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.     

计时电量法求NiCl2(bpy)3在DMF中的扩散系数和速率常数

讨论了NiCl₂(bpy)₃(bpy：2,2-联吡啶)在DMF中的电化学行为. 控制电位使电极过程处于扩散控制下, 采用计时电量法求得了29 ℃时NiCl₂(bpy)₃在DMF中的扩散系数为5.99×10^－6 cm²•s^-1, 不同温度下的扩散系数随温度升高而增大. 选择合适的电极电位, 使电极过程处于扩散和电化学混合控制下, 采用计时电量法求得了不同电极电位下的反应速率常数k_f, 以及不同温度下的标准速率常数k⁰, 求得了表观活化能为14.4 kJ•mol^-1.     

Application of the Kelvin equation to vaporization of silver and gold in electrothermal atomic absorption spectrometry

In order to obtain additional insight into the release mechanism of the metals in electrothermal atomization atomic absorption spectrometry, a quantitative relation between the heat of vaporization and the size of the released particles is proposed on the basis of the Kelvin equation. The applicability of the equation for the investigation of silver and gold vaporization is demonstrated and the limits in which the model is valid are determined.According to the present considerations the activation energy could be equal to the heat of vaporization of the silver and gold droplets. An explanation of the observed dependence of activation energy on analyte mass is given. The proposed relation provides a possibility for definition and evaluation of an “effective” radius/size of the droplets on the basis of their heat of vaporization. A correlation between the mass of the injected sample and the “effective” radius of the droplets, obtained at higher temperature is found. The minimum and maximum “effective” radii of the droplets, following the proposed equation are calculated for Ag on pyrolytic graphite coated electrographite (PGC) and Au on PGC, uncoated electrographite (EG) and glassy carbon (GC) tubes. The results obtained are indirect evidence for the island structure of precursor metal layer and for the existence of silver and gold microdroplets on the graphite support.     

Influence of the tube surface on atomization behavior in a graphite tube furnace

Summary Sensitivity is compared for a number of elements in tubes for electrothermal atomic absorption spectrometry made from pyrolytic graphite coated and uncoated polycrystalline electrographite and from glassy carbon. The best atomization efficiency is always obtained in a pyrolytic graphite coated tube, independent of the volatility of the element and the atomization mechanism involved. The pronounced sticking properties of metal atoms to active sites of uncoated polycrystalline electrographite and glassy carbon are believed to be responsible to this behavior. This theory is supported by the observed shifts in peak maxima, peak broadening, and the higher atomization temperatures that have to be applied in these tubes. For elements forming stable carbides, these effects can be enhanced by chemical reactions with the tube material.

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Einfluß der Rohroberfläche auf das Atomisierungsverhalten in einem Graphitrohrofen

Zusammenfassung Für eine Reihe von Elementen wird die Empfindlichkeit in Rohren für die elektrothermische Atomabsorptionsspektrometrie aus pyrolytisch beschichtetem und unbeschichtetem polykristallinem Elektrographit und aus glasartigem Kohlenstoff verglichen. Die wirksamste Atomisierung wird stets in pyrolytisch beschichteten Graphitrohren erzielt, unabhängig von der Flüchtigkeit des Elements und von dem Atomisierungsmechanismus. Wir nehmen an, daß die ausgeprägten Hafteigenschaften von Metallatomen an aktiven Stellen von polykristallinem Elektrographit und glasartigem Kohlenstoff für dieses Verhalten verantwortlich sind. Diese Theorie wird gestützt von den beobachteten Verschiebungen der Peakmaxima, Peakverbreiterungen und den höheren Atomisierungstemperaturen, die in diesen Rohren verwendet werden müssen. Für Elemente, die stabile Carbide bilden, können diese Effekte durch chemische Reaktionen mit dem Rohrmaterial noch verstärkt werden.

     

吡啶基离子液体[C_6py][DCA]热力学性质及利用新E?tv?s方程预测其表面张力

制备了吡啶类离子液体N-己基吡啶二氰胺盐[C_6py][DCA],并用核磁共振氢谱(~1H NMR)、核磁共振碳谱(~(13)C NMR)、差热扫描量热(DSC)、傅里叶变换红外(FT-IR)光谱对其进行表征。在288.15–338.15 K温度范围内,采用标准加入法,测定其密度(ρ)、表面张力(γ)和折光率(n_D)。在测得的实验数据的基础上,得到了离子液体[C_6py][DCA]的分子体积(V_m)、表面能(E_a)、摩尔极化度(R_m)和极化率(α_p)。结果显示E_a、R_m和α_p几乎不随温度的变化而发生改变。本文还提出了摩尔表面Gibbs自由能(g_s)的概念,并改进了E?tv?s方程。同时还计算了gs、临界温度(T_c)和E?tv?s方程经验参数(kE),并预测了离子液体[C_6py][DCA]的表面张力,预测值与实验值具有较好的一致性。     

过氧硝酸乙酰酯分解反应的速率常数

在G3MP2B3结构优化和能量计算的基础上, 采用RRKM理论和疏松过渡态模型重新估算了过氧硝酸乙酰酯(PAN)的热分解反应PAN→CH₃C(O)OO+NO₂(R1)的反应速率常数, 得到与实验值吻合的结果.用同样的模型计算了PAN→CH₃C(O)O+NO₃(R2)的反应速率常数. 结果表明, 在相同的反应条件下, R1是主要的分解通道, R2是次要通道, R2的反应速率常数比R1的小两个数量级.     

HS+HO2气相反应机理及主通道速率常数的理论研究

采用CCSD(T)/6-311++G(3df, 2pd)//B3LYP/6-311+G(2df, 2p)双水平计算方法构建了HO₂+HS反应体系的单、三重态反应势能面,并对该反应主通道的速率常数进行了研究。研究结果表明,标题反应经历了八条反应通道,其中三重态反应通道R1是标题反应主通道。此通道包含路径Path 1 (R → ³IM1 → ³TS1 → P1(³O₂+H₂S))和Path 1a (R → ³IM1a → ³TS1a → P1(³O₂+H₂S))两条路径。利用经典过渡态理论(TST)与变分过渡态理论(CVT)并结合小曲率隧道效应模型(SCT),分别计算了主路径Path 1和Path 1a在200-800 K温度范围内的速率常数k^TST、k^CVT和k^CVT/SCT,在此温度区间内路径Path 1和Path 1a具有负温度系数效应。速率常数计算结果显示,对主路径Path 1和Path 1a而言,变分效应在计算温度段内有一定影响,与此同时量子力学隧道效应在低温段有显著影响。路径Path 1和Path 1a的CVT/SCT速率常数的三参数表达式分别为k₁^CVT/SCT(200-800 K) = 1.54×10^-5T^-2.70exp(1154/T) cm³ ·molecule^-1·s^-1和k_1a^CVT/SCT(200-800 K) = 5.82×10^-8T^-1.84exp(1388/T) cm³·molecule^-1·s^-1。     

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

     

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.     

Studies of critical factors in the determination of arsenic in Standard Reference Materials of Marine origin by ETAAS: ;NMKL* interlaboratory study

A study to determine factors which are known to influence the electrothermal atomic absorption (ETAAS) determination of As has been performed. The study has been carried out using five sample solutions of marine Standard Reference Materials distributed to four participating laboratories. Uncoated and pyrolytically coated graphite tubes with L'vov platform and Ni and Pd/Mg as chemical modifiers have been tested. No differences in results have been obtained between AAS instruments equipped with Zeeman correction or deuterium arc background correction. Small differences in concentration levels of arsenic as well as in characteristic mass were found when chemical modifiers were compared. Pd/Mg will be recommended in order to avoid a contamination of the graphite furnace with nickel. The characteristic mass was improved by using pyrolytically coated graphite tubes with the L'vov platform compared with uncoated graphite tubes with the L'vov platform. In the interlaboratory study, the standard addition procedure will be recommended.     

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.     

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 study of the gas-phase reactions of C2F5OC(O)H and n-C3F7OC(O)H with OH radicals at 253–328 K

The rate constants, k₁ and k₂ for the reactions of C₂F₅OC(O)H and n-C₃F₇OC(O)H with OH radicals were measured using an FT-IR technique at 253–328 K. k₁ and k₂ were determined as (9.24 ± 1.33) × 10⁻¹³ exp[−(1230 ± 40)/T] and (1.41 ± 0.26) × 10⁻¹² exp[−(1260 ± 50)/T] cm³ molecule⁻¹ s⁻¹. The random errors reported are ±2 σ, and potential systematic errors of 10% could add to the k₁ and k₂. The atmospheric lifetimes of C₂F₅OC(O)H and n-C₃F₇OC(O)H with respect to reaction with OH radicals were estimated at 3.6 and 2.6 years, respectively.     

Temperature dependence of the reaction C2H (C2D) + O2 between 295 and 700 K

The rate coefficients for the reactions of C₂H and C₂D with O₂ have been measured in the temperature range 295 K T 700 K. Both reactions show a slightly negative temperature dependence in this temperature range, with k_C2H+O2 = (3.15 ± 0.04) × 10⁻¹¹ (T/295 K)^{−(0.16 ± 0.02)} cm³ molecule⁻¹ s⁻¹. The kinetic isotope effect is k_C2H/k_C2D = 1.04 ± 0.03 and is constant with temperature to within experimental error. The temperature dependence and the C₂H + O₂ kinetic isotope effect are consistent with a capture-limited metathesis reaction, and suggest that formation of the initial HCCOO adduct is rate-limiting.     

Determination of palladium and platinum by electrothermal atomic absorption spectrometry after deposition on a graphite tube

The inner wall of a pyrolytically coated graphite tube served as the surface for adsorptive accumulation and/or for electrodeposition of palladium and platinum. A flow system for this preconcentration was constructed. For the electrodeposition a three-electrode arrangement was used. The flow rate for deposition, the medium and deposition potentials were optimized. After the deposition step, the graphite tube was placed into the graphite furnace and an atomization programme was applied. The procedure was applied for the determination of Pd and Pt in airborne particulates.     

Lead atomization in the presence of aluminum matrices by electrothermal atomic absorption spectrometry. A comparative study of slurry vs solution sample introduction

The atomization behaviour of Pb introduced to the electrothermal atomizer in a slurry of Al₂O₃ and in a solution of Al(NO₃)₃ has been studied. The characteristics of the absorbance vs time profiles are highly dependent on the way in which Pb is incorporated in the matrix. In particular, Pb adsorbed on the surface of Al₂O₃ particles behaves differently from Pb entrapped within the particles. Atomization using platform and wall atomization from both pyrolytic and non-pyrolytic graphite surfaces is described. Pyrolytic platforms do not always provide complete recovery and better results are obtained with atomization from a non-pyrolytic platform surface within a pyrolytically coated tube. With wall atomization, large differences in the absorbance vs time profiles precludes its use for quantitative determinations. However, wall atomization should be useful for future studies of the kinetic and thermodynamic processes of Pb release. Finally, the feasibility of using the non-resonance Pb line at 261.4 nm to reduce the analytical sensitivity is examined. The strong dependence of the lower energy level population on temperature makes this difficult, but the absorbance profiles with non-pyrolytic platform atomization show that all forms of Pb can be made to atomize at similar temperatures.     

Rates of the reactions CN + H2CO and NCO + H2CO in the temperature range 294–769 K

The rate coefficients of the reactions: (1) CN + H₂CO → products and (2) NCO + H₂CO → products in the temperature range 294–769 K have been determined by means of the laser photolysis-laser induced fluorescence technique. Our measurements show that reaction (1) is rapid: k₁(294 K) = (1.64 ± 0.25) x 10⁻¹¹ cm³ molecule⁻¹ s⁻¹; the Arrhenius relation was determined as k₁ = (6.7 ± 1.0) x 10⁻¹¹ exp[(−412 ± 20)/T] cm³ molecule⁻¹ s⁻¹. Reaction (2) is approximately a tenth as rapid as reaction (1) and the temperature dependence of k₂ does not conform to the Arrhenius form: k₂ = 4.62 x 10⁻¹⁷T^1.71 exp(198/T) cm³ molecule⁻¹ s⁻¹. Our values are in reasonable agreement with the only reported measurement of k₁; the rate coefficients for reaction (2) have not been previously reported.     

Study on the Atomization Mechanism of Hydrides in Graphite Furnace Atomizers

The influence of the surface state of the graphite furnace atomizer on the atomization of hydrides has been studied by means of surface film coating and quantum chemistry CN-DO/2 calculations. The results of the study prove that the atomization of AsH3, SbH3 and BiH3 in the graphite furnace atomizer is not a simple gas phase pyrolytic process, but a surface catalysis pyrolytic process.     

Gradient expansion of the kinetic energy density functional: Local behavior of the kinetic energy density

Calculations with Hartree—Fock electron densities for the rare gas atoms He through Xe show that the gradient expansion for the kinetic energy functional, T[] = T₀[] + T₂[] + T₄[] + … = ∫t() dτ, approximates the kinetic energy by averaging over the shell structure present in the true local kinetic energy density, t(), and that the accuracy of the gradient expansion improves with increasing atomic number. Components of t(), t₀(), t₂() and t₄(), are exhibited and discussed. The defined function t() is everywhere positive.     

石墨炉原子化器中原子化过程的研究(Ⅲ)——石墨炉中锗的原子化机理

有关锗的石墨炉原子吸收光谱分析,文献报道较少,对锗的原子化机理,亦有不同的看法^[1~3].本文基于右墨炉中锗原孚化行为的观察,对锗原子形成的过程进行了讨论,认为原子化过程中,并存着二种还原反应:GeO₂(s)+C→GeO(g)+CO(g),GeO₂(s)+CO→GeO(g)+CO₂,而锗原子的形成是GeO(g)热分解的结果。