Determination of the temperature of the graphite probe surface in graphite probe furnace atomic absorption spectrometry

An apparatus for determining the temperature of a graphite probe in graphite probe furnace atomic absorption spectrometry has been developed and tested. By measuring the change in the reflection of a laser beam from various pure metals which are deposited on the probe surface at the usual location for sample deposition, it has been found that the heating of the graphite probe surface occurs in two stages. When the probe is inserted into a pulse-heated, commercial graphite furnace after it has been heated to a steady-state temperature, the probe surface is initially rapidly heated by the radiation from the heated graphite tube wall, and thereafter the probe maintains that steady-state temperature for a short time. For a given graphite probe, the heating rate at the initial stage and the corresponding steady-state temperature at the final stage are mainly determined by the final tube wall temperature; the steady-state temperature of the probe is considerably lower than the final tube wall temperature because of thermal conduction by the probe to that part of its body which is lying outside the tube wall. The higher the final tube wall temperature, the higher is the heating rate of the probe at the initial stage, the higher is its steady-state temperature at the final stage, and the less is the difference between the final tube wall temperature and the steady-state temperature of the probe surface. The heating rate of the probe surface at 1600 K is 180 K s⁻¹, whereas at 2300 K it is 3600 K s⁻¹; the differences between the probe surface and tube wall temperatures at the former temperature is 700 K, whereas at the latter temperature it is 250 K.     

The determination of manganese by electrothermal atomic absorption spectrometry with a graphite furnace at constant temperature

Many of the interferences reported earlier for the determination of manganese in a graphite furnace were not found when a modern graphite furnace was used. At high levels of chloride matrix, an interference which was observed in the modern furnace was reduced when manganese was determined under constant temperature conditions. In this work, the sample was introduced on a tungsten wire after the graphite furnace had reached a constant, preset temperature. Drying and ashing were accomplished outside the atomization furnace, reducing contamination from matrix materials.     

Mechanism of atomization at constant temperature in capacitive discharge graphite furnace atomic absorption spectrometry

At constant temperature (isothermal) maintained throughout in the capacitive discharge technique, the measured absorbance at any time t due to concentration of analyte atoms can be given by: absorbance = p[A]₀{k₁/(k₁?k₂)}[exp(?k₂t)-exp(?k₁t)], where p is a function of the oscillator strength (a constant) and the efficiency with which the analyte atoms are produced, [A]₀ is the initial concentration of the analyte atoms, k₁ and k₂ are first-order rate constants for formation and decay of analyte atoms, respectively. This technique yields k₁?k₂ and k₁t?k₂t; and so the above equation reduces to: absorbance ?p[A]₀, resulting in large enhancement in sensitivity. In the case of lead, the immediate precursor of the gaseous lead monomer is the gaseous lead dimer, which is partly lost by diffusion of the lead dimer with a first-order rate constant, k₃. The kinetic parameters k₁, k₂ and k₃ have been evaluated, and the values of k₁ at different temperatures used to draw the Arrhenius plots, from which activation energies of the rate-determining steps have been determined. The activation energies have been used to elucidate atomization mechanisms by extensive correlation of the experimental energy values with the literature values.     

Determination of kinetic parameters for atom formation at constant temperature in graphite furnace atomic absorption spectroscopy

A new method for the simultaneous determination of the kinetic order and activation energy for atom release under isothermal condition in a graphite furnace has been developed. Tungsten wire probe atomization was employed to examine the validity of the present method. By means of this model, the kinetic parameters for the atomization of Bi, Ge, Pb and Mn at constant temperatures were successfully determined. The values of the kinetic order and activation energy were found to be 0.67 ± 0.01 and 302 ± 8 kJ mol⁻¹ for Bi, 1.01 ± 0.08 and 109 ± 2 kJ mol⁻¹ for Ge, 0.46 ± 0.01 and 159 ± 2 kJ mol⁻¹ for Pb and 0.97 ± 0.03 and 372 ± 5 kJ mol⁻¹ for Mn, respectively. The atomization mechanism for these four elements from the tungsten probe surface was also discussed.     

石墨炉内石墨探针表面上的原子化机理研究 III. 铬的原子化机理

本文利用探针原子化技术, 研究了普通管式石墨炉内石墨探针表面上铬化合物的原子化过程。X射线衍射分析(XRD)、俄歇电子能谱(AES)、化学分析光电子能谱分析(ESCA)与石墨炉原子吸收光谱(GFAAS)测量的综合结果表明, 铬化合物在灰化阶段即可转化为稳定的碳化物, 最后由碳化物的热分解生成气态铬原子。     

Aspects of the pyrometric measurement of graphite furnace temperature

Experimental results are presented to show that the radiation leaving the centre hole of tubular graphite furnace atomisers can deviate significantly from ideal blackbody behaviour. Pyrometric measurements at various wavelengths of the apparent, or brightness, temperatures of two commercial graphite tube atomisers (Varian GTA-95 and CRA-90) showed that the measured brightness temperature decreased appreciably with increasing wavelength, in accordance with the predicted behaviour for a cavity radiator with an emissivity less than unity. Measurement of the sample hole emissivity from the apparent melting temperatures of certain metal and metal carbide samples indicated mean effective hole emissivities of pyrocoated GTA-95 tubes in the range 0.76–0.92, depending on temperature and wavelength. Implications of the results for pyrometric temperature measurement in graphite furnaces are also discussed.     

Reduction of temperature variation in the atomic absorption graphite furnace

The temperature variation that is experienced along the length of the graphite furnace tube of the Massmann design can be reduced by the use of a contoured tube. An analytical model of the steady-state temperature distribution along the graphite tube has been developed and has been shown to agree quite well with experimental data. Steady-state and time-dependent measurements along the length of the graphite tube are reported for different conditions. With conventional tubes at thermal equilibrium, there is a temperature difference exceeding 1000°C between the center and the ends when the center of the tube is at 2500°C. With a contoured tube this temperature gradient has been reduced to 100°C.     

A new furnace design for constant temperature electrothermal atomic absorption spectroscopy

A new concept is described for the production of atomic vapour with a constant temperature graphite furnace. Samples are manually or automatically dosed into a graphite cup which is fastened tightly to an aperture of a graphite tube. For atomization, the tube is heated to a preselected temperature followed by heating of the cup by means of a separate power supply. Experimental results are given to characterize the performance of the furnace with regard to sensitivity, non-specific absorption and interference effects. Possible applications of the system are discussed.     

Effect of magnesium nitrate vaporization on gas temperature in the graphite furnace

The vaporization of magnesium nitrate was observed in longitudinally-heated graphite atomizers, using pyrocoated and Ta-lined tubes and filter furnace, Ar or He as purge gas and 10–200-μg samples. A charge coupled device (CCD) spectrometer and atomic absorption spectrometer were employed to follow the evolution of absorption spectra (200–400 nm), light scattering and emission. Molecular bands of NO and NO₂ were observed below 1000°C. Magnesium atomic absorption at 285.2 nm appeared at approximately 1500°C in all types of furnaces. The intensity and shape of Mg atomization peak indicated a faster vapor release in pyrocoated than in Ta-lined tubes. Light scattering occurred only in the pyrocoated tube with Ar purge gas. At 1500–1800°C it was observed together with Mg absorption using either gas-flow or gas-stop mode. At 2200–2400°C the scattering was persistent with gas-stop mode. Light scattering at low temperature showed maximum intensity near the center of the tube axis. Magnesium emission at 382.9, 383.2 and 383.8 nm was observed simultaneously with Mg absorption only in the pyrocoated tube, using Ar or He purge gas. The emission lines were identified as Mg ³P°–³D triplet having 3.24 eV excitation energy. The emitting species were distributed close to the furnace wall. The emitting layer was thinner in He than in Ar. The experimental data show that a radial thermal gradient occurs in the cross section of the pyrocoated tube contemporaneously to the vaporization of MgO. This behavior is attributed to the reaction of the sample vapor with the graphite on the tube wall. The estimated variation of temperature within the cross section of the tube reaches more than 300–400°C for 10 μg of magnesium nitrate sampled. The increase of gas temperature above the sample originates a corresponding increase of the vaporization rate. Fast vaporization and thermal gradient together cause the spatial condensation of sample vapor that induces the light scattering.     

Effect of beryllium nitrate vaporization on surface temperature in the pyrocoated graphite furnace

The vaporization of 20–50 μg beryllium from nitrate solution was observed in graphite furnace atomizers using pyrocoated and Ta-lined tubes. A charge coupled device (CCD) spectrometer was employed to follow the evolution of absorption spectra (200–475 nm), the light scattering and emission. Molecular bands of NO and NO₂ were observed below 1000°C. Beryllium absorption at 234.9 nm was prominent in spectra above 2200°C and 1900°C, respectively, in Ta-lined and pyrocoated tubes. The evolution profile of Be atomic absorption and of some bands indicated a faster vapor release in the pyrocoated tube. Light scattering occurred only in the pyrocoated tube, increasing with the tube age. When purge gas mini flow was applied, the scattering was observed at 1900–2200°C simultaneously with Be atomic absorption and emission continuum at long wavelength. The emission continuum showed the wavelength distribution characteristic of black body radiation. The temperature increase, due to the vaporization of the sample, was estimated using Planck’s equation. The maximum temperature increase reached 400°C, when the most intense Be atomic absorption, light scattering and emission was observed. According to the hypothesis proposed, the black body radiation was induced by the formation of Be carbide in the pyrographite layer. Low heat capacity across the pyrographite prevented the heat dissipation, and led to increase of surface temperature. This induced an increase of sample evaporation rate and the formation of a thermal gradient in the cross section of the tube. Both factors originated vapor supersaturation in the tube center, spatial condensation and, accordingly, light scattering. The results, together with those already obtained with Mg nitrate place limitations to the atomization theories based on the concept of isothermal equilibrium or on Arrhenius kinetic approach.     

The gas temperature in and the gas expulsion from a graphite furnace used for atomic absorption spectrometry

A simplified model for heat transfer based on thermal conduction is used to calculate the radial gas temperature distribution inside a semi-enclosed, commercial graphite tube furnace used for atomic absorption spectrometry. In the absence of a forced convective flow of a purge gas, the gas temperature inside the graphite furnace during its heating is lower than the wall temperature. After the wall temperature has attained a steady-state value, the gas temperature approaches the wall temperature and the radial temperature gradient in the gas decreases. The difference between the wall temperature and the gas temperature depends on the temperature program used, the thermal properties of the purge gas, and the atomizer geometry. The residence time of relatively volatile analyte elements is largely controlled by expulsion when wall atomization at high heating rates and high atomization temperatures are used. Analytical sensitivities are often enhanced by vaporizing the analyte into a gas having an approximately constant temperature.     

The graphite furnace and its role in atomic spectroscopy

The diversity of applications of the graphite furnace is extraordinary, encompassing the fields of physics, thermochemistry, spectroscopy and analytical chemistry. In this respect, the graphite furnace has been used on a continuous basis as a research tool for nearly a century. Following its introduction as an atomization source for atomic absorption spectrometry by Lvov in 1959, its role in atomic spectrometry expanded considerably to encompass analytical applications in emision, fluorescence, absorption and mass spectrometry. In addition to its conspicuous use as an atomization source in these areas, it is frequently employed as a vaporizer when used in the format of combined and tandem sources with other instrumentation. The unique physico-chemical micro-environment which can be attained within the graphite furnace has also been used to advantage in a number of investigations, including the determination of gas- and solid-phase diffusion coefficients of high-temperature metal vapours, the heats of sublimation of refractory metals, fundamental optical constants and the measurement of the heats of desorption of adatoms from high-temperature surfaces. The range of such applications remains to be more fully explored. The attractive features of this source, viz., the high atomization/vaporization efficiency, comparatively long atomic vapour residence times, controllable chemical and thermal environment and its ability to handle high dissolved solids content samples (100%) serve to ensure its place in analytical atomic spectroscopy for years to come.     

The graphite furnace and its role in atomic spectroscopy

The diversity of applications of the graphite furnace is extraordinary, encompassing the fields of physics, thermochemistry, spectroscopy and analytical chemistry. In this respect, the graphite furnace has been used on a continuous basis as a research tool for nearly a century. Following its introduction as an atomization source for atomic absorption spectrometry by L'vov in 1959, its role in atomic spectrometry expanded considerably to encompass analytical applications in emision, fluorescence, absorption and mass spectrometry. In addition to its conspicuous use as an atomization source in these areas, it is frequently employed as a vaporizer when used in the format of combined and tandem sources with other instrumentation. The unique physico-chemical micro-environment which can be attained within the graphite furnace has also been used to advantage in a number of investigations, including the determination of gas- and solid-phase diffusion coefficients of high-temperature metal vapours, the heats of sublimation of refractory metals, fundamental optical constants and the measurement of the heats of desorption of adatoms from high-temperature surfaces. The range of such applications remains to be more fully explored. The attractive features of this source, viz., the high atomization/vaporization efficiency, comparatively long atomic vapour residence times, controllable chemical and thermal environment and its ability to handle high dissolved solids content samples (相似文献   

Hydride atomization in graphite furnace atomizers

Summary A new type of graphite furnace atomizer — the graphite paper atomizer (GPA) — is described. The tube dimensions are large: l=92 mm, i.d.=9 mm. Analytical determinations of hydride forming traces were carried out in the absence and in the presence of hydride forming matrices using both a normal quartz tube atomizer (QTA) and the new GPA. The analyticalk results for pure solutions of the traces are the same for both atomizers. In the presence of hydride forming matrices the GPA gave improvements of 1 to 3 orders of magnitude for GPA. Chemical matrix modifications were employed to reduce matrix interferences of Se, Te (with Cu²⁺ ions) and Bi (with EDTA). It was possible to explain the types of interference by hydride forming elements. Thermodynamic calculations were made of the equilibrium composition in hydride vapours. The influence of temperature and hydrogen on the atomization in both atomizers has been studied. The AsSb molecule could be detected spectroscopically in appropriate hydride vapours.It was concluded that the main source of matrix interference by hydride forming matrices is the formation of diatomic molecules of the AsSb type between trace and matrix elements. At higher temperatures (about 2000–2300 K) the thermal atomization of the hydrides plays the main role.

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Hydridatomisierung in Graphitrohratomisatoren

Zusammenfassung Ein neuer Typ eines Graphitrohratomisators — der Graphitpapieratomisator (GPA) — wird beschrieben. Die Rohrdimensionen dieses Atomisators sind relativ groß: l=92 mm, i =9 mm. Analytische Bestimmungen von hydridbildenden Spuren wurden durchgeführt in Abwesenheit und in Gegenwart von hydridbildenden Matrices. Dabei wurde sowohl ein normaler Quarzrohratomisator als auch der neue Graphitpapieratomisator eingesetzt. Die analytischen Resultate waren, wenn nur reine Lösungen eingesetzt wurden, für beide Atomisatoren gleich. In Gegenwart von hydridbildenden Matrices ergaben sich jedoch für den Graphitpapieratomisator analytische Verbesserungen von 1–3 Größenordnungen. Chemische Matrixmodifikation wurde eingesetzt zur Verminderung der Matrixeinflüsse von Se, Te (durch Cu²⁺-Ionen) und von Bi (durch EDTA). Die Mechanismen der Störungen durch hydridbildende Matrices konnten geklärt werden. Thermodynamische Berechnungen der Gleichgewichtszusammensetzung in Hydrid-Dämpfen wurden ausgeführt. Verschiedene Experimente werden beschrieben zum Studium des Einflusses der Temperatur und des Wasserstoffs auf die Atomisierung von Hydriden in beiden Atomisatoren. Das AsSb-Molekül konnte auf spektroskopischem Wege in entsprechenden Hydrid-Dämpfen nachgewiesen werden. Die Schlußfolgerungen aus diesen Berechnungen und Experimenten sind:Der Hauptgrund für die Matrixeffekte hydridbildender Matrices ist die Bildung von zweiatomigen Molekülen zwischen Spur und Matrixelement. Bei höheren Temperaturen (2000–2300 K) spielt die thermische Atomisierung der Hydride die Hauptrolle.

     

The atomic absorption determination of zinc with a graphite furnace

The use of a heated graphite furnace has been evaluated for the atomic absorption determination of zinc. Interferences were found to occur with most elements when present in large amounts; solvent extraction procedures have been investigated to avoid such effects. Results are reported for the solvent extraction and determination of zinc in the range 0.002–1 p.p.m.     

The present and future of graphite furnace atomic absorption spectroscopy

Graphite furnace atomic absorption spectroscopy (AAS) technology has been greatly improved since the late 1970s and the new technology is now being used widely. The chief characteristic of the new technology is its remarkable freedom from interference while retaining the high sensitivity of graphite furnace AAS. Thus, an important goal of continuing furnace research is to identify interferences that persist and, by understanding the causes of residual interference, make further improvements to the system. Several background correction systems have been used.Furnace AAS remains a slow analytical technique, typically about 2 min per analyte and sample. One way to speed throughput is to use simultaneous multielement analysis although this is not easily compatible with the modern furnace. Inherently, the photometric range of furnace AAS has been more limited than, say, inductively coupled plasma but there are now ways to improve the range of furnace measurements. Alternatively, furnace emission provides a potential multielement opportunity, especially if the emission signal is enhanced with an electric discharge.The reduction in matrix interferences increases the opportunity of analyzing solid samples in the furnace. Solid samples may be handled by using well stirred aqueous slurries of finely ground materials. Flow injection analysis, already widely used with flame AAS, provides real potentiality. Perhaps the furnace AAS may become an 'absolute' technique. This will require some changes in the design of the spectrometer and electronic signal handling.     

Fast analysis with Zeeman graphite furnace AAS

Stabilized Temperature Platform Furnace (STPF) methods have been adapted and altered to reduce the analytical time to less than 1 min per sample with no loss of analytical precision or accuracy. It is shown that this could be further reduced to about 30 s per sample if certain changes in instrumentation are implemented, especially in the software and firmware that control the autosampler. The sample uptake rate for the autosampler should overlap the cooldown of the tube from the prior determination. Also, the sample should be deposited onto a heated platform. In this work the pyrolysis step and, in most cases, the use of a matrix modifier has been omitted. Since backgrounds were therefore larger, the use of Zeeman correction was usually required, but continuum background correction was not tried. To confirm that these fast analytical methods might be practical, more than 10 standard reference materials were analyzed for several elements including Pb, Cd, Cu, Ni, As and Cr. The paper is not primarily intended to provide routine and reliable methods; it is intended to test the feasibility of these fast methods.     

Atomization efficiency of a Massmann-type graphite furnace

The atomization efficiency of the Perkin-Elmer HGA-400 for the production and containment of atomic vapour has been determined for Al, Co, Au, Ni, Fe, Ag, Ga and Cu by a method developed under non-isothermal conditions. The method takes into account the residence time and the shape of the absorbance signal profile. It is based on the entire absorption signal profile rather than on any part of it. The uncertainty in the method arises from calculation of the rate constant of atom loss due to temperature gradient which has a pronounced effect on the "effective" length of the analysis path when the atomizer is operated under non-isothermal conditions. The average value of the atomization efficiency for the eight elements used for this study was found to be about 0.10.     

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.     

Determination of cadmium in paint samples by graphite furnace atomic absorption spectrometry with optical temperature control

A graphite furnace atomic absorption spectrometry (GFAAS) method with optical temperature control for the determination of trace cadmium in paint samples is described. Optical temperature control was superior in many respects to current temperature control. The sensibility increased by 60%, the linear range widened by 60%, and the life of graphite tube showed a 200–300% increase because atomization temperature was lowered distinctly and atomization time was shortened. Use of lanthanum chloride as a matrix modifier was investigated. The linear range of calibration curve was 0–24 ng mL⁻¹. The detection limit was 9.6 ng L⁻¹. The characteristic mass was 3.0 pg. The method also resulted in excellent reproducibility (≤2.5% R.S.D.) at such low levels, and the recovery of added cadmium in paint samples was from 94.6% to 102%. This method is readily applicable to the determination of cadmium in paint samples.