Automated determination of tin by hydride generation using in situ trapping on stable coatings in graphite furnace atomic absorption spectrometry

Conditions have been studied for the determination of Sn by coupling of hydride generation and graphite furnace atomic absorption spectrometry. Sequestering and in situ concentration of Sn hydride in the graphite furnace requires just a single application of a long-term stable trapping reagent for automated analyses. In a systematic study it is shown that effective trapping of stannane is possible on graphite tubes or platforms coated with a carbide-forming element such as Zr, Nb, Ta, or W at trapping temperatures of 500 to 600°C. Trapping temperatures should not be higher than 600°C (the “critical temperature”) because otherwise at temperatures higher than 700°C errors in absorbance values could occur by an adsorptive “carry-over effect”. Signal stability and reproducibility are tested over more than 400 complete trapping and atomization cycles, and a precision of 2% is observed. Narrow peaks are obtained for all coatings except for Nb- and Ta-coated platforms where double peaks occur. Ir- or Pd/Ir-coated surfaces allow trapping of stannane at lower temperatures but the signal stability (especially in the case of Pd/Ir coating) is lower than with the carbide-forming element coatings. The highest sensitivity is found for Zr- and W-coated tubes with a characteristic mass of about 17 and 20 pg, respectively, and the calibration curves are linear up to 2 ng Sn on Zr-treated tubes (peak height) and 4 ng on Zr-coated platforms (integrated absorbance) using the 286.3 nm line. The detection limit is 25 pg for a 1 ml sample volume, and the reagent blank is still significant with the purest available chemicals. The method is tested by determination of Sn in low alloy steel samples.     

Studies on the sensitivity of yttrium by electrothermal atomization from metallic and metal-carbide surfaces of a heated graphite atomizer in atomic absorption spectrometry

Atomization of yttrium in tube-type electrothermal atomizers was studied using various atomization surfaces: pyrocoated graphite surface, carbidized graphite surface and tantalum or tungsten metal surfaces. Carbidizing of pyrocoated graphite tubes with other carbide-forming metals (Ta, Zr or La) produces refractory metal-carbide surfaces thereby preventing the carbide-forming yttrium to come in physical contact with the reactive graphite surface. The result is an enhancement in the analytical sensitivity (peak height absorbance) of yttrium. The atomization of Y from a metal surface (Ta or W) gives better analytical sensitivity, lower atomization temperature, and negligible memory effect compared with those from metal-carbidized surfaces.     

Investigations of reactions involved in flameless atomic absorption procedures : Part II. An experimental study of the rôle of hydrogen in eliminating the interference from chlorine in the determination of lead in steel

The rôle of hydrogen in eliminating the interference from chlorine in the flameless a.a.s. method for lead in steel has been experimentally studied with the Perkin-Elmer HGA 72 and Varian-Techtron CRA 63 graphite furnaces. Hydrogen is generated at high temperatures in both systems by the reaction of graphite with water left in the tube after the drying step. The amount of hydrogen formed during the ashing procedure in the Perkin-Elmer tube is about five times larger than that in the Varian tube. The removal of chlorine from graphite tubes containing dissolved steel samples has been determined as a function of temperature for (i) the CRA 63 without hydrogen added, (ii) the CRA 63 with hydrogen added, and (iii) the HGA 72 without hydrogen added. At 950 K, a significant amount of chlorine is left only in case (i); thus the amount of hydrogen formed in the HGA 72 is large enough to remove all of the chlorine through the reaction FeCl₂(g) + H₂(g) → Fe(s) + 2 HCl(g). If a sample is ashed at 950 K, losses of lead as lead chlorides only occur for the case (i) which is in accordance with theoretical prediction. The theoretically predicted optimum ashing temperature range of 900–1000 K is shown to be in good agreement with the experimental results.     

Determination of thallium by furnace atomic absorption spectrometry

The analytical conditions for the determination of thallium by graphite furnace atomic absorption spectrometry were studied and optimized using the peak-height mode. The charring-atomization curves for thallium from different atomization surfaces were constructed and the optimum charring and atomization conditions were established. These atomization surfaces included pyrolytic graphite-, tantalum-, zirconium- and tungsten-coated graphite tubes. The effects of different inorganic acids on the absorbance of thallium from different surfaces were studied. Using tungsten carbide-coated tubes, the interference effects due to hydrochloric and perchloric acids were eliminated. The matrix modification technique was also investigated for increasing the maximum permissible charring temperature for thallium. The matrix modifiers used included tungsten, zirconium, nickel and tantalum. The effect of adding these modifiers were studied in the presence of different acids. Tungsten increased the maximum permissible charring temperature from 400 to 1000 °C.     

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.     

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.     

Electrothermal atomic absorption spectroscopy - preseht understanding and future needs

A comparison is made between Massmann-type furnaces (with and without the L'vov platform) and constant temperature atomizers. It is shown that there is no major difference between these types of furnaces with regard to peak height sensitivities. On the other hand, the Massmann-type furnaces shoved to a greater extent susceptibilities towards matrix interference effects. The effect of the sample residence time on gas phase interference effects has been investigated at various constant temperatures for lead in large excesses of iron chloride and sodium sulphate, respectively. These experimental results are discussed and they are correlated to data obtained by high temperature equilibrium calculations. As a conclusion we found that there is a need for a better control of the gas phase inside graphite tubes. Advantages of separating the volatilization and atomization processes are discussed. The potentialities of constant temperature atomizers for atomic emission spectroscopy are lined out.Since its inception, conventional GFAAS has been developed considerably with regard to methodology and instrumentation. The technique has been essentially improved by the introduction of e.g., automatic sample devices, the L'vov platform technique, matrix modifications, pyrolytically coated graphite, automatic background correctors, adequate signal evaluation and rapid controlled heating of the atomizers. In spite of this progress there still remain problems in connection with the vaporization/atomization of samples. In conventional Massmann-type furnaces, the temperature at which an element is vaporized depends on its volatility and usually effective atomization temperatures are often too low for complete atomization. An additional disadvantage comprises difficulties in relating absorbance signals, which may originate from different atomization intervals, to true amounts of an element. Many of these problems inherent in Massmann-type furnaces can be eliminated by vaporizing samples into atomizers which are kept at a constant temperature. This concept was employed in the first graphite furnace ever built for analytical AAS [l], but due to the technical complexity of the isothermal approach, it has only been realized on a minor scale and therefore little is known about its limitations.By vaporizing samples from a platform [2,3] inserted into Massmann-type furnaces, the problems arising from non-isothermal atomization can often be minimized in a relatively simple way. In particular for volatile elements it is possible to approach conditions of constant temperature atomizers by the combined use of the platform technique with an element stabilizing modifier solution [4,5].The aim of this paper is to characterize isothermal as well as Massmann-type atomizers (equipped with and without platforms) with respect to sensitivity and susceptibility to interference effects as well as identifying future needs in order to develop the graphite furnace technique further.     

Critical Study of the Determination of Palladium by Graphite Furnace- Atomic Absorption Spectrometry

Abstract

The atomic absorption spectroscopic behaviour of palladium in a graphite furnace has been investigated. The determination is several ordes of magnitude more sensitive than the flame technique. The use of Tantalum coated tubes does not improve considerably the sensitivity but prolongs the lifetime of the furnaces.     

Factors Affecting Selenium Atomization Efficiency in Graphite Furnace Atomic Absorption

Abstract

The effect of graphite furnace surface treatment, and the addition of “matrix modifiers” such as nickel or lanthanum on the observed selenium atomization siqnal in electrotherval atomic absorption analysis has been investiqated. The results indicate that the removal of signal depression caused by the addition of metal solution to analyte solution is not simply a devolatilization of the selenium. The effect appears to be a modification of the graphite surface which leads to more efficient atom formation. The role of the surface was investigated monitoring the atomic absorption signal generated from graphite furnaces which were untreated, pyrolytic graphite-coated, zirconium-or tantalum-coated, and metal-coated followed by pyrolytic graphite coating. The dependence of the analyte signal on the concentration of added metal was investigated for these surfaces. The optimum results obtained were the metal-coated/pyrolytic graphite-coated cuvettes. These cuvettes showed reduced effect of “matrix modifier,” suggesting that the surface treatment can replace the “matrix modifer,” Surface chemistry consistent with the atomic absorption observations and surface analysis data is presented.     

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.     

石墨炉内石墨探针表面上的原子化机理研究——Ⅷ 锡的原子化机理

利用探针原子化技术,研究了在普通石墨管中锡化合物的原子化过程中所发生的化学反应,阐述了锡的原子化机理。结果表明,锡试样首先转化成为氧化物,氧化物发生石墨碳还原而生成气态原子。     

Chemical modification of graphite surfaces for the determination of chromium by electrothermal atomic absorption spectrometry

Different kinds of graphite surfaces (electrographite, pyrolytic graphite, zirconium and tungsten carbide-coated) have been tested for optimization of analytical conditions for the determination of chromium using electrothermal atomic absorption spectrometry. The effect of mineral acids on the peak absorbance signal of chromium has been investigated. Considering pyrolysis temperature and sensitivity, atomization from pyrolytic graphite coated surface showed the best performance.     

The L'vov platform for furnace atomic absorption analysis

The L'vov Platform converts the Massmann-type graphite furnace into a furnace that provides a constant temperature environment for the more volatile metals. A very great reduction of matrix interferences for Pb, Cd and Tl is reported, so long as the matrix concentration provides a background signal during atomization that is small enough to be controlled by the deuterium arc background corrector. Interferences for Cu, Mn and Sn are also reduced to some extent. L'vov's recent theory relating to vapor phase thermodynamics makes it possible to predict both qualitatively and semi-quantitatively many of the remaining interference effects. This provides potential solutions to remaining problems which are herein explored. The continued test of interferences reported in the literature indicates that most are not present in the modern furnaces when they are used in an optimum manner.     

镧涂层塞曼效应石墨炉原子吸收法测定铜基合金中微量锡

本文采用塞曼石墨炉原子吸收法结合石墨管涂镧技术测定了铜基合金中微量锡。文章对比了涂镧管与非涂镧管测定锡时的原子化曲线对于镧涂层对实验结果的影响及酒石酸的作用进行了讨论，提出了镧化合物可能在管内起到催化作用的观点。实际样品测定相对标准偏差为４．４％，加标回收率１０１％。     

Some observations on the atomic absorption spectrometry of gallium with electrothermal atomizers

Signal strength can be doubled in normal graphite furnaces compared to pyrolytically coated ones, thus proving the essential role of carbon in the atomization of gallium. Platform vaporization enhances both peak height and area signals. The use of ascorbic acid as matrix modifier improves sensitivity considerably, with enhancements being more pronounced in pyrolytically coated furnaces.     

Hydride generation AAS performed at 1800–2300°C atomization temperature using a graphite furnace

Summary Mutual interferences of As, Bi, Sb, Se, Sn and Te in the determination of As, Sb, Se and Sn by hydride generation AAS performed at atomization temperatures between 1800 and 2300°C were investigated. The sensitivities for As, Sb and Se were similar to that obtained with a conventional quartz tube, whereas Sn showed a completely different behaviour. Radiotracer experiments indicated that tin was strongly adsorbed on the graphite inlet tube. In processing a typical amount of ca. 50 ng of the element to be determined, the addition of 10–100 g of any of the interfering elements, under the optimized conditions, does not cause a signal depression greater than 12%. In this way, the acceptable concentration range of the interfering elements can be increased by up to three orders of magnitude as compared with the conventional quartz tube technique. However, problems to be solved include reactions of water and hydrogen with heated graphite which give rise to an increased background.     

Glassy carbon tubes for electrothermal atomic absorption spectrometry

Summary The use of glassy carbon as a tube material in electrothermal atomic absorption spectrometry requires modifications to the power supply if temperatures and heating rates comparable to those for graphite tubes are to be obtained. Glassy carbon tubes frequently have a longer lifetime than pyrolytic graphite coated tubes made of polycrystalline electrographite. Peak height sensitivity for glassy carbon is better by a factor of two for some volatile elements, but up to a factor of five inferior for less volatile elements than that for pyrolytic graphite coated tubes. Peak area sensitivity is generally inferior by about a factor of two. Sample volume is limited to 5–10 l because of the smooth surface.From the signal shape it can be deduced that adsorption of analyte atoms at the tube wall plays an important role in glassy carbon, and is responsible at least in part for the lower sensitivity. Non-spectral interferences can be less pronounced in glassy carbon tubes for those interferents which interact with graphite tube surfaces. Glassy carbon is, however, no alternative to pyrolytic graphite coated tubes.

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Glasartiger Kohlenstoff als Rohrmaterial für elektrothermische Atomabsorptionsspektrometrie

     

Mechanism of cobalt atomization from different atomizer surfaces in graphite-furnace atomic-absorption spectrometry

The mechanism of cobalt atomization from different atomizer surfaces in graphite-furnace atomic-absorption spectrometry has been investigated. The atomizer surfaces were pyrolytically coated graphite, uncoated electrographite, and glassy carbon. The activation energy of the rate-determining step in the atomization of cobalt (taken as the nitrate in aqueous solution) in a commercial graphite furnace has been determined from a plot of log k_s vs. 1/T (for T values greater than the appearance temperature), where k_s is a first-order rate constant for atom release, and T is the absolute temperature. The activation energy E_a, can be correlated either with the dissociation energy of CoO_(g) or with the heat of sublimation of Co_(s), formed by carbon reduction of CoO_(s), the latter being the product of the thermal decomposition of Co(NO₃)₂. The mechanism for Co atomization seems to be the same for the pyrolytically coated graphite and the uncoated electrographite surfaces, but different for the glassy carbon surface. The suggested mechanisms are consistent with the chemical reactivity of the three atomizer surfaces, and the physical and thermodynamic properties of cobalt and its chemical compounds in the temperature range involved in the charring and atomization cycle of the graphite furnace.     

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.     

The Use of Solid Permanent Modifiers for ETA-AAS

Noble metals such as iridium as well as certain other metals may be sputtered onto the inner graphite surface of a graphite tube for use as atomization surface in electrothermal atomization atomic absorption spectrometry. This is accomplished by a low pressure argon discharge process described earlier. It has also been shown previously that the use of such tubes may reduce background absorption when determining lead, cadmuim and selenium in a complex matrix such as whole blood.