Two-Stage Atomizer for Electrothermal Atomic Absorption Spectrometry: Dynamics of the Spatial Temperature Distribution

A two-stage atomizer for atomic absorption spectrometry is proposed. Its distinctive feature is the introduction of an extra stage of the fractional condensation of analyte atoms and the carrying out of the analytical cycle by a vaporization–condensation–atomization scheme. A special computer-driven power supply unit allows the heating of the upper and lower parts of the graphite furnace to be controlled independently. The dynamics of the temperature of the inner surface of the furnace for various temperature programs is studied. Using aqueous solutions of lead as an example, it is shown that one can control the processes of condensation–revaporization of elements to be determined proceeding in the atomizer volume.     

Direct determination of lead in sweet fruit-flavored powder drinks by electrothermal atomic absorption spectrometry

A simplified method for direct determination of lead in sweet fruit-flavored powder drinks, syrups and honeys by electrothermal atomic absorption spectrometry without sample digestion is proposed. Samples were dissolved in water, acidified to 0.2% (v/v) HNO₃, and directly injected into an end-capped transversely heated graphite atomizer (THGA). Building up of carbonaceous residue inside the atomizer was effectively precluded for sugar solutions not exceeding 8.0% (m/v) when a heating program with two pyrolysis steps (600 and 1000°C) was carried out without air-ashing. Under these conditions one atomizer supported about 250 firings. Among various chemical modifiers tested, better recovery and repeatability results were obtained with a 5 μg Pd + 3 μg Mg(NO₃)₂ mixture. Tests carried out with individual concomitants containing up to 1.0 μg Na, K, Ca or Cl, and up to 10.0 μg phosphate or sulphate, and several mixtures of these six concomitants, did not reveal significant interferences on lead atomization. Characteristic mass and detection limit based on integrated absorbance were 15 and 11 pg Pb, respectively. The relative standard deviation based on 10 measurements for typical samples (20–60 ng g⁻¹ Pb) was always lower than 5.5%. The detection limit of 7.0 ng g⁻¹ Pb attained the Codex recommendation for the maximum allowed lead contents in the sugar samples. Application of t-test to the results obtained by the proposed direct analysis, and the official method adopted by Food Chemical Codex, demonstrated that there were no significant differences at the 5% probability level.     

Atomization Characteristic of Chromium on a Molybdenum Tube Atomizer by Electrothermal Atomic Absorption Spectrometry and Determination of Chromium in Biological Materials

Atomization characteristics of chromium have been studied by electrothermal atomic absorption spectrometry with a molybdenum tube atomizer. The appearance temperature (T_app) of chromium nitrate was 1300°C. TheT_appwas independent of the heating rate of atomizer, but the temperature at the peak of the Cr AA signal increased with the heating rate. A sensitive absorption chromium signal was obtained in pure argon purge gas. The chromium signal decreased as the ratio of hydrogen in the purge gas increased. The optimal gas flow rate was Ar 480 ml min⁻¹+ H₂20 ml min⁻¹because of the avoidance of oxidation of the atomizer. The absolute characteristic mass (the mass of element giving 0.0044 abs.) of chromium by the atomizer was 0.35 pg and the detection limit was 23 pg ml⁻¹(3S/N). The interferences caused by large amounts of interferents were evaluated. The addition of thiourea served to eliminate the severe interferences. The accuracies of the recommended method were considered almost satisfactory for the determination of chromium in biological materials, compared with the certified values of NIST materials. The recovery of spiked chromium in biological materials was in the range from 93.5 to 102%.     

On-line coupling of electrochemical preconcentration in tungsten coil electrothermal atomic absorption spectrometry for determination of lead in natural waters

A flow injection system was coupled to a tungsten coil electrothermal atomizer (150 W) for on-line separation and preconcentration of lead based on its electrochemical reduction on the atomizer surface. The electrochemical cell is built up inside the furnace by using a Pt flow-through anode and the atomizer itself as the flow-through cathode. The manifold and the tungsten coil power supply were controlled by a computer running a program written in Visual Basic, which was utilized in synchronism with the original software of the atomic absorption spectrometer. The flow-through anode (50 mm long, 0.7 mm i.d.) was inserted in tip of the autosampler arm by replacing the last section of the PTFE sample delivering tube. The tungsten coil atomizer and the counter electrode were easily connected to a d.c. power supply. An enrichment factor of 25 was obtained for lead after a 120-s electrodeposition for a sample flowing at 1.0 ml min⁻¹. The method detection limit was 0.2 μg l⁻¹ Pb and the R.S.D.<5% (n=10 for 5 μg l⁻¹ Pb). Up to 2% m/v NaCl or KCl and 5% m/v CaCl₂ or MgCl₂ did not interfere on the separation and atomization of 5 μg l⁻¹ Pb.     

Temperature distribution in a tungsten electrothermal atomizer

Temperature distribution in the tube surface of a tungsten electrothermal atomizer (WETA-82) has been studied both theoretically and experimentally; also, the temperature distribution in the gas inside the atomizer has been studied theoretically. A theoretical model of the temperature distribution in the tube surface and in the gas inside the atomizer is proposed. It is considered that thermal radiation to the surroundings and beat conduction towards the water-cooled electrode supports are the only paths for heat loss, and electrical resistive heating and conduction from adjacent nodes are the only sources of heat gain. The time rate of change in temperature of the atomizer is expressed in the form of a differential equation. A finite-difference form of the differential equation is used in a computer program to calculate the temperature at each time step. Optical pyrometry was used to determine the surface temperature of the tube atomizer. The model predicts that an increase in the heating rate of the tube atomizer will slightly decrease the temperature gradient over the tube circumference; however, unambiguous observation of this decrease could not be made in the sequentially measured temperature difference between two selected point using optical pyrometry. Variation in the hydrogen content of the hydrogen-argon mixture purge gas did not have a significant effect on the temperature gradient of the tungsten tube surface. The results of this study have indicated possible improvements in the atomizer design with a view to making the temperature distribution over the circumference of the tube atomizer more uniform.     

Application of atmospheric pressure dielectric barrier discharge plasma for the determination of Se,Sb and Sn with atomic absorption spectrometry

The atomization of hydride-forming elements, Se, Sb and Sn, has been studied with an atmospheric pressure dielectric barrier discharge atomizer. The elements were first converted to hydride through the reaction with NaBH₄. Then the hydride were atomized in the atomizer and detected by atomic absorption spectrometry. The effects of operational parameters such as power, gas flow rate and concentrations of HCl and NaBH₄ were investigated. Compared with other hydride atomization methods, the proposed atomizer shows the following features: (1) small size, which is preferable for the miniaturization of the total analytical system; (2) low temperature, which would be helpful for further improvement in the compactness of the total analytical system; (3) low power consumption, which is also necessary for the development of analytical instrumentation for in situ detection of environmentally important elements. The analytical performance of the atomizer has also been investigated. The detection limits of Sb, Se and Sn obtained with the present method were 13.0, 0.6 and 10.6 μg l^− 1. This detector is a very promising technique for hydride detection.     

Studying element vaporization and atomization processes in electrothermal atomizers by laser-excited atomic fluorescence spectrometry: indium, matrix effects

Indium vaporization and atomization processes were studied in a variable-pressure atomizer by laser-excited fluorescence spectrometry for various analyte masses over a wide range of buffer-gas pressures. It was found that the atomization processes are of intricate nature and depend on the element mass, pressure, and heating rate. Based on the data obtained over a wide range of pressures, individual atomization processes were singled out and their parameters were determined. The effects of KC1 and CuCl₂ matrices on the kinetics and other characteristics of the processes were investigated. Possible mechanisms of indium vaporization and atomization under various experimental conditions and mechanisms of matrix effects on the analytical signals of indium and gallium are discussed.     

High current power supply for electrothermal atomic absorption spectrometry

A new power supply for electrothermal atomization — atomic absorption spectrometry (ETA-AAS) with associated equipment for temperature control is presented. The system is composed of two high current commercial batteries and a MOSFET switch which controls the temperature of the atomizer by switching the current in an on-off way through a feedback loop connected to a personal computer and suitably programmed peripherals. The delivered power can be changed from 0 to 100% in 1% steps which gives the possibility of roughly controlling both the temperature at low power when the temperature sensor is not working and the heating rate. With this system, curves for which the simultaneous presence of all atoms injected in the atomizer is claimed, have been extended to Pb, Ag, Tl and Zn. With a more conventional power supply previously used, such a claim could be made only for Cd and Hg.     

Preparation of two-dimensional molybdenum disulfide for NO2 detection at room temperature

In this work, the two-dimensional MoS₂ film was prepared by sulfuring the molybdenum atomic layer on SiO₂/Si substrate. The reaction temperature, heating rate, holding time and carrier gas flow rate were investigated comprehensively. The quality of MoS₂ film was characterized by optical microscopy, atomic force microscopy, Raman and photoluminescence spectroscopy. The characterization results showed that the optimum synthesis parameters were heating rate of 25 °C/min, reaction temperature of 750 °C, holding time of 30 min and carrier gas velocity of 100 sccm. The MoS₂ gas sensor was fabricated and its gas sensing performance was tested. The test results indicated that the sensor had a good response to both reducing gas (NH₃) and oxidizing gas (NO₂) at room temperature. The sensitivity to 100 ppm of NO₂ was 31.3%, and the response/recovery times were 4 s and 5 s, respectively. In addition, the limit of detection could be as low as 1 ppm. This work helps us to develop low power and integrable room temperature NO₂ sensors.     

Evaluation of a hydride generation-atomic fluorescence system for the determination of arsenic using a dielectric barrier discharge atomizer

A new atomizer based on atmospheric pressure dielectric barrier discharge (DBD) plasma was specially designed for atomic fluorescence spectrometry (AFS) in order to be applied to the measurement of arsenic. The characteristics of the DBD atomizer and the effects of different parameters (power, discharge gas, gas flow rate, and KBH₄ concentration) were discussed in the paper. The DBD atomizer shows the following features: (1) low operation temperature (between 44 and 70 °C, depending on the operation conditions); (2) low power consumption; (3) operation at atmospheric pressure. The detection limit of As(III) using hydride generation (HG) with the proposed DBD-AFS was 0.04 μg L⁻¹. The analytical results obtained by the present method for total arsenic in reference materials, orchard leaves (SRM 1571) and water samples GBW(E) 080390, agree well with the certified values. The present HG-DBD-AFS is more sensitive and reliable for the determination of arsenic. It is a very promising technique allowing for field arsenic analysis based on atomic spectrometry.     

Electrothermal atomization from metallic surfaces: Part 3. Some new developments in design and performance of a tungsten-tube atomizer

A new design of a work-head for a tungsten-tube atomizer as well as voltage- and temperature-feedback circuits added to the power supply of a Varian model CRA-63 atomizer are described. The importance of rapid, temperature-controlled heating of the atomizer is shown. The effect of heating rate (0.5–20 K ms^-1) on the analytical signal of many elements is investigated; experimental atomization and residence times and peak absorbance values are evaluated. The analytical performance of the tungsten-tube atomizer is tested for elements of different volatilities.     

Design considerations regarding an atomizer for multi-element electrothermal atomic absorption spectrometry

The methodology of simultaneous multi-element electrothermal atomic absorption spectrometry (ETAAS-Electrothermal Atomic Absorption Spectrometry) stipulates rigid requirements to the design and operation of the atomizer. It must provide high degree of atomization for the group of analytes, invariant respective to the vaporization kinetics and heating ramp residence time of atoms in the absorption volume and absence of memory effects from major sample components. For the low resolution spectrometer with a continuum radiation source the reduced compared to traditional ETAAS (Electrothermal Atomic Absorption Spectrometry) sensitivity should be, at least partially, compensated by creating high density of atomic vapor in the absorption pulse. The sought-for characteristics were obtained for the 18 mm in length and 2.5 mm in internal diameter longitudinally heated graphite tube atomizer furnished with 2-4.5 mg of ring shaped carbon fiber yarn collector. The collector located next to the sampling port provides large substrate area that helps to keep the sample and its residue in the central part of the tube after drying. The collector also provides a “platform” effect that delays the vaporization and stipulates vapor release into absorption volume having already stabilized gas temperature. Due to the shape of external surface of the tube, presence of collector and rapid (about 10 °C/ms) heating, an inverse temperature distribution along the tube is attained at the beginnings of the atomization and cleaning steps. The effect is employed for cleaning of the atomizer using the set of short maximum power heating pulses. Preparation, optimal maintenance of the atomizer and its compliance to the multi-element determination requirements are evaluated and discussed. The experimental setup provides direct simultaneous determination of large group of element within 3-4 order concentration range. Limits of detection are close to those for sequential single element determination in Flame AAS with primary line source that is 50-1000 times higher than the limits obtainable with common ETAAS (Electrothermal Atomic Absorption Spectrometry) instrumentation.     

Evaluation of electrodeposited tungsten chemical modifier for direct determination of chromium in urine by ETAAS

The direct determination of chromium in urine by electrothermal atomic absorption spectrometry (ETAAS) using graphite tubes modified with tungsten is proposed. Modification of the graphite is made by tungsten electrodeposition over the whole surface atomizer followed by carbide formation by heating the tube inside its own furnace. For tungsten electrocoating, the graphite tube and a platinum electrode were connected to a power supply as cathode and anode, respectively, and immersed in a solution containing 2 mg of W in 0.1% v/v HNO₃. Then, 5 V was applied between the electrodes during 20 min for tungsten electrodeposition over the whole atomizer. A SpectrAA 220 Varian atomic absorption spectrometer equipped with a deuterium background corrector was used throughout. Undiluted urine (20 μl) was delivered over the tungsten-treated tube and the chromium-integrated absorbance was measured after applying a suitable heating program with maximum pyrolysis at 1300 °C and atomization at 2500 °C. With electrodeposited tungsten modifier, the tube lifetime increased up to four times when compared to previous published methods for Cr determination in urine by ETAAS, reaching 800 firings. Method detection limit (3 S.D.) was 0.10 μg l⁻¹, based on 10 integrated absorbance measurements of a urine sample with low Cr concentration. Two reference materials of urines (SRM 2670) from National Institute of Standards and Technology (NIST) were analyzed for method validation. For additional validation, results obtained from eight human urine samples were also analyzed in a spectrometer with Zeeman effect background correction.     

Characteristics of Multi-Phase Alternating Current Arc for Glass In-Flight Melting

An innovative in-flight melting technology with multi-phase AC arc was developed for glass industry. The enthalpy probe and high speed video camera were used to characterize the temperature, velocity, and discharge behavior of multi-phase AC arc. The effects of input power and sheath gas flow rate on arc and melting behavior were investigated. Results show that the temperature and velocity on arc center are increased with input power or sheath gas flow increase. The fluctuation of luminance area ratio and coefficient of variation reflects the change of arc discharge behavior. High temperature of plasma enhances the melting of granulated raw particles during in-flight heating treatment. The shrinkage of particle and the volatilization degree of Na₂O increase under a larger flow rate of sheath gas. The characterized arc behavior agrees with the melting behavior of glass raw materials, which can provide valuable guidelines for the process control of glass melting.     

Determination of adipate plasticizers in poly(vinyl chloride) by microwave-assisted extraction

A new method based on the application of microwave radiation to the extraction of adipate plasticizers from poly(vinyl chloride) PVC plastics is described. The experimental conditions for microwave-assisted extraction (i.e. extracting solvent, temperature, time and microwave power) were evaluated in terms of recovery. The optimisation was carried out with pastes of PVC plastified with di-2-ethylhexyl adipate, and extracts were measured by gas chromatography with flame ionization detection. Six different adipate plasticizers were studied, and microwave-assisted extraction was compared with supercritical fluid extraction for the extraction of adipates and phthalates from PVC matrices. It has been observed that the microwave-assisted extraction parameters evaluated are tightly interconnected. It has been shown that the efficiency of microwave-assisted extraction depends on the kind of solvent, the temperature achieved and the heating time. Moreover, the final temperature reached depends on the microwave power, the number of vessels and the irradiation time. On the other hand, microwave-assisted extraction provides higher recovery values than supercritical fluid extraction for both phthalate and adipate plasticizers.     

石墨炉原子化器中铅吸收信号的时间位移和双峰形成

本文研究了石墨炉原子化器中铅吸收信号的时间位移和双峰出现的原因,讨论了石墨管壁附近氧的分压对铅原子化的影响。证实了氯化物基体使铅形成双峰的影响主要发生在固相和试样的蒸发过程中。     

Surface testing of a tungsten atomizer

This paper deals with the structure and surface changes of a tungsten atomizer. Atomizers made by Metallwerk Plansee for the Czech equipment WETA 90 were studied. Their effect on the height and shape of the peaks of absorption signals was investigated in dependence on number of atomization firings. The influence of various sample matrices on material changes and the evaporation of tungsten during the atomization cycle were also studied. A relationship was found between sensitivity changes and surface quality in dependence on the number of firings carried out. Good stability of sensitivity is guaranteed if the right thermal conditioning process of the atomizer is used. The influence of 4 mol dm⁻¹ HNO₃, HCl, H₂SO₄ and HC1O₄ and 5% H₂O₂ on the corrosion of the atomizer surface was investigated, and significant corrosion was observed only for H₂SO₄ and H₂O₂. The concentration of free tungsten atoms was measured from the absorbance at the tungsten 255.1 nm line (bandpass 0.1 nm) and was significantly registered over 3000°C. No influence of the concentration of hydrogen in the argon protective atmosphere on this parameter was observed.     

Mechanisms of material failure for fast heating up at the center of ultra high temperature ceramic

ZrB₂–20%SiC (ZS) and ZrB₂–20%SiC–10%graphite (ZS10C) were fabricated by hot-press sintering method. A novel method was proposed to characterize material failure for heating up at the center of ultra high temperature ceramic. Thermal stress field was calculated under different center temperatures for ZS. It showed that the material would be failed when the center temperature was higher than 2300 °C. It was consistent with the experimental results. Holding percentage of residual strength for ZS10C was higher than that for ZS, which can be attributed to flake shape graphite for ZS10C. It indicated that the material with higher crack resistance would improve security when applied under high temperatures.     

Corrosion of transversely heated graphite tubes by mineral acids

Corrosive changes of transversely heated graphite atomizer (THGA)-tube and platform surfaces were studied by scanning electron microscopy in combination with tube lifetime measurements under recommended conditions for vanadium determination. This was done for the four mineral acid matrices HNO₃, HF, HCl and HClO₄. Rising corrosion and reduced tube lifetime are observed for these matrices in the sequence HNO₃<HF≪HCl<HClO₄. Morphological changes related to the corrosive attack are different for each acid matrix and so are the effects on the analytical behaviour of the tubes. The results are compared to relevant data for vanadium and chromium measurements, which are applied for routine quality control of THGA- and LHGA-tubes by Perkin Elmer. The average mass loss of the investigated tubes per analysis cycle is also determined and is a further essential parameter of tube corrosion. Mass loss is mainly caused by carbon evaporation and particle emission during atomization and tube scavenging steps. Changes in electrical resistivity of the investigated tubes before and after the lifetime experiments were found, however, they were within the specified range for the quality control of THGA-tubes. Hence, they do not affect the temperature setting by voltage control in the relevant spectrometer systems.     

Determination of gold in biological materials by electrothermal atomic absorption spectrometry with a molybdenum tube atomizer

Electrothermal atomic absorption spectrometry (ETA-ASS) of gold with a molybdenum tube atomizer has been investigated. A sensitive ETA-AAS method was developed. The gold absorption signal became higher with the heating rate of the tube atomizer and as the ratio of hydrogen in the argon purge gas decreased. The optimal heating rate and the optimal gas flow rate were 5.5 C/msec and Ar 480 ml/min + H(2) 20 ml/min, respectively. The absolute characteristic mass (the mass of element giving 0.0044 abs.) of gold by the atomizer was 1.8 pg and the detection limit was 130 pg/ml (3S/N). These values were > 10 times better than those obtained with graphite atomizers, ICP and ICP-MS. The interferences caused by large amounts of interferents were evaluated. The addition of thiourea served to eliminate severe interferences. The recovery of spiked gold in biological materials was in the range of 101-106%.