an electrothermal atomizer with Zeeman background correction

A pulsed excimer-pumped dye laser was used to excite atomic flourescence in graphite tube electrothermal atomizer. A 60-Hz ac magnitude field was applied around the atomizer and parallel to the excitation beam, for Zeeman background correction. The correction system was found to degrade the detection limits for silver, cobalt, indium, manganese, lead, and thallium by a factor of between 1 and 10. An increase in magnetic field strength, or a decrease in laser linewidth, should improve the detection limits, but was not possible here. For copper, the application of Zeeman background correction was unsuccessfull because the instrumentation was unable to resolve the sigma components from the laser emission profile sufficiently during the background correction measurement. For elements that exhibit sufficient Zeeman splitting, the linear dynamic range was the same with or without background correction Zeeman background correction was used to correct for scatter, in the resonance flourescence determination of manganese in a zinc chloride matrix and in mouse brain tissue.     

Temperature distribution in a tungsten electrothermal atomizer

Temperature distribution in the tube surface of a tungsten electrothermal atomizer (WETA-82) has been studied both theoretically and experimentally; also, the temperature distribution in the gas inside the atomizer has been studied theoretically. A theoretical model of the temperature distribution in the tube surface and in the gas inside the atomizer is proposed. It is considered that thermal radiation to the surroundings and beat conduction towards the water-cooled electrode supports are the only paths for heat loss, and electrical resistive heating and conduction from adjacent nodes are the only sources of heat gain. The time rate of change in temperature of the atomizer is expressed in the form of a differential equation. A finite-difference form of the differential equation is used in a computer program to calculate the temperature at each time step. Optical pyrometry was used to determine the surface temperature of the tube atomizer. The model predicts that an increase in the heating rate of the tube atomizer will slightly decrease the temperature gradient over the tube circumference; however, unambiguous observation of this decrease could not be made in the sequentially measured temperature difference between two selected point using optical pyrometry. Variation in the hydrogen content of the hydrogen-argon mixture purge gas did not have a significant effect on the temperature gradient of the tungsten tube surface. The results of this study have indicated possible improvements in the atomizer design with a view to making the temperature distribution over the circumference of the tube atomizer more uniform.     

Design considerations regarding an atomizer for multi-element electrothermal atomic absorption spectrometry

The methodology of simultaneous multi-element electrothermal atomic absorption spectrometry (ETAAS-Electrothermal Atomic Absorption Spectrometry) stipulates rigid requirements to the design and operation of the atomizer. It must provide high degree of atomization for the group of analytes, invariant respective to the vaporization kinetics and heating ramp residence time of atoms in the absorption volume and absence of memory effects from major sample components. For the low resolution spectrometer with a continuum radiation source the reduced compared to traditional ETAAS (Electrothermal Atomic Absorption Spectrometry) sensitivity should be, at least partially, compensated by creating high density of atomic vapor in the absorption pulse. The sought-for characteristics were obtained for the 18 mm in length and 2.5 mm in internal diameter longitudinally heated graphite tube atomizer furnished with 2-4.5 mg of ring shaped carbon fiber yarn collector. The collector located next to the sampling port provides large substrate area that helps to keep the sample and its residue in the central part of the tube after drying. The collector also provides a “platform” effect that delays the vaporization and stipulates vapor release into absorption volume having already stabilized gas temperature. Due to the shape of external surface of the tube, presence of collector and rapid (about 10 °C/ms) heating, an inverse temperature distribution along the tube is attained at the beginnings of the atomization and cleaning steps. The effect is employed for cleaning of the atomizer using the set of short maximum power heating pulses. Preparation, optimal maintenance of the atomizer and its compliance to the multi-element determination requirements are evaluated and discussed. The experimental setup provides direct simultaneous determination of large group of element within 3-4 order concentration range. Limits of detection are close to those for sequential single element determination in Flame AAS with primary line source that is 50-1000 times higher than the limits obtainable with common ETAAS (Electrothermal Atomic Absorption Spectrometry) instrumentation.     

The use of graphite cups for introducing solid samples with an electrothermal atomizer

Because of difficulties in quantitatively transferring accurately weighed solid microsamples into electrothermal atomizer tubes, various materials were investigated for their suitability as cups or carriers. Plastics, ‘ashless’ papers and cellulose acetates, though theoretically completely destructible at the ashing step, were found to have two principal disadvantages: they contribute comparatively high and variable blank values of many common elements; the residues left in the tube after atomization of solid metal and refractory samples lead to inaccuracy of reading and to much shortened tube life.Graphite cups not only overcome these problems but have the positive advantage that their use results in a usefully higher sensitivity for the elements investigated.Some brief examples of the use of graphite cups are quoted, but it is emphasized that the method of calibration must still be carefully checked. The reasons for the improved sensitivity of the graphite cup method and its relationship to the L'vov ‘platform’ method are discussed.     

Atomic absorption determination of elements in solid samples using an electrothermal atomizer ‘crucible with separated zones’

A model of an electrothermal atomizer given the name ‘crucible with separated zones’ has been proposed. It provides the filtration of sample vapors through porous walls of a graphite tube placed in the atomization zone and heated independently of the evaporation zone. The inner volume of the tube is an isothermal analytical zone. The atomizer has been applied to the analysis of solid sea and river suspensions, DETATA concentrates of waters, and bottom sediments weighing 30–50 mg and more. As a result, the performance characteristics of the direct atomic absorption determination of Cd, Ag, and Pb in geochemical samples of complicated composition have been improved.     

Investigation of evaporation and atomization processes in a variable-pressure electrothermal atomizer by laser spectroscopy using gallium as an example

A new approach to studying the evaporation and atomization of elements in graphite furnaces has been developed that combines a highly sensitive technique of laser-induced fluorescence and variable-pressure atomization. This approach allows one to work with low concentrations of analytes corresponding to real analytical conditions, to separate concurrent processes of evaporation and atomization, and to determine their parameters. The approach has been used for studying the evaporation and atomization of gallium in isothermal graphite furnaces.     

Double resonance laser-induced fluorescence of Cadmium and Zinc in the inductively coupled plasma and electrothermal atomizer

Double resonance laser-induced fluorescence (LIF) spectrometry studies of Zn and Cd have been performed in the inductively coupled plasma (ICP) and electrothermal atomizer (ETA). Stepwise excitation of Zn is accomplished at 213.856 nm and 636.235 nm with fluorescence emissions observed near 334.5 nm. Excitation of Cd is accomplished at 228.802 nm and 643.847 nm with fluorescence emissions observed near 361.1 nm and 347.6 nm. Calibration studies demonstrate that the double resonance LIF approaches provide high sensitivity and linearity over several orders of magnitude in both atomizers. Limits of detection for Zn and Cd in the ICP are 1.7 ng/ml and 5 ng/ml, respectively. Excellent sensitivity is observed in the ETA resulting in limits of detection of 70 pg/ml (700 fg absolute mass) and 4 pg/ml (40 fg absolute mass) for Zn and Cd, respectively. The Zn content of a bovine serum standard reference material (NIST SRM #1598) has been determined by ETA-LIF and found to be 940±60 ng/g, which is in very good agreement with the certified value of 890±60 ng/g. Low-level ETA-LIF measurements of Zn in these studies are strongly limited by the high background level observed for this element.     

A method for reduction of matrix interferences in a commercial electrothermal atomizer for atomic absorption spectroscopy

A CRA-63 atomizer has been modified by replacement of the conventional center supports with 2-pronged supports that hold the atomizer tube at both ends. This arrangement allows current to flow only across the atomizer ends. The center is heated by conduction. Large initial temperature differences (750–900 K) between the sample-containing center and the ends eliminates or decreases the interferences on Pb normally observed with this atomizer. Six chloride and one sulfate matrices were studied.Longer atomizers and higher power produced the largest temperature differences and, therefore, the best lead recoveries. At a given length, recovery eventually plateaued while the signal for lead in the absence of matrices decreased when heating power was further increased. It was concluded that further improvements would require longer tubes and a combination of end and center heating.The importance of considering interferent/analyte ratios in interpreting recovery data was examined for the interference of MgCl₂ on lead.     

Kinetics of indium atomization from different atomizer surfaces in electrothermal atomic absorption spectrometry (ETAAS)

The kinetic parameters of indium atomization in electrothermal atomic absorption spectrometry (ETAAS) have been determined by a newly proposed method. Effect of the atomizer surface and the palladium modifier on the kinetics of indium atomization has been investigated. The mechanisms of indium atomization seem to be identical for the pyrolytically coated graphite and the uncoated graphite tubes, i.e. the rate-limiting step for the atomization changes from a first order kinetics at lower temperatures into a nearly 1/3 order kinetics at higher temperatures, which may suggest that the analyte moves from a dispersed state to agglomates with increasing temperature. However, for the zirconium coated graphite tube, the atomization of indium is controlled by a single mechanism with the kinetic order of near 2/3 and the activation energy of 186 ± 13 kJ/mol. Relatively weak indium—zirconium carbide interactions and the release of indium from the sphere of molten indium metal on the zirconium coated surface are suggested. In the presence of palladium, a simple mechanism, i.e. the release of indium from the solid solution of the In and the Pd on the pyrolytically coated graphite surface, is proposed to account for the observed first order kinetics and the activation energy of 421 ± 27 kJ/mol.     

Direct atomic absorption determination of mercury in drinking water and urine using a two-step electrothermal atomizer

A new method was developed for the direct electrothermal atomic absorption determination of mercury in drinking water and urine using double vaporization in a two-step atomizer with a purged vaporizer. In this method, a sample is placed in the vaporizer of a two-step atomizer, dried, and vaporized. The sample vapor is transferred to an unheated atomizer cell with a flow of argon and trapped by the inner surface of cell walls. This procedure can be performed repeatedly to preconcentrate mercury in the atomizer cell. Next, a portion of the sample transferred to the inner surface of the atomizer cell is revaporized and atomized by heating the atomizer cell of the two-step atomizer with a purged vaporizer, and the atomic absorption of mercury is measured. It was found that the degree of mercury transfer and trapping is as high as 100% at sufficiently high temperatures of primary vaporization, regardless of the material of the inner surface of the atomizer cell. The detection limits for mercury were 0.24 or 0.024 µg/L for drinking water at a sample volume of 100 µL using a single sample transfer or the procedure repeated ten times, respectively, and 2.0 µg/L for urine at a sample volume of 20 µL and a single sample transfer. The accuracy of the results was confirmed by the analysis of certified mercury samples and samples with known additives.Translated from Zhurnal Analiticheskoi Khimii, Vol. 60, No. 1, 2005, pp. 45–51.Original Russian Text Copyright © 2005 by Vilpan, Grinshtein, Akatov, Gucer.     

Determination of gold in biological materials by electrothermal atomic absorption spectrometry with a molybdenum tube atomizer

Electrothermal atomic absorption spectrometry (ETA-ASS) of gold with a molybdenum tube atomizer has been investigated. A sensitive ETA-AAS method was developed. The gold absorption signal became higher with the heating rate of the tube atomizer and as the ratio of hydrogen in the argon purge gas decreased. The optimal heating rate and the optimal gas flow rate were 5.5 C/msec and Ar 480 ml/min + H(2) 20 ml/min, respectively. The absolute characteristic mass (the mass of element giving 0.0044 abs.) of gold by the atomizer was 1.8 pg and the detection limit was 130 pg/ml (3S/N). These values were > 10 times better than those obtained with graphite atomizers, ICP and ICP-MS. The interferences caused by large amounts of interferents were evaluated. The addition of thiourea served to eliminate severe interferences. The recovery of spiked gold in biological materials was in the range of 101-106%.     

A kinetic study of diffusion in the electrothermal atomizer with a graphite filter by laser excited atomic fluorescence

Laser excited atomic fluorescence-electrothermal atomizer (LEAFS-ETA) was used to study atomization and diffusion mechanisms in a novel diffusive graphite tube atomizer. The atomizer design included a hollow graphite cylinder mounted between two graphite rods which served as electrodes. One of the rods had a small graphite insert with a sampling hollow and could move backwards and forwards. After the sample was introduced into the hollow, the electrodes tightly sealed the graphite cylinder ensuring that the insert was directly in the center of the furnace. The furnace assembly was then heated and the vaporized sample diffused through the hot graphite wall. The atomic fraction of the sample vapor was excited by a laser beam which was directed along the graphite tube surface so that no gap remained between the beam and the tube surface. Fluorescence vs. time profiles for three elements — Cu, Ag and Ni — were obtained within the temperature range of 1400–2600 K. The rate constants of specific processes were measured from the decay portions of the fluorescence signals under the assumption of first-order kinetics. The Arrhenius plots were constructed and the activation energies, E_a were evaluated from their slopes. The plots obtained for Cu and Ag consisted of two linear parts, the corresponding values of E_a were: 195 kJ/mol and 77 kJ/mol for Cu (1550 K < T < 2600 K) and 238 kJ/mol and 97 kJ/mol for Ag (1430 K < T < 2280 K). The Arrhenius plot for Ni was linear within the temperature range of 1770–2530 K resulting in an E_a equal to 161 kJ/mol. The diffusion coefficients were evaluated on the basis of a steady-state diffusion model out of a hollow cylinder. The values for the diffusion coefficients were: 3.7·10⁻⁴−2.0·10⁻³ cm²/s (1750–2600 K) for Cu, 6.5·10⁻³−1.4·10⁻³ cm²/s (1750–2280 K) for Ag and 5.6·10⁻⁵−1.5·10⁻³ cm²/s (1770–2530 K) for Ni.     

Investigations for the determination of lead by in situ hydride trapping within a graphite electrothermal atomizer for routine analysis

A new commercial system consisting of a flow injection analysis system for hydride generation coupled with a transversely heated graphite atomizer-atomic absorption spectrometer for the determination of lead is investigated in detail. The hydride generation is optimized by using an ammonium peroxodisulphate-hydrochloric acid system as oxidant and sodium borohydride as reducing reagent. The addition of sodium cumol sulphonate as surface active substance shows advantages considering efficient plumbane production. The hydride trapping and atomization in a graphite electrothermal atomizer is also optimized. The characteristic concentration was 0.74 mug/l, the detection limit was 0.70 mug/l for 500 mul sample volumes. The relative operation standard deviation of this method was smaller than 2%. Further examinations demonstrate the influence of several heavy metals on the determination of lead. Finally, the measurement of standard reference materials shows the efficiency of the method in combination with decomposition with aqua regia solutions.     

Atomic absorption determination of cadmium,lead, and mercury in sea and river suspensions using an electrothermal atomizer with two vaporization zones

It is shown that suspended forms of Cd, Pb, and Hg can be simultaneously determined in river and sea waters by direct atomic absorption spectrometry with two independent stages of the fractional evaporation of solid suspensions (at <1000°C and >1500°C) in the system crucible (vaporizer)-core (collector). An electrothermal atomizer core-cell-core with two vaporization zones and an independently heated common analytical zone is proposed for analyzing condensates. The approach ensures the determination of cadmium and lead at the level below the clarke values for suspended substances and mercury in the range of average-increased concentrations.     

Determination of traces of lead and copper in foods by electrothermal atomic absorption spectrometry with metal atomizer

Summary A method is described for the determination of traces of lead and copper in foods by a microcomputer-processed atomic absorption spectrometry with a molybdenum micro-tube atomizer. Accurate results of lead and copper are obtained by atomization in the presence of thiourea. Thiourea contributes to a highly reproducible atomization profile and reduction of interferences from concomitant elements in foods. Solid samples are digested with nitric acid in the Uni-seal decomposition vessel, while liquid or water-soluble samples are directly atomized.

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Bestimmung von Blei- und Kupferspuren in Lebensmitteln durch elektrothermische AAS mit einem Metallzerstäuber

Zusammenfassung Ein Verfahren zur Bestimmung von Blei- und Kupferspuren in Lebensmitteln mittels mikrocomputer-gesteuerter AAS mit einem Zerstäuber aus Molybdän wurde beschrieben. Die Zerstäubung in Gegenwart von Thioharnstoff gibt genaue Resultate. Dieser führt zu einem gut reproduzierbaren Zerstäubungsprofil und setzt die Störung von Begleitelementen herab. Feste Proben werden mit Salpetersäure aufgeschlossen, flüssige oder wasserlösliche Proben werden unmittelbar eingesetzt.

     

Effect of atomizer surface type on the quantitation of molybdenum and ytterbium by electrothermal atomization atomic absorption spectrometry

The effect of pyrocoated graphite, uncoated graphite, metal-carbide, and metal atomization surfaces on the quantitation of molybdenum and ytterbium by electrothermal atomization atomic absorption spectrometry was investigated. The peak shape was affected by heating rate and the different surfaces gave different shapes. Except for the case of uncoated graphite, the sensitivities and detection limits were similar for all surfaces. In a sodium chloride matrix it is preferable to use uncoated graphite for molybdenum because an ashing stage greater than the boiling point of sodium chloride can be used without loss of molybdenum. Tube lifetime depended on atomization temperature, atomization time and the matrix.     

Slurry sampling electrothermal atomic absorption spectrometric determination of copper in herbal medicine samples with a molybdenum tube atomizer

An ultrasonic slurry sampling electrothermal atomic absorption spectrometric method with a molybdenum tube atomizer has been developed for the determination of copper in herbal medicine samples. Glycerol solution (10%) was used as the slurry medium. The optimum pyrolysis temperature was 760 °C. The detection limit was 72 fg (3×S/N ratio). Matrix element interference was investigated and it was found that glycerol as a chemical modifier eliminated the interference. The amounts of copper in herbal medicines determined by the method proposed are in good accordance with those measured in dissolved acid-digested samples. The method enables rapid calibration, and simple and rapid analysis of copper in herbal medicine samples at low cost.     

Atom formation processes for cadmium in the presence of sulphur in electrothermal atomization atomic absorption spectrometry with a molybdenum tube atomizer

The atom formation process that occurs during elimination by sulphur of interferences on cadmium in electrothermal atomization was studied by using a thermodynamic-kinetic equation and measuring x-ray diffraction patterns. Activation energies for cadmium in the presence of Al, Ca, Cr, Cu, Fe, K, Na or Pb were obtained in argon-hydrogen in the presence and absence of sulphur. The atom formation process for cadmium in the presence of sulphur was CdS(g) → Cd(g) in the presence of each interferent.