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.     

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.     

石墨炉原子吸收光谱中海水的背景吸收研究

由于海水的成分复杂,含盐量高,给用石墨炉直接测定其中的痕量杂质带来许多困难^[1]。Sturgeon等用预先分离的方法来克服干扰^[2]。为了进行直接测定,研究海水背景吸收的来源、特点和消除方法是重要的。海水背景吸收的波长特性和在石墨管内蒸发行为的研究表明,海水的背景吸收主要来自氯化钠。时间特性的研究表明,背景吸收的时间分布及背景峰高与原子化阶段的加热方式和原子化温度有关。作者还研究了基体改进。剂和其它减小背景吸收的方法。     

The interference effect of more than one anion and cation in graphite furnace atomic absorption spectrometry. Part 2: Effect of sodium,magnesium, sulphate and chloride mixtures on the atomization of manganese

Although there are numerous cations and anions in real samples generally, the interference effects of a matrix consisting of one cation and one anion on the atomization of an element in graphite furnace atomic absorption spectrometry (GFAAS) have been investigated. Therefore, it would be more realistic to investigate the interference of a matrix containing more than one cation and anion. In this study, the simultaneous interference effects of sodium, magnesium, sulphate and chlorine, which are the most common and abundant ions in many samples, e.g. sea water, on the atomization of manganese in GFAAS were studied. As a first step, the individual interferences of some possible salts consisting of simple combinations of the ions studied such as sodium chloride, magnesium chloride, sodium sulphate and magnesium sulphate, were investigated. It was found that in the presence of these four ions and in their wide concentration range, sodium chloride and magnesium sulphate are the major salts formed after the drying step which were supported by X-ray diffraction (XRD) studies. Sodium chloride causes a significant depressive effect at low pyrolysis temperatures. The interference of sodium chloride originates from expulsion of the analyte with matrix and gas phase reaction between manganese and chloride ions during atomization. Magnesium sulphate does not cause any depression and in fact higher pyrolysis temperatures compared to matrix-free manganese can be applied without loss of any analyte. The depressive effect of sodium chloride on manganese markedly decreased in the presence of magnesium sulphate due to its protecting effect. The mutual interaction mechanism of these two salts and their effects on manganese have been discussed.     

Investigation of reactions involved in graphite furnace atomic absorption procedures: Part 12. A study of some factors influencing the determination of selenium

An experimental and theoretical study of various factors influencing the determination of selenium by graphite furnace atomic absorption spectrometry (g.f.a.a.s.) is reported. It is shown that the atomization efficiency can be increased as compared to the L'vov platform technique by means of a constant temperature furnace as a consequence of the possibility of choosing a higher atomization temperature. This is explained by means of high temperature equilibrium calculations, which include the formation of the thermodynamically relatively stable gaseous diselenium, hydrogen selenide and selenium sulphide. The extent of losses of selenium during thermal pretreatment was established by measurements with ⁷⁵Se for different types of selenium compounds, Se(-II)-methionine, selenite and selenate, in aqueous solutions as well as in chloride and sulphate matrices. It is shown that the addition of 20 μg of nickel is effective in stabilizing Se(IV) and Se(VI) in the presence of sodium chloride, sodium sulphate and pure water. However, in the presence of both an organic matrix (glucose) and sodium chloride, nickel is shown to lose its stabilizing effect.     

Time resolution of interferences in electrothermal atomic absorption spectrometry

A fast detector-amplifier-readout system is used for studying interferences in electrothermal graphite atomizers. The effects of different matrix components (K, B, Ca, Mg, and Cl), and graphite tube surfaces significantly alter the atomization processes of lead.     

Comparative Study of the Effect of Sample Matrix on the Atomic Absorption of High-Volatile Elements in Two-Step and Conventional Atomizers

The space–time dynamics of absorbing atomic layers of cadmium and lead and molecular layers of zinc chloride in a commercially produced transverse-heated graphite atomizer and a newly developed two-step atomizer was studied. It was shown that the limiting temperature of cadmium pyrolysis in the two-step atomizer without the use of modifiers may be as high as 1000°C, whereas in the commercial analyzer is it not higher than 300°C. Levels of nonselective absorption due to sodium chloride were compared. It was found that, for a two-step atomizer, the maximum allowable mass of sodium chloride for which the background at lead and cadmium lines can be adequately compensated is 17–30 times higher than that for the commercial atomizer. The atomization of cadmium in the presence of sodium chloride was studied using time, space, and spectral resolution. It was shown that the effect of the chloride matrix in the two-step atomizer is suppressed because of sample fractionation and distillation in the course of its evaporation and condensation.     

Morphological and spectroscopic investigation of the behavior of permanent iridium modifier deposited on pyrolytic graphite coated and zirconium treated platforms in electrothermal atomic absorption spectrometry

In order to better characterise a permanent modifier based on iridium deposited on zirconium or tungsten treated platforms of transversely heated graphite atomizer, and to gain additional information about its chemical behavior directed to an eventual further optimization, a series of experiments were carried out, both by surface techniques such as scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS or ESCA) and X-ray fluorescence (XRF) and by electrothermal atomic absorption spectrometry on the iridium release from unmodified and various other modified pyrolytic graphite platforms. Special attention was paid to the influence of the amount of iridium, zirconium carbide coating of the platform surface and the presence of citric acid on the iridium vaporization during pyrolysis and atomization. The processes of iridium losses during pyrolysis and atomization and peak maximum alignment depend on the amount of the iridium deposited on the pyrolytic graphite coated platforms in the presence of nitric acid. A fractional order of release which suggests an atom vaporization from the surface or edges of the iridium islands was estimated. In the presence of citric acid, mass independence and zero order of the atom release were found. The zirconium treatment of the platform results in change of the spatial distribution of iridium and hence its vaporization. Vaporization temperatures as high as 2100°C, and first order of the process of atom generation were obtained. While it was possible to study the iridium atomization from uncoated and zirconium coated surfaces, evidencing a different order for the release process, the same was not possible for the tungsten coated platforms due to an ‘overstabilization’ that brought the iridium atomization temperature out of the working range of the instrument used. The different chemical behavior of tungsten and zirconium was also confirmed by XPS investigations. With tungsten, evidence of both W---C and W---O bonding was found, while zirconium on the contrary shows only Zr---O bonding and no evidence of carbide bonding. The SEM revealed a highly dispersed distribution of spot-like features whose smallest average diameter was of the order of 0.1 μm. The XRF asserted the confinement of iridium in these features and a strict association with zirconium in the case of zirconium treated surfaces. It is worth mentioning that such structure was preserved also after 400 thermal cycles simulating an atomization step at 1900°C despite a quite evident deterioration of the graphite surface, thus confirming the excellent durability of this modifier.     

镧涂层表面效应初探

本实验通过对涂镧石墨管和非涂镧石墨管的电子显微分析，证实了镧涂层并未能对石墨管壁的孔洞起到有效的填充作用，这与测定金时吸收信号未明显变化的实验现象相吻合。通过对锡原子化前后石墨表面的Ｘ光电子谱分析，尝试提出了镧在管中的催化作用及影响某些元素原子化机理、从而改变其灵敏度的观点。     

锡石墨炉原子化机理研究

本文考察了盐酸、硫酸、硝酸、亚硒酸及硼酸中锡的石墨炉原子吸收特性,对锡在不同基体中的原子化过程进行了初步探讨。发现硼酸是石墨炉法测定锡时的一种较好的化学改进剂;1%的硼酸加入可克服一定含量的硫酸与盐酸的干扰。并利用XRD与XPS对硼酸与锡共存时的石墨表面进行了结构与状态分析。提出了硼酸与锡共存时,锡的原子化历程。     

Improvement of direct determination of Cu and Mn in seawater by GFAAS and total elimination of the saline matrix with the use of hydrofluoric acid

Hydrofluoric acid, added to seawater, can assist in the removal of chloride in the drying step by precipitating fluoride salts, thus suppressing the chloride interference effects induced on the atomization signals of Cu and Mn. By adding HF to seawater before the analysis, MgF₂ and CaF₂ are precipitated at the bottom of the sampling flask, without precipitating Cu and Mn, and are consequently not introduced into the graphite furnace. Because sodium salts are eliminated at the pretreatment step, the whole seawater matrix is eliminated before the atomization of Cu or Mn. Therefore, the analyzed volume of seawater can be increased by using the multi-injection procedure without degradation of the limit of detection and risks of spectral interferences. The limit of detection obtained for Cu and Mn are 0.05 and 0.01 μg L⁻¹, respectively, for a 50 μL analyzed seawater volume.     

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.     

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.     

Electrothermal atomic absorption spectrometry of silicon vaporized from different surfaces

The dependence of silicon absorbance on inert gas flow rate, ashing temperature and the amount injected was studied for uncoated, pyrolytically coated and tantalum carbide-coated graphite tubes. The reasons for the anomalous curvature of the calibration graphs and other unusual phenomena are discussed. Most sensitive results for silicon were obtained by isothermal atomization from a tantalum-treated tube and platform.     

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.     

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.     

Study of the electrothermal atomization of selenium (IV) in transversely-heated graphite atomizers in the presence of nickel compounds as chemical modifiers

The electrothermal atomization of selenium, in transversely-heated graphite atomizers, was studied with regard to the effect of the addition of nickel nitrate and nickel chloride as chemical modifiers. Particular importance was given to the behavior of the analyte in aqueous standards and in sodium chloride containing solutions, after thermal reduction of the modifiers prior to the injection of the analyte solution into the atomizer. Thermal reduction of either nickel compound brings about an enhancement in the sensitivity of selenium measurements, as compared to those obtained by the injection of both the analyte and the modifier together into the atomizer. Additionally, thermal reduction of the modifiers permits the presence of chloride, as sodium chloride, to be tolerated in amounts as high as 500 μg Cl⁻ with sensitivity losses lower than 10%.     

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。     

Atomic-absorption spectrometric determination of cadmium,lead and zinc in salts or salt solutions by hanging mercury drop electrodeposition and atomization in a graphite furnace

Atomic absorption spectrometric methods are described for the determination of trace amounts of cadmium, lead and zinc in salts or salt solutions. The metals are separated from the salt matrix by electrolysis on a hanging mercury drop electrode, the mercury is transferred to a graphite boat and removed by evaporation, and the metals are determined by atomization. The feasibility of the technique was tested by analysis of sea water and of reagent-grade potassium chloride. For comparison the three metals were also determined in the sea water by stripping voltammetry, good agreement being found.