Effect of beryllium nitrate vaporization on surface temperature in the pyrocoated graphite furnace

The vaporization of 20–50 μg beryllium from nitrate solution was observed in graphite furnace atomizers using pyrocoated and Ta-lined tubes. A charge coupled device (CCD) spectrometer was employed to follow the evolution of absorption spectra (200–475 nm), the light scattering and emission. Molecular bands of NO and NO₂ were observed below 1000°C. Beryllium absorption at 234.9 nm was prominent in spectra above 2200°C and 1900°C, respectively, in Ta-lined and pyrocoated tubes. The evolution profile of Be atomic absorption and of some bands indicated a faster vapor release in the pyrocoated tube. Light scattering occurred only in the pyrocoated tube, increasing with the tube age. When purge gas mini flow was applied, the scattering was observed at 1900–2200°C simultaneously with Be atomic absorption and emission continuum at long wavelength. The emission continuum showed the wavelength distribution characteristic of black body radiation. The temperature increase, due to the vaporization of the sample, was estimated using Planck’s equation. The maximum temperature increase reached 400°C, when the most intense Be atomic absorption, light scattering and emission was observed. According to the hypothesis proposed, the black body radiation was induced by the formation of Be carbide in the pyrographite layer. Low heat capacity across the pyrographite prevented the heat dissipation, and led to increase of surface temperature. This induced an increase of sample evaporation rate and the formation of a thermal gradient in the cross section of the tube. Both factors originated vapor supersaturation in the tube center, spatial condensation and, accordingly, light scattering. The results, together with those already obtained with Mg nitrate place limitations to the atomization theories based on the concept of isothermal equilibrium or on Arrhenius kinetic approach.     

Effect of magnesium nitrate vaporization on gas temperature in the graphite furnace

The vaporization of magnesium nitrate was observed in longitudinally-heated graphite atomizers, using pyrocoated and Ta-lined tubes and filter furnace, Ar or He as purge gas and 10–200-μg samples. A charge coupled device (CCD) spectrometer and atomic absorption spectrometer were employed to follow the evolution of absorption spectra (200–400 nm), light scattering and emission. Molecular bands of NO and NO₂ were observed below 1000°C. Magnesium atomic absorption at 285.2 nm appeared at approximately 1500°C in all types of furnaces. The intensity and shape of Mg atomization peak indicated a faster vapor release in pyrocoated than in Ta-lined tubes. Light scattering occurred only in the pyrocoated tube with Ar purge gas. At 1500–1800°C it was observed together with Mg absorption using either gas-flow or gas-stop mode. At 2200–2400°C the scattering was persistent with gas-stop mode. Light scattering at low temperature showed maximum intensity near the center of the tube axis. Magnesium emission at 382.9, 383.2 and 383.8 nm was observed simultaneously with Mg absorption only in the pyrocoated tube, using Ar or He purge gas. The emission lines were identified as Mg ³P°–³D triplet having 3.24 eV excitation energy. The emitting species were distributed close to the furnace wall. The emitting layer was thinner in He than in Ar. The experimental data show that a radial thermal gradient occurs in the cross section of the pyrocoated tube contemporaneously to the vaporization of MgO. This behavior is attributed to the reaction of the sample vapor with the graphite on the tube wall. The estimated variation of temperature within the cross section of the tube reaches more than 300–400°C for 10 μg of magnesium nitrate sampled. The increase of gas temperature above the sample originates a corresponding increase of the vaporization rate. Fast vaporization and thermal gradient together cause the spatial condensation of sample vapor that induces the light scattering.     

Vaporization of indium nitrate in the graphite tube atomizer in the presence of chemical modifiers

The CCD spectrometer coupled with the graphite tube furnace was employed to investigate the vaporization of micrograms of In (as nitrate). Fifty absorption spectra between 200 and 475 nm were collected during 10 s while the tube temperature increased from 700 to 2600–2700 K. The vaporization was carried out in the pyrocoated, Ir-sputtered and Ta-lined tubes in the presence of Cu, Co, Ni, Pd and Mg nitrates, sodium tungstate, ascorbic acid and ammonium hexachloroiridate monohydrate after thermal pretreatment. In the pyrocoated tube the vaporization of In occurred at 1350–1550 K with fast evolution of molecular vapor. The observed broad bands with maxima at 225, 290 and 275 nm were attributed to In₂O and InO, according to their thermal behavior. Cu, Co, Ni, Pd, Ir modifiers, Ta-lining and Ir sputtered surface suppressed the release of In oxides and induced the delayed appearance of In atomic lines simultaneous with a broad band at 205 nm, tentatively attributed to In dimer. Tungsten caused faster and more complete reduction of In oxide than carbon. Indium oxide bands were substituted between 1100 and 1350 K by a band at 244 nm assigned to gaseous tungsten oxide. Ascorbic acid caused the decrease of indium oxide fraction in gas phase. The presence of MgO in the tube led to the decrease of the band at 205 nm. The vaporization of micrograms of Cu, Co, Ni, Pd and MgO in the pyrocoated tube caused the appearance of absorption and emission continuum, superimposed to In atomic lines at temperatures above 1550 K. This effect had been earlier explained as induced by exothermal interaction of the vaporized substance with carbon. SEM observations of Ir deposits on the graphite surface confirmed the interaction of Pt group metals with carbon at high temperature. A similar effect is advanced for other metal modifiers.     

Energy transfer caused by reactions in a graphite tube atomizer

A spectrometer with a charge coupled device detector is employed to measure the temperature inside a graphite furnace using the wavelength distribution of the radiation continuum. For steady-state heating and blank firings, the results are good approximations to those expected from the black body theory. The calculated temperatures from 1900 to 2700 K, can be affected by the emissivity of the tube wall and radiating area by ±60 K. The vaporization of microgram quantities of Mg, Be, Pd and Cu as nitrates is accompanied by transient light scattering and an emission continuum. The effect occurs for Cu in both Ta-lined and pyrocoated tubes, and for Mg, Be and Pd only in the pyrocoated tube. Wavelength distribution of the transient radiation is also characteristic of a black body radiator, but with temperature increase of 400–600 K in comparison with that of the tube wall. The emission originates from the clouds of condensed particles formed almost simultaneously with the vaporization of the sample. The effect is accompanied by increased vaporization rate and appearance of some particularities in the vapor spectra. The concept of isothermal atomization fails to explain both phenomena. Hence, the hypothesis is advanced concerning the evolution of chemical energy during sample interaction with the tube material. Energy transfer and dissipation in the vicinity of the sample control both mass output and kinetic energy of the released atoms. The exothermic process of nucleation and aerosol formation causes release of energy through radiation. It is suggested that such phenomena can occur in the tube furnace during trace element determination in the presence of microgram quantities of matrix and chemical modifiers.     

Atomic and molecular spectra of vapors evolved in a graphite furnace. Part 4: alkaline earth chlorides

Dry residues of aqueous solutions of alkaline earth chlorides (100 μg as element) were vaporized in the Ta-lined and pyrocoated graphite furnace during temperature ramp from 400° to 2400–2500°C. Absorption and emission spectra were obtained in the 200–475 nm range using a dedicated CCD spectrometer. BeCl₂ in aqueous solutions showed an almost complete hydrolysis prior to the vaporization. The spectra were characterized by the contemporaneous presence of intense light scattering and emission continuum, in analogy with the behavior of Be(NO₃)₂ already examined. By applying BeCl₂ slurry in chloroform enabled the observation of BeCl₂ and BeCl species in the vapor phase. Aqueous solution of MgCl₂ showed the evolution of di-halide species at low temperatures and the vaporization pattern with the emission continuum and light scattering similar to that observed for Mg(NO₃)₂ at higher temperature. The hydrolysis of the di-chlorides decreases for the alkaline earth elements with increasing of atomic mass. The Ca, Sr and Ba chlorides showed a vaporization pattern of di-chloride in both a pyrocoated and Ta-lined tube with increased simultaneous appearance of mono-chloride and atomic species in the pyrocoated tube. The proposed explanation indicates a trend to the decomposition due to the sample vapor interaction with graphite.     

Design considerations regarding an atomizer for multi-element electrothermal atomic absorption spectrometry

The methodology of simultaneous multi-element electrothermal atomic absorption spectrometry (ETAAS-Electrothermal Atomic Absorption Spectrometry) stipulates rigid requirements to the design and operation of the atomizer. It must provide high degree of atomization for the group of analytes, invariant respective to the vaporization kinetics and heating ramp residence time of atoms in the absorption volume and absence of memory effects from major sample components. For the low resolution spectrometer with a continuum radiation source the reduced compared to traditional ETAAS (Electrothermal Atomic Absorption Spectrometry) sensitivity should be, at least partially, compensated by creating high density of atomic vapor in the absorption pulse. The sought-for characteristics were obtained for the 18 mm in length and 2.5 mm in internal diameter longitudinally heated graphite tube atomizer furnished with 2-4.5 mg of ring shaped carbon fiber yarn collector. The collector located next to the sampling port provides large substrate area that helps to keep the sample and its residue in the central part of the tube after drying. The collector also provides a “platform” effect that delays the vaporization and stipulates vapor release into absorption volume having already stabilized gas temperature. Due to the shape of external surface of the tube, presence of collector and rapid (about 10 °C/ms) heating, an inverse temperature distribution along the tube is attained at the beginnings of the atomization and cleaning steps. The effect is employed for cleaning of the atomizer using the set of short maximum power heating pulses. Preparation, optimal maintenance of the atomizer and its compliance to the multi-element determination requirements are evaluated and discussed. The experimental setup provides direct simultaneous determination of large group of element within 3-4 order concentration range. Limits of detection are close to those for sequential single element determination in Flame AAS with primary line source that is 50-1000 times higher than the limits obtainable with common ETAAS (Electrothermal Atomic Absorption Spectrometry) instrumentation.     

Investigation of aging processes of graphite tubes modified with iridium and rhodium used for atomic spectrometry

UV spectrometry (187–380 nm) with charge coupled device (CCD) detection was used to study the evolution of absorption spectra during the vaporization of various species in the pyrocoated graphite furnace, with electrodeposited Ir and Rh as modifiers. In order to mimic a typical matrix composition, various salts of aluminum, manganese, copper, magnesium, sodium, and lead were used in microgram amounts. Changes in spectra and vapor release rate, along with aging of the tubes in the repetitive temperature cycles, were observed.     

Feasibility of employing permanent chemical modifiers for the determination of cadmium in coal using slurry sampling electrothermal atomic absorption spectrometry

Iridium and ruthenium, alone and in combination with tungsten, thermally deposited on the platform of a transversely heated graphite tube, were investigated for their suitability as permanent chemical modifiers for the determination of cadmium in coal slurries by electrothermal atomic absorption spectrometry (ET AAS). The conventional mixed palladium and magnesium nitrates (Pd–Mg) modifiers, added in solution, were also investigated for comparison. The latter one showed the best performance for aqueous solutions, and the mixed W–Ir and W–Ru permanent modifiers had the lowest stabilizing power. All of the investigated modifiers lost some of their stabilizing power when coal slurries were investigated. The Pd–Mg modifier, pure Ir and Ru, and a mixture of 300 μg W + 200 μg Ir could stabilize Cd at least to a pyrolysis temperature of 600 °C, whereas all the other combinations already failed at temperatures above 500–550 °C. Additional investigations of the supernatant liquid of the slurries supported the assumption that the high acid concentration of the slurries and/or a concomitant leaching out of the coal might be responsible for the reduced stabilizing power of the modifiers. The maximum applicable pyrolysis temperature of 600 °C was not sufficient to reduce the background absorption to a manageable level in the majority of the coal samples. High-resolution continuum source ET AAS revealed that the continuous background absorption was exceeding values of A = 2, and was overlapping with the analyte signal. Although the latter technique could correct for this background absorption, some analyte was apparently lost with the rapidly vaporizing matrix so that the method could not be considered to be rugged. A characteristic mass of 1.0 pg and a detection limit of 0.6 ng g^− 1 could be obtained under these conditions.     

Determination of cadmium and lead in river water by sequential metal vapor elution analysis using a step-heated column

Determination of cadmium and lead in river water by sequential metal vapor elution analysis (SMVEA, column temperature; >1210 K) with argon carrier gas and an atomic absorption detector (AA) is reported. The column was a molybdenum tube inserted a tungsten coil. The flow rate of carrier gas was 1.8 ml min^–1. Cadmium and lead were separated from Ca, Fe, K, Na, and Zn metal vapours by SMVEA with the step-heated column (1210–1520 K) at an atomization temperature of 1830 K. Under the optimal experimental conditions, the recoveries of spiked-cadmium and lead in river waters were in the range of 91 to 106%. It is to determine cadmium and lead in river water without the interferences by matrix elements observed by electrothermal AAS, after only the addition of hydrochloric acid to the sample.     

Migration of Ag, Cd and Cu into highly oriented pyrolytic graphite and pyrolytic coated graphite

Migration of Cd, Cu and Ag from solution deposited samples of the respective nitrates into highly oriented pyrolytic graphite (HOPG) was studied using electrothermal atomic absorption spectroscopy (ETAAS) with platform vaporization. Metal migration was verified by removing the top layers of the HOPG platform after the sample had dried and performing the analysis using ETAAS. The results obtained suggest that the metals or their salts migrate into HOPG only when the sample solutions are deposited on those areas of the platform that have surface imperfections. The surface blemishes can be seen as tiny lines on the otherwise smooth surface of the HOPG platform. One possible driving force for the migration could be simple capillary action; however, additional information is needed to establish the true mechanism.

The effect of metal migration into graphite on atomic absorption profiles was also evaluated. This effect was studied by comparing the signals obtained after the sample had been deposited either on the imperfections or on the smooth areas of the HOPG platform. In addition, samples were atomized from both sides of a pyrolytic coated platform in which one of the sides had been roughened with an abrasive material to expose the electrographite. The main effect of metal migration on the absorption profiles seems to be an increased tailing of the back edge of the signals. This could suggest a secondary generation function limited by the rate of diffusion of the metal back to the substrate surface and subsequent vaporization.     

Atomic and molecular spectra of vapors evolved in a graphite furnace. Part 5: gallium,indium and thallium nitrates and chlorides

The vaporization behavior and vapor spectra of Ga, In and Tl nitrates and chlorides in the tube furnace was investigated using an UV spectrometer with CCD detector. Fifty spectra in the wavelength range of 200–475 nm were collected in each experiment during the vaporization step, with temperature increase from 473 to 2673 K. The vaporization patterns were compared for the pyrocoated, non-pyrocoated graphite tubes, and Ta-lined tubes. Nitrate and chloride aqueous solutions and chloride slurries in chloroform were used to distinguish the impact of hydrolysis on the vaporization behavior of these chlorides. The spectra of oxygen and chlorine containing molecules, presumably of suboxides, chlorides and dichlorides were identified upon variation of the experimental conditions. The release of suboxide vapors due to the reduction of oxides by carbon was promoted after the decomposition of nitrates. The presence of other elements on the vaporization surface, or the isolation of the sample from the graphite surface by Ta-lining, impeded the vapor release and reduced the intensity of molecular bands. Adsorption of chlorine onto graphite caused a decrease of chloride and dichloride bands. The suboxide bands were observed in the spectra of Ga and In chlorides introduced in the tube as aqueous solutions, due to partial hydrolysis.     

Graphite filter atomizer in atomic absorption spectrometry

Graphite filter atomizers (GFA) for electrothermal atomic absorption spectrometry (ETAAS) show substantial advantages over commonly employed electrothermal vaporizers and atomizers, tube and platform furnaces, for direct determination of high and medium volatility elements in matrices associated with strong spectral and chemical interferences. Two factors provide lower limits of detection and shorter determination cycles with the GFA: the vaporization area in the GFA is separated from the absorption volume by a porous graphite partition; the sample is distributed over a large surface of a collector in the vaporization area. These factors convert the GFA into an efficient chemical reactor. The research concerning the GFA concept, technique and analytical methodology, carried out mainly in the author's laboratory in Russia and South Africa, is reviewed. Examples of analytical applications of the GFA in AAS for analysis of organic liquids and slurries, bio-samples and food products are given. Future prospects for the GFA are discussed in connection with analyses by fast multi-element AAS.     

Investigation of artifacts caused by deuterium background correction in the determination of phosphorus by electrothermal atomization using high-resolution continuum source atomic absorption spectrometry

The artifacts created in the measurement of phosphorus at the 213.6-nm non-resonance line by electrothermal atomic absorption spectrometry using line source atomic absorption spectrometry (LS AAS) and deuterium lamp background correction (D₂ BC) have been investigated using high-resolution continuum source atomic absorption spectrometry (HR-CS AAS). The absorbance signals and the analytical curves obtained by LS AAS without and with D₂ BC, and with HR-CS AAS without and with automatic correction for continuous background absorption, and also with least-squares background correction for molecular absorption with rotational fine structure were compared. The molecular absorption due to the suboxide PO that exhibits pronounced fine structure could not be corrected by the D₂ BC system, causing significant overcorrection. Among the investigated chemical modifiers, NaF, La, Pd and Pd + Ca, the Pd modifier resulted in the best agreement of the results obtained with LS AAS and HR-CS AAS. However, a 15% to 100% higher sensitivity, expressed as slope of the analytical curve, was obtained for LS AAS compared to HR-CS AAS, depending on the modifier. Although no final proof could be found, the most likely explanation is that this artifact is caused by a yet unidentified phosphorus species that causes a spectrally continuous absorption, which is corrected without problems by HR-CS AAS, but which is not recognized and corrected by the D₂ BC system of LS AAS.     

Determination of vanadium in petroleum and petroleum products using atomic spectrometric techniques

Vanadium is recognized worldwide as the most abundant metallic constituent in petroleum. It is causing undesired side effects in the refining process, and corrosion in oil-fired power plants. Consequently, it is the most widely determined metal in petroleum and its derivatives. This paper offers a critical review of analytical methods based on atomic spectrometric techniques, particularly flame atomic absorption spectrometry (FAAS), electrothermal atomic absorption spectrometry (ET AAS), inductively coupled plasma optical emission spectrometry (ICP OES), inductively coupled plasma mass spectrometry (ICP-MS). In addition an overview is provided of the sample pretreatment and preparation procedures for vanadium determination in petroleum and petroleum products. Also included are the most recent studies about speciation and fractionation analysis using atomic spectrometric techniques.     

Release of selenium and tin from the graphite support in the presence of tungsten and palladium modifiers

Electrothermal atomic absorption spectrometry is applied to investigate the release of the selenium and tin atoms from pyrocoated graphite support. The Coats and Redfern and Sturgeon approaches are used for estimation of the apparent activation energies of atom release. The addition of tungsten and palladium modifiers leads to enhancement of the activation energies and changes the kind of the vaporizing species for analytes.     

Determination of total chromium traces in tannery effluents by electrothermal atomic absorption spectrometry, flame atomic absorption spectrometry and UV-visible spectrophotometric methods

Three different analytical methods comprising colorimetric method with 1,5-diphenyl-carbazide, electrothermal atomic absorption spectrometry (ET AAS) and flame atomic absorption spectrometry were utilized in a study to determine traces of chromium (Cr) in synthetic tannery effluent from laboratory scale treatment process variations. All the results obtained using the three different methods showed good agreement and met the requirement of Brazilian regulation for total Cr for effluent discharges (<0.5 mg l(-1)). However, ET AAS has been the proposed method because it was faster, less laborious, needed smaller volume of sample and presented lower limit of quantification (LOQ=2.2 mug l(-1)).     

Atomic and molecular spectra of vapours evolved in a graphite furnace. Part 1. Alkali halides

Molecular Absorption Spectrometry (MAS) with electrothermal vaporization was applied to the measurement of absorption by alkali halides. The MAS system, consisting of a deuterium lamp primary source, a tubular graphite furnace, a grating polychromator and a linear array of Charge-Coupled-Device (CCD) detectors, allowed the simultaneous determination of atomic and molecular absorption in the range 200–400 nm. Vaporization was carried out in a pyrocoated graphite tube and absorption was measured during the heating of the furnace from 500°C to 2000°C in 100 s.Alkali halides vaporize as molecular compounds which absorb radiation in the whole ultraviolet range. The complexity of the molecular bands as well as the extent of the absorption increases from fluorides to iodides. The limit of absorption at long wavelengths is 254 nm for NaF, 287 nm for NaCl, 320 nm for NaBr and 370 nm for NaI. The appearance of vapors was observed between 680°C (RbI) and 1220°C (LiF), while the maximum absorption was reached between 800°C (CsI) and 1440°C (LiF); the characteristic temperatures of the vaporization peak were shifted towards lower values going from fluorides to iodides.     

Determination of As,Cd, Cu,Hg and Pb in biological samples by modern electrothermal atomic absorption spectrometry

Pollution from heavy metals has increased in recent decades and has become an important concern for environmental agencies. Arsenic, cadmium, copper, mercury and lead are among the trace elements that have the greatest impact and carry the highest risk to human health. Electrothermal atomic absorption spectrometry (ETAAS) has long been used for trace element analyses and over the past few years, the main constraints of atomic absorption spectrometry (AAS) methods, namely matrix interferences that provoked high background absorption and interferences, have been reduced. The use of new, more efficient modifiers and in situ trapping methods for stabilization and pre-concentration of these analytes, progress in control of atomization temperatures, new designs of atomizers and advances in methods to correct background spectral interferences have permitted an improvement in sensitivity, an increase in detection power, reduction in sample manipulation, and increase in the reproducibility of the results. These advances have enhanced the utility of Electrothermal atomic absorption spectrometry (ETAAS) for trace element determination at μg L⁻¹ levels, especially in difficult matrices, giving rise to greater reproducibility, lower economic cost and ease of sample pre-treatment compared to other methods. Moreover, the recent introduction of high resolution continuum source Electrothermal atomic absorption spectrometry (HR-CS-ETAAS) has facilitated direct solid sampling, reducing background noise and opening the possibility of achieving even more rapid quantitation of some elements. The incorporation of flow injection analysis (FIA) systems for automation of sample pre-treatment, as well as chemical vapor generation renders (ETAAS) into a feasible option for detection of As and Hg in environmental and food control studies wherein large numbers of samples can be rapidly analyzed. A relatively inexpensive approach with low sample consumption provide additional advantages of this technique that reaches figures of merit equivalent to Inductively coupled plasma mass spectrometry (ICP-MS). Herein is presented an overview of recent advances and applications of (ETAAS) for the determination of As, Cd, Cu, Hg and Pb in biological samples drawn from studies over the last decade.     

Prospects and problems in absolute analysis by electrothermal atomic absorption spectrometry

The most significant achievements in the development of methods of absolute analysis in electrothermal atomic absorption spectrometry (ET AAS) made in the recent five years are discussed. Problems requiring further investigation and the significance of the concept of absolute analysis in the evolution of ET AAS are pointed out.     

Critical evaluation of analytical performance of atomic absorption spectrometry and inductively coupled plasma mass spectrometry for mercury determination

The analytical performance of cold vapor atomic absorption spectrometry (CV AAS), graphite furnace atomic absorption spectrometry (GF AAS) and inductively coupled plasma mass spectrometry (ICP-MS) for mercury determination have been investigated with the use of two reference materials SRM 2710 Montana I Soil and BCR-144R (sewage sludge from domestic origin). The digestion conditions and their influence on determination of mercury have been studied. Samples were decomposed by microwave digestion in closed vessels with the use of HCl alone or mixture of HCl+HNO₃+HF. The digestion solutions were analyzed by CV AAS using NaBH₄ as a reducing agent, by GF AAS with Pd or mixture of Pd/Rh as modifiers and by ICP-MS with Rh as internal standard. In the case of CV AAS, results were not dependent on digestion conditions. In the case of GF AAS and ICP-MS, results depended significantly on digestion conditions; in both cases, the use of the mixture of acids as defined above suppressed the signal of mercury. Therefore, in those cases, the microwave digestion with HCl is recommended. Detection limits of 0.003, 0.01 and 0.2 μg g⁻¹ were achieved by ICP-MS, CV AAS and GF AAS, respectively.