Comparison of uncoated, pyro-coated and totally pyrolytic graphite tubes for the HGA-500 electrothermal atomiser

The analytical performance of three uncoated electrographite tubes, three pyro-coated electrographite tubes, one tantalum carbide (TaC) coated electrographite tube, and three totally pyrolytic graphite (TPG) tubes has been evaluated and compared. A test programme was devised to determine the useful operational lifetime of each tube, and assess the influence of tube age on the sensitivity of lead, manganese and vanadium measurements by atomic absorption spectrometry. The TPG tubes were found to be more durable than the other types studied, but the lifetime advantage depended on the thickness of the pyrolytic graphite. The best TPG tube, of 720 μm wall-thickness, lasted 2.5 times longer than the pyro-coated tubes, and 5 times longer than the uncoated electrographite tubes.The TPG tubes gave slightly poorer AAS sensitivity for lead, equivalent sensitivity for manganese, and 4 times better sensitivity for vanadium than the pyro-coated electrographite tubes. Also, with TPG, signal magnitude was more consistent throughout the lifetime of a tube. For each of the test elements studied, poorest sensitivity was encountered with the TaC-coated electrographite tube.     

Determination of vanadium in water by electrothermal atomisation atomic absorption spectrometry after extraction with 8-hydroxyquinoline in isobutyl methyl ketone

A sensitive method for the determination of vanadium in water by electrothermal atomisation atomic absorption spectrometry (ETAAS) is described. The vanadium is chelated with 8-hydroxyquinoline in isobutyl methyl ketone and determined by ETAAS after pre-heating the pyrolytic graphite coated graphite tube of a graphite furnace atomiser before injection. The effects of the pH and amount of reagent required for the extraction were studied. The precision, accuracy and interferences of the method were also investigated. The proposed method allows concentrations of vanadium of 0.16 microgram l-1 to be detected.     

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.     

Determination of vanadium in water by atomic absorption spectrometry with electrothermal atomization and using hot injection and preconcentration on the graphite tube

A sensitive method for the determination of vanadium in water by atomic absorption spectrometry with electrothermal atomization and using hot injection and preconcentration on the graphite tube is described. The water sample (200 μl) is added to the heated graphite tube in four portions over 200 s. Magnesium nitrate is used as matrix modifier. The precision, accuracy and interferences of the method were investigated. The method allows vanadium down to 0.27 μg l^?1 to be detected.     

Determination of submicrogram amounts of vanadium in biological materials by extraction with N-cinnamoyl-N-2,3-xylylhydroxylamine and flameless atomic-absorption spectrometry with an atomizer coated with pyrolytic graphite

A highly sensitive and simple method for determination of vanadium in plants and biological samples by solvent extraction and flameless atomic-absorption spectrometry with a carbon tube coated with pyrolytic graphite is described. After digestion of the sample, vanadium is separated by extraction of its N-cinnamoyl-N-2,3-xylylhydroxylamine complex into carbon tetrachloride from 6M hydrochloric acid medium. The method can be used to determine vanadium in plants and biological samples with average recovery of 94% and coefficient of variation of 14%. The sensitivity (1% absorption) is estimated to be 4 x 10(-11) g.     

Determination of barium in sea water by cation- exchange separation and electrothermal atomic absorption spectrometry

A method is described for the routine determination of barium in sea water by graphite-furnace atomic absorption spectrometry. Barium is separated from the main sea- water cations by collection on a cation-exchange resin. The barrium is extracted into nitric acid from a portion of the resin for injection into the pyrolytically-coated graphite furnace. The method is satifactory for the low μg l^?1 levels of barium present in ocean water, with recoveries ?99% and a relative standard deviation of <5%.     

Slurry sampling for the determination of thallium in soils and sediments by graphite furnace atomic absorption spectrometry

The analytical conditions for thallium determination in soils and sediments by slurry sampling graphite furnace atomic absorption spectrometry were studied and optimized. Elimination of a strong background for soils rich in organic materials by application of tungsten carbides coated graphite tubes/platforms was studied in detail. Tungsten carbides increased the maximum permissible pyrolysis temperature from 300 to 900 degrees C. The mechanism of tungsten carbide formation on different graphite surfaces was proposed. Application of a strong basic anion-exchange resin for interference elimination in thallium determination in marine sediments was described. Calibration was performed directly using aqueous standards both for soil and sediment analysis. Analysis of CRM confirmed the reliability of the approach. The precision and accuracy of thallium determination by the described method for soils and sediments was acceptable. A characteristic mass of 13.8 pg was obtained and the limit of detection for the proposed method was around 0.06 microg g(-1) Tl.     

Pyrolytic carbon coating method for contoured graphite tubes and their use in furnace atomic absorption spectrometric determination of boron and uranium

Tubes are machined from rods of suitable graphite according to a modified design so that there is a recess of 20-μl capacity in the inner surface at the tube centre; this improves reproducibility and sensitivity. Pyrolytic carbon coating is accomplished independently of the atomic absorption instrument by using a simple apparatus (dimensions are given). Tube lifetimes are considerably longer than those of both coated and uncoated commercial tubes. The coated tubes described have lifetimes long anough at high atomizing temperatures to determine uranium and boron on a routine basis. The boron memory is eliminated and that of uranium successfully controlled. The addition of organic solvents to the analyte solution to enhance sensitivity for uranium is reported.     

石墨炉原子吸收法测定钒的研究

本文比较了在标准石墨管,热解涂层和全热解石墨管中钒的吸收信号形状。用全热解石墨管和EDTA铵盐作基体改进剂,直接测定水系沉积物中痕量钒。方法的特征量为61pg/0.0044A。     

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.

---

Glasartiger Kohlenstoff als Rohrmaterial für elektrothermische Atomabsorptionsspektrometrie

     

Kinetics of indium atomization from different atomizer surfaces in electrothermal atomic absorption spectrometry (ETAAS)

The kinetic parameters of indium atomization in electrothermal atomic absorption spectrometry (ETAAS) have been determined by a newly proposed method. Effect of the atomizer surface and the palladium modifier on the kinetics of indium atomization has been investigated. The mechanisms of indium atomization seem to be identical for the pyrolytically coated graphite and the uncoated graphite tubes, i.e. the rate-limiting step for the atomization changes from a first order kinetics at lower temperatures into a nearly 1/3 order kinetics at higher temperatures, which may suggest that the analyte moves from a dispersed state to agglomates with increasing temperature. However, for the zirconium coated graphite tube, the atomization of indium is controlled by a single mechanism with the kinetic order of near 2/3 and the activation energy of 186 ± 13 kJ/mol. Relatively weak indium—zirconium carbide interactions and the release of indium from the sphere of molten indium metal on the zirconium coated surface are suggested. In the presence of palladium, a simple mechanism, i.e. the release of indium from the solid solution of the In and the Pd on the pyrolytically coated graphite surface, is proposed to account for the observed first order kinetics and the activation energy of 421 ± 27 kJ/mol.     

Determination of boron in blood,urine and bone by electrothermal atomic absorption spectrometry using zirconium and citric acid as modifiers

A comparative study of various potential chemical modifiers (Au, Ba, Be, Ca, Cr, Ir, La, Lu, Mg, Ni, Pd, Pt, Rh, Ru, Sr, V, W, and Zr), and different ‘coating’ treatments (Zr, W, and W+Rh) of the pyrolytic graphite platform of a longitudinally heated graphite tube atomizer for thermal stabilization and determination of boron was undertaken. The use of Au, Ba, Be, Cr, Ir, Pt, Rh, Ru, Sr and V as modifiers, and of W+Rh coating produced erratic, and noisy signals, while the addition of La, Ni and Pd as modifiers, and the W coating had positive effects, but with too high background absorption signals, rendering their use unsuitable for boron determination even in aqueous solutions. The atomic absorption signal for boron was increased and stabilized when the platform was coated with Zr, and by the addition of Ca, Mg, Lu, W or Zr as modifiers. Only the addition of 10 μg of Zr as a modifier onto Zr-treated platforms allowed the use of a higher pyrolysis temperature without analyte losses. The memory effect was minimized by incorporating a cleaning step with 10 μl of 50 g l⁻¹ NH₄F HF after every three boron measurements. The addition of 10 μl of 15 g l⁻¹ citric acid together with Zr onto Zr-treated platforms significantly improved the characteristic mass to m₀=282 pg, which is adequate for biological samples such as urine and bone, although the sensitivity was still inadequate for the determination of boron in blood of subjects without supplementary diet. Under optimized conditions, the detection limit (3σ) was 60 μg l⁻¹. The amount of boron found in whole blood, urine and femur head samples from patients with osteoporosis was in agreement with values previously reported in the literature.     

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

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

Graphite-tube effects on perchloric acid interferences on aluminum and thallium in the stabilized-temperature platform furnace

The determination of aluminum in the presence of perchloric acid is shown to depend upon the quality of the pyrolytic coating of furnace tubes. With new pyrolytically coated tubes, no interference was found from 0.5 M HClO₄ on 0.5 ng Al and no decrease in signal or deterioration of the pyrolytic coating was found after more than 150 firings. Very little interference was found in the determination of thallium in perchloric acid. In both cases the literature reported severe interferences. The determination of thallium and aluminum in perchloric acid appears to be more sensitive to the quality of the pyrolytic graphite coating than any of the materials studied previously here.     

Determination of vanadium in urine by electrothermal atomic absorption spectrometry

After wet ashing of the urine sample with nitric acid, vanadium is chelated with cupferron, extracted into 4-methylpentan-2-one and determined by atomic absorption spectrometry with a pyrolytically-coated graphite furnace atomizer. The sensitivity allows the precise determination of 1–500 μg V l^-1 in urine. The coefficient of variation for triplicate urine measurements is <8% for 10 μg V l^-1.     

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.     

Determination of molybdenum in infant formula and human milk by electrothermal atomic absorption spectrometry with barium difluoride as matrix modifier

A method for the determination of molybdenum in infant formula and human milk by electrothermal atomic absorption spectrometry was developed and optimized. Samples were injected directly in the graphite tube with barium difluoride as the matrix modifier. The detection limit was 0.89 μg Mo l^?1. The molybdenum levels found in infant formula and human milk were 0.09–2.23 μg Mo g^?1 and 2.32–8.38 μg Mo l^?1, respectively.     

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

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

---

Einfluß der Rohroberfläche auf das Atomisierungsverhalten in einem Graphitrohrofen

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

     

Periodic trends in sensitivity and its dependence on the properties of pyrolytic and non-pyrolytic graphite in graphite-furnace atomic-absorption spectrometry

The sensitivities for metal determination by GFAAS in the peak-height and integration modes were examined with pyrolytic graphite (PG) and non-pyrolytic graphite (NPG) tubes for 34 elements. It was found that there are periodic trends of the mole sensitivity and the elements can be classified according to whether their sensitivity of determination is enhanced by use of (a) the PG tube (alkali, alkaline-earth and transition metals); (b) the NPG tube (semi-metals); about equally by both tubes (Mg, Zn, Cd, and Pb). The mole sensitivity pM for atomic-absorption spectrometry (AAS) was defined as pM = -log(m(h)/A(w)) where m(h) is the weight of an element corresponding to 1% absorption and A(w) is the atomic weight. It was found that the pM values for graphite furnace AAS have a periodic trend similar to that for flame AAS and atomic-fluorescence spectrometry.