Shadow spectral imaging of absorbing layers in a transversely heated graphite atomizer: Part 1. Analyte atoms

The technique of shadow spectral imaging was used to investigate dynamics of formation and dissipation of Ag, In, Ga, Bi, Mn, Cu and Tl atomic layers in a transversely heated graphite tube atomizer (THGA) with and without integrated platform under gas-stop and gas-flow conditions. It is shown that non-uniform heating of the tube walls and platform surface in the radial cross section is the main reason for analyte transfer from atomizer bottom to less heated sides of the tube and platform before atomization temperature is reached. This transfer in the atomizer transverse cross section can be an additional factor that reduces matrix interferences in the THGA. In all the investigated cases, the atomic absorbing layers are not spatially uniform. Absorbance gradients grow up to 0.2 mm^− 1 even in the case of chemically inert silver atomization. Inverse atomization of In, Bi, Ga and Tl when atoms first appear in the atomizer's upper part was detected in THGA with platform. The effect of the internal gas flow on the spatial structure of analyte atoms is less pronounced in the transversely heated atomizer as compared to the end-heated furnaces.     

Shadow spectral imaging of absorbing layers in a transversely heated graphite atomizer: Part 2. Molecules and condensed-phase species

The dynamics of formation and dissipation of chloride, nitrate and sulfate matrix vapors in a transversely heated graphite tube atomizer (THGA) with and without integrated platform was investigated with the use of multi-channel atomic absorption spectrometry and the shadow spectral imaging technique. It is shown that non-uniform heating of the tube walls and platform in the furnace radial cross-section causes vapor transfer from atomizer bottom to less heated sides of the tube and platform. This transfer in the atomizer cross-section can be an additional reason for lower level of matrix interferences in the THGA and is a prerequisite for explosive atomization of some elements that appear as absorbance spikes. The cross-sectional structures of molecular layers and the cloud of condensed phase particles are highly inhomogeneous, resulting in absorbance gradients up to 0.2–0.5 mm^− 1. These structures differ significantly from those observed earlier in end-heated atomizers. Local vortices of the sheath gas, toroid-shaped and bridge-like structures of vapor layers were observed in the atomizer volume. The role of light scattering on the finally dispersed condensed phase particles in the transverse heated furnace is greater than that in the end heated atomizers because of near axis location of the cloud.     

Permanent iridium modifier deposited on tungsten and zirconium-treated platforms in electrothermal atomic absorption spectrometry: vaporization of bismuth,silver and tellurium

The stabilizing role of permanent iridium modifier deposited on tungsten-treated (WTP) and zirconium-treated (ZrTP) platforms of transversely heated graphite atomizer (THGA) was studied in detail by electrothermal atomic absorption spectrometry (ETAAS) and different surface techniques in model experiments for Ag, Bi and Te. The comparison of the stabilizing efficiency of permanent Ir modifier on WTP and ZrTP and each of the single components, reveals the better effect of Ir on WTP and Ir itself. The extent of analyte losses during pre-atomization and the strength of analyte association with the modifier were estimated by the plotting of `differential vaporization curves'. The existence of double peaks of Ag, Bi and Te in WTP and Ir on WTP was confirmed and possible reasons for their formation were discussed. The absorbance profiles presented as differential curves reveal an existence of at least two different types of precursors determining processes of atom generation. The observed differences in the behavior of Ir permanent modifier on WTP and ZrTP, respectively, were explained by the different extent of iridium–tungsten and iridium–zirconium interaction and surface distribution. XRF, ESCA and SEM studies reveal non-uniform distribution of the modifier on the graphite substrate and the presence of oxide containing species on the surface.     

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.     

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).     

Method development for the determination of lead in wine using electrothermal atomic absorption spectrometry comparing platform and filter furnace atomizers and different chemical modifiers

A method has been developed for the determination of lead in wine by electrothermal atomic absorption spectrometry without any sample preparation and calibration against aqueous standards, using 7.5 μg Pd as a chemical modifier. The results obtained for seven wines using the proposed method and an acid digestion procedure did not show any significant difference using a Student's t-test. Atomization in a transversally heated filter atomizer (THFA) was compared with atomization in a conventional transversally heated platform furnace. The former provided a 2.6-fold higher sensitivity, improving the characteristic mass from 34 to 12 pg and a 1.6-fold better limit of detection (0.3 μg L⁻¹ compared to 0.5 μg L⁻¹) for aqueous solutions using the same injection volume of 20 μL. However, the average precision, expressed as the relative standard deviation for the determination of lead in wine under routine conditions was improved from 4.6% with platform atomization to 0.6% in the THFA. The lead content found in seven arbitrarily chosen white and red wines, five from Brazil, one from Chile and one from Spain, ranged from 6 to 60 μg L⁻¹ Pb with an average content of 11.4 μg L⁻¹ Pb for the wines from South America.     

Direct As,Bi, Ge,Hg and Se(IV) cold vapor/hydride generation from coal fly ash slurry samples and determination by electrothermal atomic absorption spectrometry

Direct cold vapor and hydride generation procedures for As, Bi, Ge, Hg and Se(IV) from aqueous slurry of coal fly ash samples have been developed by using a batch mode generation system. Ir-treated graphite tubes have been used as a preconcentration and atomization medium of the vapors generated. A Plackett–Burman experimental design has been used as a strategy for evaluation of the effects of several parameters affecting the vapor generation efficiency from solid particles, vapor trapping and atomization efficiency from Ir-treated graphite tubes. The effects of parameters such as hydrochloric acid and sodium tetrahydroborate, argon flow rate, trapping and atomization temperatures, trapping time, acid solution volume and mean particle size have been investigated. The significant parameters obtained (trapping and atomization temperatures for As and Ge; trapping temperature and trapping time for Bi; argon flow rate and atomization temperature for Se) have been optimized by 2²+star central composite design. For Hg, the trapping temperature has been also significant. Optimum values of the parameters have been selected for the development of direct cold vapor/hydride generation methods from slurry particles. The accuracy of methods have been verified by using NIST-1633a coal fly ash certified reference material. Absolute detection limits of 11.5, 48.0, 600, 55.0 and 11.0 ng l⁻¹ for As, Bi, Ge, Hg and Se have been achieved, respectively. A particle size less than 50 μm has shown to be adequate to obtain total cold vapor/hydride generation of metals content in the aqueous slurry particles.     

Different platform and tube geometries and atomization temperatures in graphite furnace atomic absorption spectrometry: Cadmium determination in whole blood as a case study

In the present work the performance of different platform and tube geometries and atomization temperatures in graphite furnace atomic absorption spectrometry was investigated, using the determination of Cd in whole blood as an example. Grooved, integrated and fork platforms as well as atomization temperatures between 1200 °C and 2200 °C were investigated in a longitudinally heated graphite atomizer and compared with the performance of a transversely heated furnace. In the longitudinally heated furnace the increase of the atomization temperature in the studied range resulted in an increase of matrix effects for all platform geometries. The integrated platform exhibited slightly lower sensitivity and increased multiplicative interferences in comparison to the other two platform designs. Interference-free Cd determination was possible with all types of platforms and 1200 °C as the atomization temperature as well as with grooved and fork platforms at 1700 °C. On the other hand, lower atomization temperatures resulted in poorer limits of detection, due to the longer integration time needed. No matrix effect was observed at any atomization temperature using the transversely heated atomizer; in addition, limits of detection were better than those observed with the longitudinally heated atomizer. Best values were around 0.02 μg L^− 1 with the latter atomizer compared to values around 0.02 μg L^− 1 with the former one.     

Determination of trace elements in seawater and river water by atomic absorption spectrometry with the use of an electrothermal crucible atomizer with two evaporation zones for solid samples

Direct and sorption-atomic absorption spectrometric determination methods were developed for dissolved and suspended Cd, Pb, and Tl species in seawater and river water were developed. These methods involve the step of ashing solid matrices (suspension or suspension + concentrate) in the crucible-rod system followed by the simultaneous atomization of a thermally modified sample and a condensate of volatile components. A crucible-cell-rod three-chamber graphite electrothermal atomizer with two evaporation zones and a common isothermal analytical zone, which were heated independently, was proposed for the analysis of these matrices. The approach proposed improved the metrological characteristics of the results of determining volatile metals in samples with high organic matter contents.     

Multiple microflame quartz tube atomizer — further development towards the ideal hydride atomizer for atomic absorption spectrometry

The development of an improved type of hydride atomizer for atomic absorption spectrometry — multiple microflame quartz tube atomizer (MMQTA) — is presented. The main feature of this atomizer is recurrent analyte atomization proceeding over its whole optical tube length, which is achieved by production of H-radicals at multiple points within the tube by oxygen microflames burning in the hydrogen-containing atmosphere. The MMQTA design optimization leading to a complete filling of the observation volume with H-radicals is described. The influence of individual atomization parameters is discussed. Optimum H-radical producing oxygen intake into the MMQTA was found to correspond to a H₂:O₂ stoichiometric (3:1) ratio. The performance of the individual MMQTA tube designs is evaluated and compared to a typical externally heated quartz tube atomizer (EHQTA) — the linearity of calibration graphs for As, Se and Sb is significantly improved in all MMQTA tubes, without compromising the sensitivity, simplicity, low cost and easy operation. In fact, the free atom reactions within the tube causing calibration curvature are avoided up to an analyte concentration of at least 200 ng ml⁻¹ for Se and Sb and 100 ng ml⁻¹ for As. Tolerance limits of 0.7, 1.4, 0.2 and 0.2 μg ml⁻¹ are achieved for the atomization interferences of As on Se, Se on As, Sb on Se and Se on Sb, respectively, which is an improvement by 1–2 orders of magnitude in comparison to the conventional EHQTA with the same hydride generation system.     

Determination of Cd and Pb in food slurries by GFAAS using cryogenic grinding for sample preparation

A simple method combining slurry sampling after cryogenic grinding and the use of a permanent modification of the integrated platform inside the transversely heated graphite atomizer (THGA) was proposed for the determination of Cd and Pb in foods. Potentialities of the cryogenic grinding were evaluated for grinding different materials of difficult homogenization such as high fat and high fiber tissues. Animal and vegetal samples were cut into small pieces and ground in liquid nitrogen for 2 min. Slurries were prepared directly in the autosampler cup after cryogenic grinding by transferring an exact amount of homogeneous powdered material (5-20 mg) to the cup, followed by 1.00 mL of 0.2% (v/v) HNO3 containing 0.04% (v/v) Triton X-100 and sonication for 30 s, before transferring into the platform previously coated with 250 microg W and 200 microg Rh. Use of a tungsten carbide-rhodium permanent modifier combined with NH4H2PO4 conventional modifier improves tube lifetime and increases the pyrolysis temperature for Cd. Homogeneity tests, carried out by comparing the between- and within-batch precision for each kind of sample, showed no significant differences at the 95% confidence level, indicating good homogeneity for 5-20 mg masses. Detection limits were 3.3 ng g(-1) Cd and 75 ng g(-1) Pb for 1% m/v slurries. Results for determination of Cd and Pb in foods slurries were in agreement with those obtained with digested samples, since no statistical differences were found by the paired t-test at the 95% level.     

Palladium and magnesium nitrates,a more universal modifier for graphite furnace atomic absorption spectrometry

A mixture of palladium and magnesium nitrates was found to be a very powerful modifier for the determination of As, Bi, In, Pb, Sb, Se, Sn, Te and Tl in graphite furnace atomic absorption spectrometry. Thermal pretreatment temperatures of 900-1400°C can be used with the proposed modifier. This is in most cases substantially more than what can be applied with the modifiers recommended up to now, so that separation of the analyte from the concomitants should be easier. This is shown to be true for the determination of lead in sea water and of selenium in biological materials. Optimum atomization temperatures are more uniform and typically around 2000°C for the investigated elements when the palladium and magnesium nitrates mixed modifier is used. This modifier therefore allows the use of common conditions for all the investigated elements with a minimum sacrifice in sensitivity, an important pre-requirement for multi-element furnace techniques. The proposed mixed modifier also minimizes the risk of contamination because palladium as well as magnesium nitrate can be obtained in high purity, and both elements are infrequently determined in the graphite furnace.     

Direct determination of cadmium and copper in seawater using a transversely heated graphite furnace atomic absorption spectrometer with Zeeman-effect background corrector

Methods for the direct determination of copper and cadmium in seawater were described using a graphite furnace atomic absorption spectrometer (GFAAS) equipped with a transversely heated graphite atomizer (THGA) and a longitudinal Zeeman effect background corrector. Ammonium nitrate was used as the chemical modifier to determine copper. The mixture of di-ammonium hydrogen phosphate and ammonium nitrate was used as the chemical modifier to determine cadmium. The matrix interference was removed completely so that a simple calibration curve method could be applied. This work is the first one with the capability of determining cadmium in unpolluted seawater directly with GFAAS using calibration curve based on simple aqueous standards. The accuracy of the methods was confirmed by analysis of three kinds of certified reference saline waters. The detection limits (LODs), with injection of a 20-mul aliquot of seawater sample, were 0.06 mug l(-1) for copper and 0.005 mug l(-1) for cadmium.     

Spatiotemporal Dynamics of Vapors of Chloride Matrices in a Transversely Heated Graphite Furnace for Atomic Absorption Spectrometry

The dynamic of the formation and dissipation of sodium and nickel chloride vapors in the THGA transversely heated graphite furnace atomizer of a SIMAA 6000 multielement spectrometer was investigated by shadow spectral filming. The spatial distribution for the entire vaporization cycle of vapors is strongly inhomogeneous, due mainly to the transverse non-isothermal conditions of the atomizer. It also depends on the addition of a chemical modifier and the rate of the internal flow of the sheath gas. At the normal brightness of the spectral lamps, the spectrometer's Zeeman-based background corrector reliably compensates for the nonselective light absorption by molecular vapors and condensed particles with an amplitude of up to 1. For the minimum allowable brightness of lamps, this value decreases to as low as 0.6 because of an increase in the noise of the measurement path.     

Investigation of interferences in the determination of thallium in marine sediment reference materials using high-resolution continuum-source atomic absorption spectrometry and electrothermal atomization

A high-resolution continuum-source atomic absorption spectrometer with a xenon short-arc lamp as the radiation source, a compact double echelle monochromator with a focal length of 302 mm and a spectral resolution of λ/Δλ≈110 000, and a UV-sensitive charged-coupled device (CCD) array detector was used to investigate the spectral interferences found with a conventional line-source atomic absorption spectrometer in the determination of thallium in marine sediment reference materials. A transversely heated graphite furnace was used as the atomizer unit, and the samples were introduced in the form of slurries. A strong iron absorption line at 276.752 nm, which was observed at atomization temperatures >2000 °C in the vicinity of the thallium resonance line at 276.787 nm, could be responsible for some of the interferences observed with low-resolution continuum-source background correction. The outstanding feature at atomization temperatures <2000 °C was the electron excitation spectrum of the gaseous SO₂ molecule that exhibited a pronounced rotational fine structure, and is for sure the main reason for the observed spectral interferences. The molecular structures could be removed completely by subtracting a model spectrum recorded during the atomization of KHSO₄, using a least squares algorithm. The same results, within experimental error, were obtained for thallium in a variety of marine sediment reference materials using ammonium nitrate as a modifier, ruthenium as a permanent modifier in addition to ammonium nitrate, and without a modifier, using aqueous standards for calibration, demonstrating the ruggedness of the method. A characteristic mass of 15–16 pg Tl was obtained, and a limit of detection of 0.02 μg g⁻¹ Tl was calculated from the standard deviation of five repetitive determinations of HISS-1, the sediment with the lowest thallium content.     

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.     

Factors influencing the free oxygen content in an electrothermal atomizer

A considerable decrease in the partial pressure of free oxygen in the gas phase within a graphite atomizer during atomization with the use of ascorbic acid as a matrix modifier was proved by using the method suggested by L'vov and Ryabchuk. A similar change in the gas-phase composition was observed in graphite tubes treated with compounds of refractory metals. A mechanism for this oxygen binding is suggested. It is shown that the sensitivity of the determination of tin is improved by decreasing the free oxygen content of the gas phase.     

Atomic absorption determination of elements in solid samples using an electrothermal atomizer ‘crucible with separated zones’

A model of an electrothermal atomizer given the name ‘crucible with separated zones’ has been proposed. It provides the filtration of sample vapors through porous walls of a graphite tube placed in the atomization zone and heated independently of the evaporation zone. The inner volume of the tube is an isothermal analytical zone. The atomizer has been applied to the analysis of solid sea and river suspensions, DETATA concentrates of waters, and bottom sediments weighing 30–50 mg and more. As a result, the performance characteristics of the direct atomic absorption determination of Cd, Ag, and Pb in geochemical samples of complicated composition have been improved.     

Electrothermal atomization from metallic surfaces: Part 1. Design and Performance of a tungsten-tube atomizert

A tungsten-tube atomizer has been developed and tested for a number of elements. The atomizer can be accommodated in the support of a commercial Varian carbon rod atomizer (CRA-63 or CRA-90) and operated as an alternative to the graphite tubes and cups. Characteristic concentrations, detection limits, optimum atomization temperatures, reproducibility and profiles of calibration curves obtained for Ag, Al, Ba, Cd, Co, Cu, Mg, Mn, Ni, Pb and V are similar to those obtained with the graphite-tube atomizer. The main advantage of this atomizer is its simple construction, low cost, long lifetime and excellent analytical performance, particularly for elements forming refractory carbides.     

Some aspects of electrothermal atomization of elements from large amounts of involatile matrices

Non-volatile matrices which cannot diffuse into the graphite walls of an electro-thermal atomizer (as obtained by direct sampling of solids, injection of solutions into tubes with pyrolytic coatings or with a hydrophobic film) make a “miniplatform” in the centre of the atomizing tube. A delay in the temperature increase of this platform at the beginning of the atomization step compared to that of the atomizer walls causes enhancement of the peak height sensitivity. This was proven by atomization of nanogram quantities of copper and iron from solutions of aluminium salts.