Terminology for the modification of graphite tubes with high-melting carbides used in electrothermal atomic absorption spectrometry

New terminology is suggested to replace the IUPAC-recommended “tantalum (or other element) carbide-coated tube.”     

Graphite atomizers modified with high-melting carbides for electrothermal atomic absorption spectrometry. II. Practical aspects

Data relating to practical applications of carbide-modified graphite atomizers (CMGAs) published mainly since 1985 are critically discussed. All elements being determined by means of electrothermal atomic absorption spectrometry are divided into ten groups according to their characteristics in graphite atomizers. CMGAs are the most effective for the determination of the elements forming rather stable oxides, such as Ge, Sn, Ga, In, as well as B and Si. The application of CMGAs to the determination of carbide-forming elements (Cr, Mo, V, Ti, etc.) may cause a significant decrease in their sensitivity. Concerning the high-volatility analytes (semi-metals and boron), CMGAs are usually effective only in the presence of regular chemical modifiers, primarily the salts of Pd and Ni. CMGAs may be successfully used for the determination of some organoelement (with Sn, Pb, Se, As) compounds, as well as for the trapping of volatile hydrides. CMGAs seem to be especially promising for analysis of biological samples, organic extracts, solid samples and samples containing high concentrations of mineral acids. High-melting carbides are prospective permanent modifiers.     

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.     

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

     

Determination of Cd by electrothermal atomic absorption spectrometry after electrodeposition on a graphite probe modified with palladium

Traces of Cd were determined by electrothermal atomic absorption spectrometry after electrochemical preconcentration on a commercial graphite ridge probe modified with Pd. The Pd electrochemically deposited on the probe surface served not only as the modifier but it also protected the graphite surface. Cd was electrodeposited at a controlled potential − 1.2 V (vs. saturated calomel electrode) using the Pd-modified graphite probe as a working electrode. The sensitivity of Cd determination remained unchanged for 300 electrodeposition and atomization cycles. The detection limit (3σ_blank) was improved with increasing time of electrolysis and was 1.2 ng l^− 1 for a 10 min electrolysis time in the presence of 0.1 mol l^− 1 NaNO₃. The procedure was applied for the determination of Cd in (1 + 1) diluted seawater and in (1 + 1) diluted urine samples using the standard addition method.     

Surface and subsurface examination of graphite tubes after electrodeposition of noble metals for electrothermal atomic absorption spectrometry

The modification phenomena of noble metals (Pd, Ir, Rh) electrodeposited onto the inner surface of pyrolytic graphite (PG) coated furnaces were investigated mainly by electron microprobe analysis with energy dispersive X-ray emission detection. The conditions of electrodeposition were optimized in order to achieve the best analytical performance of atomic absorption measurements. Investigations concerning the distribution of noble metals on the tube surface and in-depth were performed at different stages of the tube history. It was found that the noble metals used for the modification do not form a compact layer on the surface but penetrate into the pyrolytic graphite structure already at the deposition step. When two metals were deposited together, both penetrated into the graphite structure. The degree of penetration of the pyrolytic graphite at high temperature differs for various metals. It was also demonstrated that electrodeposited noble metals remain in sub-surface domains of the graphite for hundreds of atomization cycles, which means that they can be used as permanent modifiers.     

Electrolytic hydride generation electrothermal atomic absorption spectrometry – in situ trapping of As on different pre-conditioned end-heated graphite tubes

An automated analytical system for the determination of As combining an electrolytic hydride generator and a graphite furnace atomic absorption spectrometer has been developed. To investigate the trapping efficiency of permanent modifiers, the end-heated graphite tubes have been impregnated with Ir and mixed Pd/Ir pre-reduced modifiers, respectively, or pre-coated with Ir by electron beam evaporation under high vacuum. Furthermore, the influence of the modifier mass on the shape of the absorption signal has been studied and the performance of the modifier has been discussed. Using the pre-coated graphite tube the calculated detection limit (3s criteria ) for As was 3 pg and 15 ng/L (200 μL sample volume, two preconcentration steps) for the absolute mass and the concentration, respectively. The long-term stability of the permanent modifiers and their physical and or chemical changes during the lifetime of the tube have been observed.     

Analytical evaluation of totally pyrolytic graphite cuvettes for electrothermal atomic absorption spectrometry

The performance characteristics of electrothermal atomiser cuvettes made of totally pyrolytic graphite (TPG) are compared to those of coated and uncoated electrographite. An analytical programme was devised to determine the useful operational lifetime of each cuvette and assess the effect of cuvette age on the sensitivity and precision of the determinations of lead, manganese and vanadium by atomic absorption spectrometry. The main advantages of TPG are enhanced cuvette durability and improved sensitivity and precision, especially for involatile elements. The characteristics of uncoated electrographite were generally unsatisfactory compared to the pyro-coated and TPG cuvettes studied.     

Tube-probe atomisation in electrothermal atomic absorption spectrometry

A method is described in which liquid samples are deposited by a conventional autosampler on to a tubular graphite probe positioned inside a 7.5 mm i.d. Pye Unicam SP-9 atomiser cuvette. The tube-probe has certain advantages in comparison with the flat probe systems described previously. In particular, the precision of determinations in acid media is improved since the sample solution is better confined within the atomiser and hence the effects of droplet spreading are curtailed. Also, the tube-probe is shown to reduce diffusional loss effects at high atomiser temperatures, in comparison with flat probe operation. The characteristic mass values obtained for volatile and medium volatile elements are similar for tube and flat probes. Since a greater sample volume (up to 40 μl) can be deposited and dried in the tube-probe, improved detection limits are anticipated for these elements. For more involatile elements, the greater mass of the tube-probe results in poorer sensitivity by a factor of × 3 for V and × 4 for Cr.     

Electrodeposition of lead on to a graphite probe for electrothermal atomic absorption spectrometry

A simple system for controlled potential electrodeposition on to a graphite probe electrode is described. Totally pyrolytic graphite was found to be better for electrodeposition than microporous glassy carbon or electrographite coated with pyrolytic graphite. Lead can be deposited by anodic and cathodic processes as PbO₂ and Pb, respectively. Potentials of + 1.2 to + 2.0 V were best for anodic deposition and – 0.8 to– 2.0 V were best for cathodic deposition. With an electrodeposition time of 120 s, AAS sensitivity gains of × 9 and × 3.5 were achieved for anodic and cathodic deposition, respectively, in comparison with the results obtained by direct injection of 20 1 sample volumes on to the probe. The lead cathodic process was unaffected by NaCl concentrations up to 10^–2 M, but only 10^–3 M NaCl could be tolerated by anodic deposition.     

Metal speciation by coupled in situ graphite furnace electrodeposition and electrothermal atomic absorption spectrometry

The technique of coupled in situ electrodeposition–electrothermal atomic absorption spectrometry (ED–ETAAS) is applied to the analytes Bi, Pb, Ni and Cu. Bi, Pb, Ni and Cu are deposited quantitatively from their EDTA complexes at E_cell=1.75, 2.0, 3.0 and 2.5 V, respectively (E_cell=E_anode−E_cathode+iR). By varying the cell potential, selective reduction of free metal ions could be achieved in the presence of the EDTA complexes. For Bi³⁺ and Pb²⁺ this utilised the voltage windows E_cell=0.6–1.0 and 1.8–2.0 V, respectively. For Ni, deposition at E_cell=1.7–2.0 V achieved substantial, but not complete, differentiation between Ni²⁺ (ca. 90–100% deposition) and Ni(EDTA)²⁻ (ca. 12–20% deposition). An adequate voltage window was not obtained for Cu. The ability of ED–ETAAS to differentiate between electrochemically labile and inert species was demonstrated by application of both ED–ETAAS and anodic stripping voltammetry to the time-dependent speciation of Pb in freshly mixed Pb²⁺–NaCl media. Application to natural water samples is complicated by adsorption of natural organic matter to the graphite cathode.     

Plasma afterglow atomization in electrothermal atomic absorption spectrometry

Potentialities of an Ar/H₂ microwave induced plasma afterglow at 8.2 mbar as an atomization source in electrothermal atomic absorption spectrometry have been examined. More specifically the atomization efficiency, as shown from appearance temperatures, and the reaction mechanisms of the atomization of the oxides and chlorides of alkaline earth and transition metals have been investigated and compared with conventional electrothermal atomization. For all the investigated metal chlorides and alkaline earth oxides, a considerable decrease in appearance temperature (some 500 K), is observed in the plasma afterglow. Such enhanced atomization is believed to be linked to reactions with H atoms. No plasma enhancement, however, is measured for the atomization of the transition metal oxides. All metal oxides are effectively reduced to free metal in the solid state by the Ar/H₂ afterglow, and as a consequence the supply rate is governed by the metal sublimation for these compounds. For metal chlorides, however, strong evidence is found for the atomization process to proceed via gas phase reactions.     

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.     

Improved cesium sensitivity in electrothermal atomic absorption spectrometry

The sensitivity for cesium determination by electrothermal atomic absorption spectrometry is improved four-fold by the addition of a large excess of potassium nitrate. Zeeman background corrections is used to compensate for the large non-specific absorption signal resulting from the potassium. The characteristic concentration and detection limit are 0.44 and 2 μg l^?1, respectively, and the coefficient of variation is 2% at the 50 μg l^?1 level. The procedure is suitable for the rapid determination of cesium in leach solutions from nuclear waste fixation experiments.     

Comparison of palladium and zirconium treated graphite tubes for in-atomizer trapping of hydrogen selenide in hydride generation electrothermal atomization atomic absorption spectrometry

Zirconium treated graphite tubes were investigated and compared with non-treated and palladium coated ones for in situ trapping of selenium hydride generated in a flow injection system. Selenium was effectively trapped on zirconium treated tubes at trapping temperatures of 300–600°C, similar to those observed for palladium, whereas trapping temperatures higher than 600°C had to be used with non-treated tubes. Zirconium treated tubes used in this work showed good stability up to 300 trapping/atomization cycles, with precision better than 5%, characteristic masses of 42 (peak height) and 133 pg (peak area) of selenium were obtained. Sensitivity of zirconium and palladium treatments were similar, but zirconium offered the advantage of a single application per tube. Detection limits were 0.11 (peak height) and 0.23 ng (peak area) for a 1 ml sample volume.     

The tube-in-tube technique in electrothermal atomic absorption spectrometry

Summary A new tube-in-tube is presented. It consists of a small graphite tube which is inserted into a usual Perkin-Elmer graphite oven. Three bars, 1×1 mm over the whole length of the tube, prevent direct contact with the outer one and smoothe the temperature profile within the tube. It causes a thermal delay in the atomization of the atomic cloud. Thus, expulsion of the atomic cloud by the thermally expanding inert gas grows less important compared to its growth. A significant increase of sensitivity is the result.

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Die Rohr-in-Rohr-Technik bei der elektrothermalen Atomabsorptions-SpektrometrieII. Das Dreisteg-Rohr

Zusammenfassung Ein neuer Rohr-in-Rohr-Atomisator wird vorgestellt. Er besteht aus einem kleinen Graphitrohr, welches in ein übliches Perkin-Elmer-Graphitrohr hineingebracht wird. Drei Stege (1×1 mm) über die gesamte Rohrlänge verhindern den direkten Kontakt mit dem Außenrohr und glätten das Temperaturprofil im Innenrohr. Die Atomisierung erfolgt thermisch verzögert. Deshalb verliert das Austreiben der Atomwolke durch das sich thermisch ausdehnende Intergas im Verhältnis zu deren Anwachsen an Bedeutung. Ein deutlicher Anstieg der Meßempfindlichkeit ist das Ergebnis.

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Herrn Prof. Dr. W. Fresenius zum 70. Geburtstag gewidmet

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Part I: Atomspektrometrische Spurenanalytik, Bd. 2 (B. Welz, Ed.). Verlag Chemie Weinheim — in print     

Scanning electron microscopy studies on surfaces from electrothermal atomic absorption spectrometry: II. Total pyrolytic graphite platforms in pyrographite coated polycrystalline electrographite tubes

Morphological studies on graphite surfaces by scanning electron microscopy are presented for platforms made from total pyrolytic graphite, and for polycrystalline electrographite tubes with pyrographite coating in which the pyrolytic graphite platforms are inserted. Platforms made of pyrolytic graphite have a very long lifetime and can actually survive several tubes. But also the lifetime of polycrystalline electrographite tubes with pyrographite coating is substantially longer when the sample is deposited on a platform and not on the tube wall. The analytically useful lifetime of platform and tube comes close to 1000 determinations at an atomization temperature of 2650°C, and even in the presence of corrosive matrices 500 determinations can be performed without significant loss in sensitivity. Intercalation appears to be no problem with pyrolytic graphite platforms. Substantial deposition of graphite was found on the platform under different analytical conditions, leading to a secondary coating of the surface. The sensitivity of carbide-forming elements depends very much upon the quality of the pyrographite coating of the polycrystalline electrographite tubes in which the platforms are mounted.     

Nanotopographical changes on graphite tube surfaces in electrothermal atomic absorption spectrometry experiments as studied by atomic force microscopy

Atomic force microscope images of surfaces of graphite tubes used in chromium determinations in electrothermal atomic absorption spectrometry (ET-AAS) reveal severe topographical modifications on both the micrometer and the nanometer scale after only a few atomisation cycles. A previously undescribed nanostructure has been found on unused and also on used graphite tubes. During the first atomisation cycles the protrusion size distribution is found to become more uniform, which is in agreement with the observed increase in reproducibility of ET-AAS measurements after some 10 to 20 analysis cycles. The homogenisation of the graphite surface is interpreted as the initial phase of a secondary coating.     

A new approach to the problem of atomization in electrothermal atomic absorption spectrometry

We established that the partial pressure of oxygen in the graphite furnace is several orders of magnitude higher than is explained by the thermodynamic equilibrium of the O₂ + 2C = 2CO reaction. Taking this into account has led us to some new conclusions for thermal destruction, atomization and dissociation of the oxygen-containing compounds. It explains why some elements are reduced to the metal on graphite, while other elements are vaporized as the oxide. For the elements vaporized as the oxide, it is shown that there is good agreement between the calculated thermal dissociation temperature of the metal oxide and the observed appearance temperature. For these metals, the atomization of the oxides proceeds by their thermal dissociation without direct participation of carbon in the reduction process. The presence of oxygen in the purge gas accounts for anomalous curvature in, for example, the Sn calibration curve, large variations in sensitivity for some elements (Ga, Ge, Sb, Se, Sn) comparing gas-stop and full-flow modes of atomization and the enhancement of sensitivity for some elements in oxygen-activated graphite furnaces.