Determination of gallium by graphite furnace atomic absorption spectrometry with combined use of a tungsten-coated L'vov platform tube and a chemical modification technique

Tungsten-coated non-pyrolytic graphite (NPG), pyrolytic graphite (PG) and pyrolytic L'vov platform graphite (PPG) tubes were prepared, and their analytical performances were compared. The coating process simply involved injecting 100 μl of a sodium tungstate solution (0.01 mol l⁻¹) into each graphite tube, followed by heating according to a temperature programme similar to an atomisation cycle for the determination of gallium. This procedure for coating was repeated at least 12, 25, and 7 times towards NPG, PG, and PPG tubes, respectively. Among these tubes, the tungsten-coated PPG tube showed excellent performance for the determination of gallium. By combined use of a chemical modifier such as aluminium(III) or nickel(II) a detection limit (3σ) of 6 pg and sensitivity (1% absorption) of 3–4 pg were achieved. The practical potential of the proposed technique was demonstrated for the determination of gallium in several samples of alloys and fresh water.     

Evaluation of end-capped tubes for transverse heated graphite atomizer electrothermal atomic absorption spectrometry

Transverse heated graphite tubes with (EC-THGA) and without (THGA) ends caps have been tested with respect to characteristic mass, detection limits and reproducibilities at two levels of concentration for four different types of analytes. Compared for Cd, Pb and Cr with a standard THGA tube, the EC-THGA tube exhibits a gain of sensitivity by a factor of about 1.4 in terms of characteristic masses. Also detection limits are significantly improved for the end-capped tube design tested. The presence of end caps increases the mass of the tube and decreases consequently the heating rate achieved. As shown on the molybdenum example, the atomization efficiency of refractory metals is not so good as with standard THGA tubes. Interference effects studied on the Cd, Pb and Cr determinations in environmental samples (sediments, plants and animal tissues) are similarly negligible for the two tubes tested.     

Thermal-mechanical behavior of styrene-based shape memory polymer tubes

Styrene-based shape memory polymer (SMP) tubes were fabricated and their basic mechanical properties in different deformation states were investigated. The tensile, compression, bending and twisting shape memory properties of the tubes were analyzed and discussed, and the results indicated that SMP tubes exhibit good shape fixity ratio and shape recovery ratio. In addition, the shape recovery behavior was investigated at different heating rates. These experimental results will provide guidance for future applications of SMP tube structures.     

Production and crosslinking of multi-layer tubes (PE & metal) by E-beam

Irradiation crosslinking of PE-tubes has been used for heating floors for about 25 years. Such tubes are also used today for drinking water supply. A further development has been the coating of such tubes with Ethylene-Vinyl-Alcohol-Copolymers (EVAL), in order to prevent oxygen diffusion into the water through the PE tube. For about 15 years composite tubes made of PE and aluminum have been available. These tubes are crosslinked with electron beams. The energy of the accelerated electrons must be adjusted for the particular tube configuration, so that the inner PE-layer will be crosslinked. This paper will concern itself with the manufacture and the crosslinking of composite tubes.     

Recent developments in atomizers for electrothermal atomic absorption spectrometry

This review first describes general requirements to be met for suitable base materials used to produce electrothermal atomizers (ETAs). In this connection the physical and chemical properties of adequate types of graphite and metals are discussed. Further, various atomizer designs, their temperature dynamics during atomization and general performance characteristics are critically reviewed. For end-heated Massmann-type atomizers, discussions are focused on recent developments of, e.g., contoured tubes to achieve improved temperature homogeneity over the tube length, second surface atomizers to realize temporally isothermal atomization and tubes with graphite filters to reduce interference effects. The state-of-the-art of platform equipped, side-heated atomizers with integrated contacting bridges are characterized mainly with respect to heating dynamics, as well as susceptibility to interference- and memory effects. In contrast to end-heated ETAs, the tube ends of side-heated ETAs are freely located in the furnace compartment and, as a consequence of this configuration, convective gas flows can easily appear. The magnitude and effect of these flows on analytical performance are discussed and measures are suggested, permitting operation under diffusion controlled conditions. A critical comparison of classical constant temperature atomizers with state-of-the-art platform equipped ETAs is made and from this it is concluded that future ETA developments are likely to involve only minor modifications aiming at, e.g., the reduction of cycling times or the improvement of tube surface properties.     

Recent developments in atomizers for electrothermal atomic absorption spectrometry

This review first describes general requirements to be met for suitable base materials used to produce electrothermal atomizers (ETAs). In this connection the physical and chemical properties of adequate types of graphite and metals are discussed. Further, various atomizer designs, their temperature dynamics during atomization and general performance characteristics are critically reviewed. For end-heated Massmann-type atomizers, discussions are focused on recent developments of, e.g., contoured tubes to achieve improved temperature homogeneity over the tube length, second surface atomizers to realize temporally isothermal atomization and tubes with graphite filters to reduce interference effects. The state-of-the-art of platform equipped, side-heated atomizers with integrated contacting bridges are characterized mainly with respect to heating dynamics, as well as susceptibility to interference- and memory effects. In contrast to end-heated ETAs, the tube ends of side-heated ETAs are freely located in the furnace compartment and, as a consequence of this configuration, convective gas flows can easily appear. The magnitude and effect of these flows on analytical performance are discussed and measures are suggested, permitting operation under diffusion controlled conditions. A critical comparison of classical constant temperature atomizers with state-of-the-art platform equipped ETAs is made and from this it is concluded that future ETA developments are likely to involve only minor modifications aiming at, e.g., the reduction of cycling times or the improvement of tube surface properties.     

A method for reduction of matrix interferences in a commercial electrothermal atomizer for atomic absorption spectroscopy

A CRA-63 atomizer has been modified by replacement of the conventional center supports with 2-pronged supports that hold the atomizer tube at both ends. This arrangement allows current to flow only across the atomizer ends. The center is heated by conduction. Large initial temperature differences (750–900 K) between the sample-containing center and the ends eliminates or decreases the interferences on Pb normally observed with this atomizer. Six chloride and one sulfate matrices were studied.Longer atomizers and higher power produced the largest temperature differences and, therefore, the best lead recoveries. At a given length, recovery eventually plateaued while the signal for lead in the absence of matrices decreased when heating power was further increased. It was concluded that further improvements would require longer tubes and a combination of end and center heating.The importance of considering interferent/analyte ratios in interpreting recovery data was examined for the interference of MgCl₂ on lead.     

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.     

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

     

Adsorption of multilamellar tubes with a temperature tunable diameter at the air/water interface

The ethanolamine salt of 12-hydroxy stearic acid is known to form tubes having a temperature tunable diameter. Here, we study the behavior of those tubes at the air/water interface by using Neutron Reflectivity. We observed that tubes indeed adsorbed at this interface below a fatty acid monolayer and exhibit the same temperature behavior as in bulk. There is however a peculiar behavior at around 50 °C for which the increase of the diameter of the tubes at the interface yields an unfolding of those tubes into a multilamellar layer. Upon further heating, the tubes re-fold and their diameter re-decreases after which they melt into micelles as observed in the bulk. All structural transitions at the interface are nevertheless reversible. This provides to the system a high interest for its interfacial properties because the structure at the air/water interface can be tuned easily by the temperature.     

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.     

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.     

Hot-injection procedures for the rapid analysis of biological samples by electrothermal atomic-absorption spectrometry

The combination of palladium/hydrogen matrix-modification and injection of samples into a graphite tube at 120 degrees has allowed the accurate determination of copper, iron, lead and nickel in biological reference materials (urine, milk powder and bovine liver). Palladium modification allowed the use of a standard ashing temperature of 1000 degrees for all four elements. Direct aqueous calibration was applied without the need for standard additions. The total heating cycle, from the start of sample injection, took 45 sec.     

Feasibility of filter atomization in high-resolution continuum source atomic absorption spectrometry

A prototype spectrometer for high-resolution continuum source atomic absorption spectrometry (HR-CS AAS), built at ISAS Berlin, Germany, was combined with a graphite filter atomizer (GFA), earlier developed at TUT, Pretoria, South Africa. The furnace and auto-sampler units from a commercial AA spectrometer, model AAS vario 6 (Analytik Jena AG, Jena, Germany), were employed in the instrument. Instead of conventional platform tube, the GFA was used to provide low measurement susceptibility to interferences and short determination cycle. The GFA was modified according to the design of the furnace unit and optimal physical parameters of its components (filter and collector) found. Afterwards, optimal GFA was replicated and tested to outline analytical performances of the HR-CS GFA AA spectrometer in view of prospects of multi-element analysis. In particular, reproducibility of performances, repeatability of analytical signals, lifetime, temperature limit and duration of the measurement cycle were examined, and elements available for determination justified. The results show that the peak area of the atomic absorption signal is reproduced in various GFA copies within ± 4% deviation range. The GFA can stand temperatures of 2800 °C with 6 s hold time for 55 temperature cycles, and 2700 °C (8 s) for about 200 cycles. Only the external tube is prone to destruction while the filter and collector do not show any sign of erosion caused by temperature or aggressive matrix. Analytical signals are affected insignificantly by tube aging. Repeatability of the peak area remains within 1.1–1.7% RSD over more than hundred determination cycles. Peak areas are proportional to the sample volume of injected organic and inorganic liquids up to at least 50 μL. The drying stage is combined with hot sampling and cut down to 15–20 s. The list of metals available for determination with full vapor release includes Al, Co, Cr, Ni, Pt as well as more volatile metals. Characteristic masses at optimal atomization temperature are close to or slightly better than those found in HR-CS AAS with PIN-platform tubes. In addition, fast sequential determination of multiple elements and truly simultaneous determination of Cd, Fe and Ni using nearby analytical lines were performed with the GFA controlled by a single temperature program and an atomization temperature of 2700 °C.     

At-column heating and a resistively heated, liquid-cooled thermal modulator for a low-resource bench-top GC x GC

A transportable GC x GC instrument is under development for on-site applications that would benefit from the enhanced resolution and powers of detection, which can be achieved by this method. In the present study, a low-resource GC x GC instrument using an electrically heated and liquid-cooled single-stage thermal modulator that requires no cryogenic materials is evaluated. The instrument also uses at-column heating, thus eliminating the need for a convection oven to house the two columns. The stainless-steel modulator tube is coated with PDMS, which can be heated to 350 degrees C for sample injection into the second-dimension column. The modulator is cooled to -30 degrees C by a 100 mL/min flow of PEG by means of a commercial liquid chiller and a small recirculating pump. Resistive heating of the modulator tube is provided by a programmable power supply, which uses a voltage program that results in increasing modulator temperature during an analysis. This, together with more rapid cooling by the use of a liquid cooling medium, results in reduced solute breakthrough following each heating cycle as the modulator cools to a temperature where quantitative trapping resumes. As a result, modulated peak widths at half-height of less than 40 ms are observed. Design and performance details are presented along with chromatograms of gasoline and an essential oil sample.     

Fire behaviour of hybrid filament-wound single and adhesively bonded composites tubes under static pressure

The present work aims to experimentally investigate the fire behaviour of water-filled E glass reinforced thermoset resin hybrid filament-wound composites tubes under static pressure. Heretofore, fire endurance tests have been conducted on single and adhesively bonded tubes manufactured by CTRA Company. Furthermore, internal pressure tests until failure have been performed on the burnt single and burnt joined tubes in order to quantify their abilities to contain the fluid after being exposed to heat flux. A comparison between the pressure behaviour of exposed to fire (burnt) and non-exposed tubes (single and joined) was also inspected. The identification of the fire-induced damage mechanisms of the tubes was performed through optical microscopy, Scanning Electron Microscopy (SEM) and X-ray tomographic observations. Finally, the thermal analysis was carried-out on burnt specimens in order to better understand the multiphysical phenomenon taking place during the fire endurance tests. The experimental results have revealed that the combustion process of both single and joined tubes was described in four steps namely tube heating, resin degradation, ignition and flame decay. Moreover, it was found that no leakage was witnessed on the tubes (single and joined) outer surfaces during the fire endurance tests. The comparison between the pressure behaviour of the burnt single tube and the burnt joined one has proved that the single tube is much resistant under internal pressure loading than the burnt joined tube. Finally, the fire-induced damage included matrix cracking and delamination between the tube plies which was noticed from microscopic observations.     

Factors influencing the atomization of vanadium in graphite furnace AAS

We have made the determination of V conform to the requirements of the modern (stabilized temperature) furnace technology where the integrated absorbance (A·s) signals are used to quantitate analyte volatilized into a chamber whose temperature is relatively constant during the period when the analyte peak is measured. Graphite tubes with good pyrolytic coating and fast (maximum power) heating are required. We explored the advantages of specially designed tubes, of a cool-down procedure between the char and atomization steps and of very thin platforms. We found that Mg(NO₃)₂ was advantageous as a matrix modifier. With these conditions we found no problems with several matrices reported by earlier workers to be troublesome, for example HNO₃, phosphate, Fe, Mg and Ca. However metals that form very refractory carbides, such as La, Mo, W and Zr may remain troublesome for V, probably because mixed carbides result which include VC. A group of geological samples was analysed for V. Our recommendation is the use of wall atomization from tubes with good pyrolytic coatings, Mg(NO₃)₂ as a matrix modifier and the cool-down procedure to establish a nearly constant temperature along the tube.     

Influence of diameter on the degradation profile of multiwall carbon nanotubes

Nanofluid and coiled tubes have been employed as two passive methods for enhancing the heat transfer. In the present study, the turbulent flow of CuO–water nanofluid in helical and conical coiled tubes was numerically investigated with constant wall temperature through mixture model. The thermophysical properties of base fluid (water) were considered as temperature-dependent functions, while Brownian effects were adopted in thermal conductivity and dynamic viscosity of nanofluid. Simulation results were validated using experimental data for heat transfer coefficient and pressure drop in helical coiled tube for different Reynolds numbers. Four different geometries were simulated and compared. The first one was a conical coiled tube; the others were helical coiled tubes whose coil diameters were minimum, maximum, and median of the conical coiled tube pitch coil diameter. The velocity profiles indicated stronger secondary flow in conical coiled tube at a specified Dean number. The obtained results also showed higher heat transfer enhancement in the conical coiled tube in comparison with helical coiled tube with the same average pitch coil diameter. Moreover, the nanoparticle-induced heat transfer enhancement was more effective in conical coiled tube.

     

Diffusion vapour transfer modelling for an end-capped atomizer. Part 1. Atomizer with closed injection port

For end-cap equipped transverse-heated graphite atomizers (THGA) with integrated contacts used for analytical atomic spectrometry, a model equation describing the diffusional losses of analyte atomic vapour through the tube ends was constructed. The model assumes that the atomic density distribution is stepwise linear along the tube axis and the absence of a sample injection hole. With a quartz tube system, providing controlled experimental conditions at room temperature, the time constant of the diffusion removal function (T_R) of mercury vapour was determined for various open and end-capped tube geometries. These results were also described by an empirical multiple regression equation with a residual standard deviation of 5%. The theoretically predicted T_R values, corrected with an empirical factor of 1.33, agreed well (correlation coefficient = 0.996) with the experimentally obtained T_R values for the endcapped quartz tubes. For the Perkin-Elmer THGA tubes, the diffusional transfer model was evaluated using the integrated atomic absorbance ratio between various end-capped and open tubes. This is meaningful because the signal ratio for graphite atomizers is closely equal to the corresponding T_R ratio. For recommended atomization temperatures the average deviation between these experimental signal ratios and the theoretically predicted ratios for the elements Ag, In, Cd, Co, Hg and Cu was 1–5% for various end-capped tube geometries. The results for the individual elements deviated more from the theoretically predicted ratios mainly because of small differences in the mean gas-phase temperature between open and end-capped tubes. For elements which tend to form molecules in the gas phase at low temperatures and for which the atomization efficiency is increased with the atomization temperature, the experimental ratios tended to be higher than the theoretically predicted values (In, Al, Se, Sn, As), whereas experimental ratios were slightly lower for other elements (Cd, Co, Cu).