Pyrolytic Graphite Platform Research Articles

Modeling the interaction of molybdenum species adsorbed on a pyrolytic graphite platform and correlations with XPS spectra at different ETAAS stages

Adsorptions of molybdenum species and interactions on a model pyrolytic graphite platform (PGP) sites were calculated employing quantum methods (density functional theory (DFT) and parametric method number 6 (PM6)). The aim of this work is to propose, at the molecular level, the structure of different chemisorbed species on the PGP surface used in the electrothermal atomic absorption spectroscopy (ETAAS) process for a Mo analyte. The carbon surface was modeled by a seven-ring polycyclic aromatic system (coronene). Molybdenum species (Mo, Mo2, MoO, MoO2, MoO3, and MoO4) adsorbed on a coronene surface were studied on different adsorption sites. Calculations of these species showed that chemisorption strengths are stronger on dehydrogenated and decarbonized edge sites than on sites located in the flat central region of the model graphite. It was confirmed that Mo-oxygenated species interactions are very strong at the edge sites and therefore the reduction of these species must occur before the atomization (evaporation of Mo species) takes place, in agreement with experimental results. Comparisons between experimental XPS spectra for ETAAS process (drying, ashing, and atomization) and calculated adsorption energies of Mo species allow an interpretation of which species are present on the PGP at each stage. The presence of very strong bonded molybdenum carbide species on decarbonized sites may explain the memory effects. Experimentally observed migration of Mo oxide species to the edges of the PGP is also corroborated by their small chemisorption energies on central sites of the PGP model.

An X-ray photoelectron spectroscopy study of the atomization of Mo from pyrolytic graphite platforms in Electrothermal Atomic Absorption Spectroscopy

X-ray photoelectron spectroscopy (XPS) was employed as the analytical tool for the identification of the solid state species formed during the atomization of molybdenum on a pyrolytic graphite surface used for electrothermal atomization in atomic absorption spectroscopy (ETAAS). As XPS is capable of probing the amounts of materials employed in actual ETAAS measurements, it was anticipated that it would allow obtaining more realistic results than those reported in previous works by means of techniques such as X-ray diffraction, which requires thousand-fold amounts of the analyte. The results obtained by XPS showed a distribution pattern of the oxidation states of Mo on the graphite surface as a function of the temperatures reached at each stage of the ETAAS procedure, which was correlated with the possible species formed during the drying, ashing and atomization steps. In light of the present results, a previously proposed atomization mechanism for Mo was revisited, which allowed to disprove the alleged presence of phases such as Mo4O11 at the ashing stage and of metallic Mo as a stable, intermediate phase during the Mo2C low temperature carbide formation at 1200–1900°C. It is concluded that the XPS technique allows gaining an insight into the atomization mechanism of the analyte.

Overcoming interference from the alumina matrix on the determination of arsenic at 189 nanometers using electrothermal atomic absorption spectrometry

A method is described enabling to eliminate the spectral interference from alumina matrix onto As determination at the wavelength 189 nm by electrothermal atomic absorption spectrometry with deuterium background correction. Matrix modification was performed by the addition of ammonium fluoride to protect the formation of aluminium oxide implicated in causing spectral interference and to increase volatility of alumina matrix via the formation of AlF 3. Pre-treating of the pyrolytic graphite platform with a solution of rhodium and citric acid has enabled to stabilize the analyte up to temperature of 1300 °C at which most of AlF 3 could be removed from the graphite furnace. The application of 2 μg of Rh + 20 μg of citric acid + 200 μg of NH 4F has enabled an accurate and interference-free determination of As up to 40 μg of Al in the form of AlCl 3 as verified by analytical recoveries study and resulted in characteristic mass and LOD value in the original sample 15 pg and 50 ng g −1, respectively (10-μL aliquots of sample).

Single drop micro-extraction with O, O-diethyl dithiophosphate for the determination of lead by electrothermal atomic absorption spectrometry

Lead was extracted as the O, O-diethyldithiophosphate (DDTP) complex from aqueous solution into a drop of CHCl 3 immersed in the solution. Unlike previously reported procedures using single drop micro-extraction (SDME) for the extraction of inorganic analytes, the complexation reaction was conducted in the aqueous phase, as the ammonium salt of DDTP is soluble in water. The concentration of DDTP was optimized as 0.01% (m/v). Experimental parameters such as extraction time (7 min) and organic drop volume (3 μL) were optimized and selected as a compromise between sensitivity and stability of the organic drop in the aqueous solution. The sensitivity with electrothermal atomic absorption spectrometry (ET AAS) was low, probably due to infiltration of the organic drop into the totally pyrolytic graphite platform. To overcome this problem, tungsten (400 μg) was thermally deposited onto the platform surface. A short pyrolysis stage at 700 °C was included to reduce background absorption. Under these conditions, five certified reference materials with different characteristics were analyzed using calibration against aqueous standards submitted to the SDME procedure, resulting in good agreement between certified and found concentration values at a 95% confidence level. Two real water samples have also been analyzed, with recoveries ranging from 85 to 92% after enrichment with Pb. An enhancement factor of 52 allowed a detection limit of 0.2 μg L −1 or 0.04 μg g −1, demonstrating the high detection capability of the proposed procedure, with a relative standard deviation typically below 4%.

Mercury speciation in environmental samples by cold vapour atomic absorption spectrometry with in situ preconcentration on a gold trap

A method has been developed for the determination of total and organic mercury in biological materials and sediments. A microwave assisted mineralization of the organic mercury, after its extraction from the matrix, is described. This procedure warrants complete transformation of Hg(II) and, consequently, the quantitative reduction to Hg(0). The conditions for mercury reduction were optimized by a central composite design. The preconcentration of the analyte has been achieved by amalgamation on a trap system, consisting in a pyrolytic graphite platform wound by a gold wire. Mercury was determined by cold vapour atomic absorption spectrometry. The method was validated by the analysis of two certified reference materials and applied to the determination of total and organic mercury species in mussel tissues and sediments. The method is simple and practical, and offers the advantage of not requiring special equipment to measure inorganic and organic mercury simultaneously.

Spectrometric study of condensed phase species of thorium- and palladium-based modifiers in a complex matrix for electrothermal atomic absorption spectrometry

The chemical and morphological transformations of condensed phase species of a thorium-based modifier were studied over the temperature range 200–2500 °C, without and with the presence of aluminium and silicon as matrix components, and in some instances, arsenic as an analyte element. A similar study was also conducted with palladium as the modifier, for comparison. Results were derived using scanning electron microscopy (SEM), energy dispersive (ED) X-ray spectrometry, Raman microanalysis and attenuated total reflectance (ATR) Fourier transform-infrared (FT-IR) spectrometry. Comparable results were found using pyrolytic and non-pyrolytic graphite platforms, with processes occurring at slightly higher temperatures on the pyrolytic graphite platform. With thorium as the modifier, metal oxides were the predominant species on the platform surface at relatively low temperatures (<1500 °C), whereas metal phases became prevalent at high temperatures, when thorium and aluminium tended to behave independently from one other. Some spatial variations in the composition of the salt residues on different regions of the platform were observed (from the region closest to the slot in the tube, to the region furthest from the slot). Nonetheless, thorium metal remained on the graphite platform to higher temperatures than did aluminium metal. In the presence of arsenic, the existence of mixtures of thorium and arsenic oxides, just before the appearance temperature of gas phase arsenic atoms, was confirmed by SEM studies, ED X-ray spectra and Raman microanalysis. This suggests that any modifying effect of thorium on arsenic occurs while the modifier is in the oxide phase rather than in the metal phase. The presence of silicon added as silica, did not influence significantly the thermochemical behaviour of mixtures of thorium and aluminium. However, coexistence of silicon and arsenic oxides at the appearance temperature of the atomic absorption signal of arsenic was obtained, confirming that silicon can act as an internal modifier for arsenic. In the presence of palladium, aluminium exhibited greater interaction with the modifier; consequently, aluminium metal was retained on the platform surface to higher temperatures than thorium, which could explain how interference effects of aluminium on e.g. arsenic are avoided or reduced. Similarly, there was evidence for interaction of palladium and arsenic in the reduced state. However, when aluminium and silicon were present, the transformation of the palladium oxide to the metallic state was affected, which could diminish the modifying benefits of palladium for arsenic in the presence of aluminium.

Determination of Lead in Coal Using Direct Solid Sampling and High-Resolution Continuum Source Graphite Furnace Atomic Absorption Spectrometry

A method has been developed for the determination of Pb in coal using direct solid sampling (SS) and high-resolution continuum source graphite furnace atomic absorption spectrometry (HR-CS GF AAS). The certified coal reference materials used in this study were ground in an agate mortar to particle size ≤50 µm. Mass aliquots between 0.1 and 1 mg were weighed onto pyrolytic graphite platforms, and inserted directly into the graphite tube. For pyrolysis temperatures lower than 600 °C, a continuous background due to radiation scattering at solid particles preceded the atomic signal, as the coal matrix could not be sufficiently eliminated under these conditions. An optimized pyrolysis temperature of 700 °C was adopted for Pb without the use of a modifier. The optimum atomization temperature was found to be 1700 °C. Under these conditions, interference-free determination could be performed with calibration against aqueous standards in 0.5% v/v HNO3. A total of six certified reference coal samples were analyzed, and the results obtained were all in good agreement with certified and informed values, respectively, according to a Student t-test at a 95% confidence level. A detection limit of 0.008 µg g−1 for Pb was obtained at the 217.000 nm resonance line, a value comparable to those obtained by other sensitive, but much more sophisticated techniques, showing that the absence of dilution in the SS-HR-CS GF AAS method allows at least equivalent detection performance.

X-ray photoelectron spectroscopy study of pyrolytically coated graphite platforms submitted to simulated electrothermal atomic absorption spectrometry conditions

The present work is part of an ongoing project aiming to a better understanding of the mechanisms of atomization on graphite furnace platforms used for electrothermal atomic absorption spectrometry (ETAAS). It reports the study of unused pyrolytic graphite coated platforms of commercial origin, as well as platforms thermally or thermo-chemically treated under simulated ETAAS analysis conditions. X-ray photoelectron spectroscopy (XPS) was employed to study the elements present at the surfaces of the platforms. New, unused platforms showed the presence of molybdenum, of unknown origin, in concentrations up to 1 at.%. Species in two different oxidations states (Mo 6+ and Mo 2+) were detected by analyzing the Mo 3d spectral region with high resolution XPS. The analysis of the C 1s region demonstrated the presence of several signals, one of these at 283.3 eV related to the presence of Mo carbide. The O 1s region showed also various peaks, including a signal that can be attributed to the presence of MoO 3. Some carbon and oxygen signals were consistent with the presence of C O and C–O– (probably C–OH) groups on the platforms surfaces. Upon thermal treatment up to 2900 °C, the intensity of the Mo signal decreased, but peaks due to Mo oxides (Mo 6+ and Mo 5+) and carbide (Mo 2+) were still apparent. Thermo-chemical treatment with 3 vol.% HCl solutions and heating up to 2900 °C resulted in further diminution of the Mo signal, with complete disappearance of Mo carbide species. Depth profiling of unused platforms by Ar + ion etching at increasing time periods demonstrated that, upon removal of several layers of carbonaceous material, the Mo signal disappears suggesting that this contamination is present only at the surface of the pyrolytic graphite platform.

Determination of cadmium in coal using solid sampling graphite furnace high-resolution continuum source atomic absorption spectrometry

This work describes the development of a method to determine cadmium in coal, in which iridium is used as a permanent chemical modifier and calibration is performed against aqueous standards by high-resolution continuum source atomic absorption spectrometry (HR-CS AAS). This new instrumental concept makes the whole spectral environment in the vicinity of the analytical line accessible, providing a lot more data than just the change in absorbance over time available from conventional instruments. The application of Ir (400 microg) as a permanent chemical modifier, thermally deposited on the pyrolytic graphite platform surface, allowed pyrolysis temperatures of 700 degrees C to be used, which was sufficiently high to significantly reduce the continuous background that occurred before the analyte signal at pyrolysis temperatures <700 degrees C. Structured background absorption also occurred after the analyte signal when atomization temperatures of >1600 degrees C were used, which arose from the electron-excitation spectrum (with rotational fine structure) of a diatomic molecule. Under optimized conditions (pyrolysis at 700 degrees C and atomization at 1500 degrees C), interference-free determination of cadmium in seven certified coal reference materials and two real samples was achieved by direct solid sampling and calibrating against aqueous standards, resulting in good agreement with the certified values (where available) at the 95% confidence level. A characteristic mass of 0.4 pg and a detection limit of 2 ng g(-1), calculated for a sample mass of 1.0 mg coal, was obtained. A precision (expressed as the relative standard deviation, RSD) of <10% was typically obtained when coal samples in the mass range 0.6-1.2 mg were analyzed.

Electrothermal atomic absorption spectrometric determination of arsenic in essential lavender and rose oils

Analytical procedures for electrothermal atomic absorption spectrometric (ETAAS) determination of arsenic in essential oils from lavender ( Lavendula angustifolia) and rose ( Rosa damascena) are described. For direct ETAAS analysis, oil samples are diluted with ethanol or i-propanol for lavender and rose oil, respectively. Leveling off responses of four different arsenic species (arsenite, arsenate, monomethylarsonate and dimethylarsinate) is achieved by using a composite chemical modifier: l-cysteine (0.05 g l −1) in combination with palladium (2.5 μg) and citric acid (100 μg). Transverse-heated graphite atomizer (THGA) with longitudinal Zeeman-effect background correction and ‘end-capped’ graphite tubes with integrated pyrolytic graphite platforms, pre-treated with Zr–Ir for permanent modification are employed as most appropriate atomizer. Calibration with solvent-matched standard solutions of As(III) is used for four- and five-fold diluted samples of lavender and rose oil, respectively. Lower dilution factors required standard addition calibration by using aqueous (for lavender oil) or i-propanol (for rose oil) solutions of As(III). The limits of detection (LOD) for the whole analytical procedure are 4.4 and 4.7 ng g −1 As in levender and rose oil, respectively. The relative standard deviation (R.S.D.) for As at 6–30 ng g −1 levels is between 8 and 17% for both oils. As an alternative, procedure based on low temperature plasma ashing in oxygen with ETAAS, providing LODs of 2.5 and 2.7 ng g −1 As in levender and rose oil, respectively, and R.S.D. within 8–12% for both oils has been elaborated. Results obtained by both procedures are in good agreement.

Pyrolytic and Non-Pyrolytic Boron Nitride Platforms in Electrothermal AAS for the Determination of Cadmium in Sea and Estuarine Water without Chemical Modification

The analytical performance of pyrolytic and non-pyrolytic boron nitride (PBN and NBN) platforms, attached to a commercially available graphite tube furnace, in electrothermal atomic absorption spectrometry (ETAAS) for Cd was studied. Although the tolerable pyrolysis temperature was 300 degrees C with the conventional pyrolytic graphite platform, it increased to 600 and 950 degrees C with the PBN and NBN platforms, respectively. The lifetime of the ceramic platform was 500 firings. The NBN platform provided an enhanced sensitivity with a better reproducibility than others. Using the NBN platform allowed the LOD, based on the variability of the blank (3sigma), to be 0.1 microg l(-1) within a seawater matrix (20,000 mg NaCl l(-1)) and a constant sensitivity in the range 0-30,000 mg NaCl l(-1). Good recovery in the range of 90-105% was observed for Cd (2.0 microg l(-1)) spiked into sea, estuarine and river water samples using the recommended procedure. This work proposes that using the NBN platform allows the direct monitoring and control of contaminated water for Cd by ETAAS without any chemical modification.

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 NH 4F 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 m 0=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.

Low-temperature transformations of sodium sulfate and sodium selenite in the presence of pre-reduced palladium modifier in graphite furnaces for electrothermal atomic absorption spectrometry

The low-temperature interaction (up to 550°C) of a pre-reduced palladium modifier with sodium sulfate and sodium selenite on the pyrolytic graphite platform was studied using X-ray photoelectron spectroscopy (XPS) and electron microprobe analysis. The equipment applied allowed the introduction of samples heated in an argon flow into the analytical chamber of the XPS spectrometer without contact with the air. Electron microprobe analysis showed that palladium and sulfur preferably occupy different areas on the platform surface. On the contrary, selenium from sodium selenite tends to occupy areas of the graphite surface covered with palladium. The most probable reason for this is the chemisorption of selenium (IV) on the palladium surface at the drying stage. No changes in the XPS spectra of metallic Pd and S 6+ were observed when Na 2SO 4 and Pd were heated together on the graphite platform in the range 100–550°C. The reduction of sodium selenite on the graphite surface already starts during drying. Pre-reduced palladium intensifies this process. The rate of the reduction is proportional to the amount of palladium, and in the presence of palladium at an atomic ratio of Pd/Se=7.5, the transformation of Se 4+ into Se 0 completes at 250°C.

Morphological and spectroscopic investigation of the behavior of permanent iridium modifier deposited on pyrolytic graphite coated and zirconium treated platforms in electrothermal atomic absorption spectrometry

In order to better characterise a permanent modifier based on iridium deposited on zirconium or tungsten treated platforms of transversely heated graphite atomizer, and to gain additional information about its chemical behavior directed to an eventual further optimization, a series of experiments were carried out, both by surface techniques such as scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS or ESCA) and X-ray fluorescence (XRF) and by electrothermal atomic absorption spectrometry on the iridium release from unmodified and various other modified pyrolytic graphite platforms. Special attention was paid to the influence of the amount of iridium, zirconium carbide coating of the platform surface and the presence of citric acid on the iridium vaporization during pyrolysis and atomization. The processes of iridium losses during pyrolysis and atomization and peak maximum alignment depend on the amount of the iridium deposited on the pyrolytic graphite coated platforms in the presence of nitric acid. A fractional order of release which suggests an atom vaporization from the surface or edges of the iridium islands was estimated. In the presence of citric acid, mass independence and zero order of the atom release were found. The zirconium treatment of the platform results in change of the spatial distribution of iridium and hence its vaporization. Vaporization temperatures as high as 2100°C, and first order of the process of atom generation were obtained. While it was possible to study the iridium atomization from uncoated and zirconium coated surfaces, evidencing a different order for the release process, the same was not possible for the tungsten coated platforms due to an ‘overstabilization’ that brought the iridium atomization temperature out of the working range of the instrument used. The different chemical behavior of tungsten and zirconium was also confirmed by XPS investigations. With tungsten, evidence of both WC and WO bonding was found, while zirconium on the contrary shows only ZrO bonding and no evidence of carbide bonding. The SEM revealed a highly dispersed distribution of spot-like features whose smallest average diameter was of the order of 0.1 μm. The XRF asserted the confinement of iridium in these features and a strict association with zirconium in the case of zirconium treated surfaces. It is worth mentioning that such structure was preserved also after 400 thermal cycles simulating an atomization step at 1900°C despite a quite evident deterioration of the graphite surface, thus confirming the excellent durability of this modifier.

Scanning Tunneling Microscope Study of Etch Pits on Highly Oriented Pyrolytic Graphite Heated in an Atomic Absorption Electrothermal Analyzer

Scanning tunneling microscopy (STM) was used to elucidate monolayer etch pits that form on highly oriented pyrolytic graphite (HOPG) heated in an electrothermal analyzer. Pits form at elevated temperatures due to reactions between oxygen and exposed carbon edge atoms (defects) and additionally with intraplanar carbon atoms (through abstraction). Samples of HOPG without analyte or matrix modifier were placed in the depression of a pure pyrolytic graphite platform and heated by using standard analysis furnace programs. Under argon stop-flow conditions, pits form in less than a second at atomization temperatures equal to and above 1200 °C. With low argon flow rates (40 mL/min), pits formed at atomization temperatures equal to and greater than 1750 °C in less than a second. Quantitative pit formation rates were used to indicate oxygen partial pressure, which may be as high as ∼ 10−3 atm at 1200 °C. Reaction rates were used to predict surface degradation due to oxygen attack and determine that 1-μm depth normal to the surface would be removed by 200 successive 5-second-period furnace firings at 1200 °C. Implications for increases in surface reactivity and analyte intercalation are discussed.

Effects of atomization surfaces and modifiers on the kinetics of copper atomization in electrothermal atomic absorption spectrometry

Atomization of copper from different atomization surfaces and in the presence of chemical modifiers has been investigated. The atomization surfaces are molybdenum and tungsten carbide-coated platforms obtained by physical vapour deposition and pyrolytic graphite coated platforms. Molybdenum, tungsten and palladium have been used as chemical modifiers. The kinetic parameters of copper atomization in electrothermal atomic absorption spectrometry have been determined by three different methods. Although in all cases, differences in the atomization profiles are observed, the appearance temperatures when the carbide-coated platform and chemical modifiers are used, are not significantly different from those observed with conventional pyrolytic graphite platforms. Nevertheless, a longer tailing of the atomization profile and a slight thermal stabilization of the copper atomic vapour are found when tungsten and palladium modifiers are employed. In these later cases, two different activation energies are determined for each absorbance profile by the different models employed, indicating the presence of two possible precursors in the atomization mechanism. The activation energy ( E a ) values calculated in these cases are 75–104 kJ mol −1 when the atomic vapour starts to appear and 170–300 kJ mol −1 at high temperatures. The lower E a values are comparable to the single E a value obtained with pyrolytic graphite platform and the other cases studied, in which a single precursor mechanism is proposed. In these cases, the copper atomic vapour seems to desorb from the surface as highly dispersed atoms. When the palladium modifier is employed, the high E a and changes in the reaction order found at high temperatures, would indicate that the atomic vapour is additionally originating from the evaporation of aggregates. These aggregates could be formed by redistribution and agglomeration at higher temperatures and possibly trapping by the modifier. In the case of the tungsten modifier, dispersed atoms present in the cooler places of the platform could be responsible for the apparent increase of the E a value found. This second release could provide the longer tailing observed in the absorbance profiles when palladium and tungsten modifiers are used. A first-order kinetic release found in some of the cases studied could suggest a strong interaction with the surface; however, the low frequency factor value found gives no evidence for any substrate-analyte interaction in any of the cases studied.

Scanning Tunneling Microscope Images of Graphite Substrates Used in Graphite Furnace Atomic Absorption Spectrometry

Scanning tunneling microscopy (STM) was used to elucidate the submicrometer defect structures on the graphite substrates used in graphite furnace atomic absorption spectrometry (GFAAS). Images were obtained on pristine pyrolytic coated and uncoated polycrystalline graphite tubes and on pure pyrolytic graphite platforms. For comparison, images of highly oriented pyrolytic graphite, not used in GFAAS, were also obtained. Polycrystalline tubes were characterized by disordered surfaces with extensive oxidation. Pyrolytic coated tubes and pure pyrolytic graphite platforms were characterized by scaled structures, island columns, or smoothly varying contours. Scaled structures and island columns presented the greatest abundance of exposed carbon edge sites and seemed probable areas for analyte reactivity and intercalation. In areas displaying smoothly varying contours, atomic imaging revealed basal plane surface layers with moderate curvature but regular spacing between the carbon atoms within the layers. The results imply that the weak ordering between graphite layers along the c-axis does not preclude good atomic ordering within the layers, and this factor may be sufficient for discouraging analyte—substrate interactions.

Utilization of metallic platforms in electrothermal vaporization inductively coupled plasma mass spectrometry

A method to improve the analytical performance of hydride trapping on Pd inside a graphite furnace and subsequent determination by ICP-MS was investigated. Strips of tungsten, tantalum, molybdenum and rhenium were coated (electroplated/sputtered) with palladium and used as platforms inside the graphite furnace. A typical pyrolytic graphite-coated graphite platform was also coated with Pd in the same manner, for performance comparisons. Scanning electron microscopy (SEM) and X-ray fluorescence spectrometry were used to characterize the surfaces. SEM showed a smoother Pd layer covering the metallic substrates when compared with the graphite. An analyte signal reproducibility study revealed that Ta, Re and Mo were not ideal substrates for this purpose. The As peaks obtained from trapping on the graphite platform were broadened with a decrease in intensity after a few firings. The Pd-sputtered tungsten platform showed superior performance over the other platforms. The linear dynamic range obtained using this platform was improved by one order of magnitude in the upper limit (from 1 to 100 ng ml–1) over that reported using a graphite platform in previous work. The limit of detection of arsenic was found to be 0.01 ng ml–1, within the same range obtained from the graphite platform.

Capabilities and limitations of different techniques in electrothermal atomic absorption spectrometry for direct monitoring of arsenic, cadmium and lead contamination of sea-water

Three atomization techniques for electrothermal atomic absorption spectrometry (ETAAS), namely, wall, platform and graphite probe have been tested and critically compared for the direct determination of arsenic, cadmium and lead in sea-water. The best analytical performance was obtained using totally pyrolytic graphite platforms inserted in pyrolytic graphite coated graphite tubes, peak height measurement mode and the addition of chemical modifier (10 µg of palladium as nitrate). For arsenic determination the modifier was injected together with the sample (WET mode), while for cadmium and lead determination the modifier was previously reduced to Pd0 by in situ thermal decomposition of the palladium nitrate (DRY mode). With regard to sample pre-treatment, the sea-water was injected directly onto the platform for arsenic measurements, while dilution with ultrapure water (1 + 1) was preferable for lead and a dilution (1 + 3) with 25% ammonium nitrate solution was needed for cadmium. The limits of detection (LODs) for the three toxic elements, relative to the real samples (undiluted sea-water) analysed, were 1.7 µg l–1 for arsenic, 1.0 µg l–1 for cadmium and 1.3 µg l–1 for lead. The recoveries obtained for all three metals in sea-water samples were 100 ± 5% using the recommended ETAAS procedures. The analytical performances of the proposed methods are given. Quantification limits observed (5 × LOD) warrant the application of these methods to direct monitoring and control of contaminated waters for arsenic, cadmium and lead. However, when ‘normal’ levels of the metals in sea-water have to be determined adequate preconcentration techniques are needed.

Determination of total chromium in whole blood, blood components, bone, and urine by fast furnace program electrothermal atomization AAS and using neither analyte isoformation nor background correction.

Fast furnace program (total furnace time < 45 s) electrothermal atomization atomic absorption spectrometric (ETA-AAS) determinations of total Cr in several clinical materials were carried out in conventional (DABC) and transverse (ZEBC) heated graphite atomizers. Before spectrometric determination, test portions of the samples were diluted at different ratios in appropriate solvents: (a) whole blood (WB), blood plasma (BP), blood serum (BS), and red blood cells (RBC), 1 + 4 in 0.1% (v/v) Triton X-100; (b) urine (U), 1 + 4 in 0.1% (v/v) Triton X-100 + 0.01 mol/L nitric acid; and (c) bone (B) specimens and the certified reference materials after microwave mineralization, 1 + 9 in 0.01 mol/L nitric acid. The refractoriness of Cr allowed pyrolysis at a high temperature (approximately 1650 degrees C). As a consequence, two facts arose: first, isoformation was unnecessary; and second, background correction, independent of use of continuum source (DABC design) or Zeeman effect (ZEBC design) correction, was not required. For these reasons, the fast furnace program ETA-AAS analyses were simply done by automatic injection of 10-microL aliquots of the diluted test portions (or aqueous Cr standards) into either pyrolytic graphite-coated graphite tubes (DABC design; wall atomization performed) or pyrolytic graphite-coated graphite tubes with integrated pyrolytic graphite platforms (ZEBC design; integrated platform atomization performed), using neither analyte isoformation nor background correction; wall atomization in coated tubes was preferred. Under these experimental conditions, the limits of detection (3 sigma, micrograms/L Cr) and the characteristic masses (pg of Cr) were 0.03 and 2.7 (DABC design) and 0.2 and 5.0 (ZEBC design), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)