Kinetic studies on the mechanism of atomization in electrothermal atomic absorption spectrometry with and without chemical modifiers

Over the past decade a few methods for determining kinetic data for atom formation from the absorption signal in electrothermal atomic absorption spectrometry (ETAAS) have been developed in the author’s laboratory. These approaches include improvement to the Smets method, and development of new methods for simultaneous determination of kinetic order and activation energy for atom formation at increasing or constant temperatures. The steady-state approximation and first-order kinetic assumption for atom formation have been avoided during their derivation. One of the most distinct features of these new methods is their suitability for quantitative determination of the kinetic order for atom formation from absorption signals under normal analytical conditions, even for atomization processes with fractional reaction orders and/or with multiple mechanisms. The application of the developed methods to the study of the mechanism of atomization in the graphite-furnace atomizer, with and without chemical modifiers, is reviewed with emphasis on research work in the authors’ laboratory.     

Determination of vanadium in mussels by electrothermal atomic absorption spectrometry without chemical modifiers

A method was developed for the quantitative determination of total vanadium concentration in mussels via electrothermal atomic absorption spectrometry (ETAAS). After the microwave digestion of the samples, a program using temperatures of 1600 °C and 2600 °C for ashing and atomization respectively, without any matrix modifiers, allowed us to obtain results that were satisfactory since they agreed closely with certified reference material values. The detection limit was 0.03 mg kg^–1 (dry weight), indicating that the method is suitable for the analysis of mussel samples. This determination was compared with matrix modifiers that have been reported previously. The method was applied to various cultivated and wild mussels from the Galician coast, yielding levels below 1 mg kg^–1 (wet weight).     

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.     

电热原子吸收光谱法中原子形成过程的探讨

本文提出了测定电热原子吸收光谱法中原子形成过程动学参数的新方法, 并用此法测定了铜、铁、铝、钴和钼等原子形成过和的动力学级数和原子化能。结果表明这些元素的原子形成均为零动力学。通过比较原子化能的实验值与有关过程的焓变探讨了这些元素的原子化过程 。     

Kinetics of selenium atomization in electrothermal atomization atomic absorption spectrometry (ETA-AAS). Part 2: selenium with palladium modifiers

The kinetics of the atomization process of selenium with prereduced and unreduced palladium nitrate modifiers were investigated. It was found that stabilization, in both forms, occurred by principally physical processes, as opposed to compound formation. For the unreduced modifier, it was shown that higher pyrolysis temperatures resulted in a higher activation energy of atomization and that the selenium vapour-surface interaction was increased. The importance of the second high temperature step in the stabilization mechanism was stressed, and an additional conditioning step in the furnace program was proposed for the unreduced palladium modifier. The reaction order for the unreduced modifier was near first order (1.29) with an activation energy of 330 kJ mol⁻¹ and a frequency factor in the order of 1 × 10⁹. For the reduced palladium modifier, a reduction temperature of 500°C–700°C was shown to be most effective. The reduced palladium modifier showed second order kinetics and the activation energy of 500 kJ mol⁻¹, was nearly 50% higher than that of the unreduced form. This and the large frequency factor (ca. 1 × 10¹⁵) indicated strong surface interactions, thus providing an explanation to the better stabilization properties observed for the reduced form of the modifier.     

Behavior of thorium,zirconium, and vanadium as chemical modifiers in the determination of arsenic by electrothermal atomization atomic absorption spectrometry

The comparative effect of thorium, zirconium and vanadium used as chemical modifiers in wall and platform atomization of arsenic using two sample introduction modes (aerosol vs. liquid drop) was studied. The effect of proton concentration (as hydrochloric acid and nitric acid) on the behavior of these modifiers was also tested. A beneficial effect on the stabilization of arsenic from all the chemical modifiers was essentially derived in the presence of nitric acid mainly for the aerosol introduction mode. Nonetheless, the highest sensitivity and the best stabilization of arsenic were obtained using zirconium and thorium as a chemical modifier, because pyrolysis temperatures as high as ∼1500°C can be used. The mechanistic action of the chemical modifiers used is also studied, resulting in the formation of a mixture of oxides stabilized at high temperatures.     

Boron in natural waters by atomic absorption spectrometry with electrothermal atomization

Rapid scanning of numerous rock samples when prospecting for uranium and thorium ores can be facilitated by using the shorter-lived nuclides. The samples are activated during short epithermal neutron irradiations and the 20-min activities of ²³⁹U and ²³³Th are observed instrumentally with a small Ge(Li) detector. The detection limits for uranium and thorium are less than 1 ppm and 20 ppm, respectively.     

Determination of arsenic in urine by atomic absorption spectrometry with electrothermal atomization

A cheap device which can be used to follow transient signals with a chart recorder is described.     

Determination of lead in waters by atomic absorption spectrometry with electrothermal atomization

Summary The electrothermal atomization of lead has been investigated for accurate determination of lead in water samples. Thiourea served to lower the atomization temperature of lead and to eliminate the interferences from chloride matrix. The addition of thiourea also allowed the accurate determination of lead irrespective of its chemical form. The absolute sensitivity (1% absorption) was 1.1 × 10^–12g of lead. The method permits the direct rapid determination of lead in water samples including sea water.

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Bestimmung von Blei in Wässern durch Atomabsorptions-Spektrometrie mit elektrothermischer Atomisierung

Zusammenfassung Zur Verringerung der Atomisie-rungstemperatur und zur Beseitigung von Störungen durch die Chloridmatrix wird ein Zusatz von Thioharnstoff empfohlen. Dieser Zusatz gestattet außerdem eine genaue Bestimmung unabhängig von der chemischen Form, in der das Blei vorliegt. Die absolute Empfindlichkeit (1% Absorption) beträgt 1,1 · 10^–12 g Pb. Das Verfahren wurde zur direkten Bleibestimmung in Wässern, einschl. Meereswasser, angewendet.

     

Determination of cadmium in hair and blood by tungsten coil electrothermal atomic absorption spectrometry with chemical modifiers

Three chemical modifiers ((NH(4))(2)HPO(4), NH(4)H(2)PO(4), and Pd as Pd(NO(3))(2)) were evaluated for the determination of Cd in acid-digested solutions of hair and blood using electrothermal atomic absorption spectrometry in a tungsten coil atomizer (TCA). All modifiers caused some thermal stabilization of Cd when compared to the behavior observed in nitric acid medium. The best effects were observed in 15 mug ml(-)(1) Pd medium; the characteristic mass of Cd was 0.3 pg and the method detection limits were 0.009 mug g(-)(1) in hair and 0.2 mug l(-)(1) in blood. In addition to a slight thermal stabilization effect, Pd also increased the sensitivity for Cd by ca. 40% and the tungsten coil lifetime by 20% (i.e. from 300 to 360 heating cycles), reduced background signals, and eliminated condensed phase interferences caused by concomitants. The accuracy (3.2% as mean relative error in the Pd modifier) was checked for the determination of Cd in acid-digested solutions of certified reference materials of human hair and blood and by recoveries of Cd in spiked hair and blood samples by both TCA and a graphite furnace procedure. All results obtained in chemical modifiers are in agreement at a 95% confidence level.     

Evaluation and application of internal standardization in atomic absorption spectrometry with electrothermal atomization

A new dual-channel system developed for use in atomic absorption spectrometry is used to assess the internal standard technique for electrothermal atomization. Cobalt was found to be a suitable internal standard for iron determinations, and is used for determination of 7—330 ng Fe ml^-1 in water samples. With use of the internal standard technique, fluctuations caused by atomizer variables are reduced and interferences from many cations are also decreased.     

Determination of germanium in different media by atomic absorption spectrometry with electrothermal atomization

Effectsof acids, alkalis or salts on the atomic absorption spectrometry (electrothermal atomization) of germanium are described. Most of the species tested interfere with the absorption. The mechanisms of the interferences were investigated by using tantalum-coated and standard graphite tubes. The results showed that positive interferences are due to inhibition of the reaction of germanium dioxide with carbon to form volatile GeO₃ and that suppressive interferences are due to physical occlusion effects. The addition of sodium hydroxide to the sample solution provides a very sensitive analysis (0.004 ppm for 1% absorption) for germanium without matrix interferences. This is probably due to the formation of stable Na₂GeO₃ during the ashing step in the presence of sodium hydroxide.     

Behaviour of chemical modifiers in the determination of arsenic by electrothermal atomic absorption spectrometry in petroleum products

Most comparative studies on the efficiency of chemical modifiers have been conducted in aqueous media. In the present work, we proposed a detailed study of the use of different chemical modifiers for direct determination of arsenic in complex organic matrices by electrothermal atomic absorption spectrometry (ETAAS). Palladium, rhodium, tungsten, silver, lanthanum and a mixture of palladium and magnesium were tested. The figures of merit used for evaluation and comparison were acquired in the optimal conditions for each modifier, established by multivariate optimization of the main variables based on Doehlert designs. Singular features were observed for the chemical behaviour of some modifiers in organic matrices compared to aqueous media, such as the worse performance of Pd + Mg modifier and no notice of severe tube corrosion from La application. Lanthanum was chosen as the best chemical modifier for the present application, according to predefined criteria. Lanthanum showed the minimum limit of detection, characteristic concentration and blank signal among all tested species and no effect of the concomitants usually present in petrochemical feedstocks. Using a 200 mg L⁻¹ lanthanum solution as a chemical modifier, the average relative standard deviations of 7 and 16% (at 3-15 μg L⁻¹ level) and characteristic concentrations of 0.47 and 0.77 μg L⁻¹ for naphtha and petroleum condensates, respectively, were observed.     

Some observations on the mechanisms of atomization in atomic absorption spectrometry with atom-trapping and electrothermal techniques

The mechanisms of collection and release of sixteen elements in atom-trapping atomic absorption spectrometry with a water-cooled silica trap in an air-acetylene flame are examined. Ag, Au, Cd, Co, Cu, Fe, Ni, Pb, Se and Zn appear to accumulate as metals whilst K, Li, Na, Cr, Mg and Mn are trapped as silicates or oxides. Al and V are also trapped as oxides, but were not studied further. No evidence could be found that the surface temperature of the trap exceeds 1700 K during the release cycle. Plots of appearance time of atoms vs. m.p. suggest that while direct evaporation can play a part in atomization, sputtering by energetic species in the scouring flame gases may explain the appearance of gaseous atoms at the relatively low temperatures involved. The atomization phenomena are related to those observed with electrothermal atomizers based on carbon and tantalum. It is suggested that sputtering processes may also be involved in such atomizers.     

Determination of the kinetic parameters for the electrothermal atomization of gold with and without chemical modifiers

The effect of various experimental parameters and the presence of chemical modifiers on the atomization kinetics of gold have been investigated. The dissipation process of the atomic vapour is also studied and the diffusion parameters calculated in the absence and in the presence of chemical modifiers. The chemical modifiers studied are ascorbic acid, rhenium, palladium and rhodium. In the absence of chemical modifiers, a two-precursor atomization mechanism is observed in distinctive different temperature regions. When a long pyrolysis step and low masses of gold are employed, an atomization from dispersed particles with low E_a value is observed in the low temperature region (LT region). At high masses of gold, a fractional order atomization from gold agglomerates with high, mass-dependent, E_a values, approaching the heat of vaporization, ΔH_vap, is observed in the high temperature region (HT region). In the presence of ascorbic acid, a high E_a value is obtained in the LT region, suggesting a fast atomization from surface particles at the active sites produced by the pyrolysis of ascorbic acid and a low E_a value is obtained in the HT region, with first-order kinetics, which indicates a desorption process through the micropores of the amorphous carbon residue of ascorbic acid. In the presence of 5 μg Re, a two-precursor mechanism is also found, with a high E_a value in the LT region, suggesting vaporization from small clusters, and a low E_a value in the HT region with a first-order kinetics, indicating vaporization of disperse particles from the graphite surface. In the presence of 0.1–1 μg Pd, a two-precursor mechanism is also observed. The first process, in the LT region, has a low E_a value, which indicates vaporization of disperse particles from the available free active sites of the graphite surface. The second process, in the HT region, begins at the appearance temperature of Pd and shows a high E_a value and first-order kinetics, which means that release of gold atoms occurs only after the vaporization of Pd has begun. In the presence of Rh, a mass-dependent E_a value is found in the LT region, suggesting atomization from gold clusters. However, in the HT region, the generation of atomic vapour of Au is kinetically controlled by the release of the Rh modifier.     

A new approach to the problem of atomization in electrothermal atomic absorption spectrometry

We established that the partial pressure of oxygen in the graphite furnace is several orders of magnitude higher than is explained by the thermodynamic equilibrium of the O₂ + 2C = 2CO reaction. Taking this into account has led us to some new conclusions for thermal destruction, atomization and dissociation of the oxygen-containing compounds. It explains why some elements are reduced to the metal on graphite, while other elements are vaporized as the oxide. For the elements vaporized as the oxide, it is shown that there is good agreement between the calculated thermal dissociation temperature of the metal oxide and the observed appearance temperature. For these metals, the atomization of the oxides proceeds by their thermal dissociation without direct participation of carbon in the reduction process. The presence of oxygen in the purge gas accounts for anomalous curvature in, for example, the Sn calibration curve, large variations in sensitivity for some elements (Ga, Ge, Sb, Se, Sn) comparing gas-stop and full-flow modes of atomization and the enhancement of sensitivity for some elements in oxygen-activated graphite furnaces.     

Determination of submicrogram amounts of tin by atomic absorption spectrometry with electrothermal atomization

The determination of tin is described with particular reference to the addition of organic compounds to the graphite tube for the suppression of interferences of other ions. Most were suppressed by adding 20 μl of 10% ascorbic acid to 20 μl of sample in the furnace. The method was used for the determination of tin in waste-waters and sediments.     

Kinetics of selenium atomization in electrothermal atomization atomic absorption spectrometry (ETA-AAS). Part 3: Chemical interference of sulphate using palladium modifiers

The interference caused by sulphate (as the sodium salt) in the electrothermal atomization atomic absorption analysis of selenium was investigated for prereduced and unreduced palladium nitrate modifiers. Kinetic parameters of the selenium atomization were calculated for both types of modifier with varying amounts of sulphate added. Prereduced palladium was a better modifier since it tolerated higher amounts of interferent. For high levels of interferent, the kinetic parameters approached that of selenium without modifier. It was postulated that the interference was caused by the formation of palladium sulphate which reduces the number of active palladium sites available for selenium stabilization. The poorer performance of the unreduced modifier was explained in that the formation of stabile palladium sulphate hindered the reduction of Pd(II) to palladium metal which was needed for the selenium stabilization. Sulphate only interfered on the high temperature stabilization process; the low temperature stabilization, linked to the formation of a [Pd,Se,O] compound, was unaffected. The results support earlier literature reports that selenium loss occurs by covolatilization with the matrix and gives a reason why palladium modifiers are rendered useless by the sulphate interferent.