Determination of selenium in water by electrothermal atomic absorption spectrometry

Summary The electrothermal atomization of selenium has been investigated for the accurate determination of selenium in water samples. Hydrogen seriously affects the atomization temperature of selenium in a molybdenum micro-tube atomizer. The atomization of selenium also suffers from serious interferences caused by salts and other elements. The extraction of selenium diethyldithiocarbamate complex serves to eliminate the interferences from the matrix. The addition of copper allows the suppression of interferences from elements extracted with selenium. The method permits the determination of selenium(IV) and selenium(VI) separately.This research was in part funded by the Ministry of Education, Science and Culture, Japan, under Grant-in-Aid for Scientific Research, for which we express our appreciation.     

Investigation of reactions involved in graphite furnace atomic absorption procedures: Part 12. A study of some factors influencing the determination of selenium

An experimental and theoretical study of various factors influencing the determination of selenium by graphite furnace atomic absorption spectrometry (g.f.a.a.s.) is reported. It is shown that the atomization efficiency can be increased as compared to the L'vov platform technique by means of a constant temperature furnace as a consequence of the possibility of choosing a higher atomization temperature. This is explained by means of high temperature equilibrium calculations, which include the formation of the thermodynamically relatively stable gaseous diselenium, hydrogen selenide and selenium sulphide. The extent of losses of selenium during thermal pretreatment was established by measurements with ⁷⁵Se for different types of selenium compounds, Se(-II)-methionine, selenite and selenate, in aqueous solutions as well as in chloride and sulphate matrices. It is shown that the addition of 20 μg of nickel is effective in stabilizing Se(IV) and Se(VI) in the presence of sodium chloride, sodium sulphate and pure water. However, in the presence of both an organic matrix (glucose) and sodium chloride, nickel is shown to lose its stabilizing effect.     

Mechanism of interference elimination by thiourea in electrochemical atomic absorption spectrometry

The mechanism of interference elimination by thiourea in electrothermal atomization is discussed. Activation energies of atomization were measured. The experimental values for bismuth, lead, copper and cadmium were not altered in the presence of concomitants, provided that thiourea was added before atomization. These elements from complexes with thiourea which are converted to sulphides during the charring stage. Atom formation occurs from the sulphides without compound formation between analyte and concomitants.     

Peak profile characteristics and atomization mechanisms for selenium in the presence of mercury for graphite furnace atomic absorption spectrometry

Summary Time resolved absorbance profiles, as well as the effects of inorganic acids and mercury salts, have been studied for selenium graphite furnace atomic absorbance signals. Conventional furnace operating procedures and wall atomization were used. Absorbance profiles were found to be sensitive to the mass of the counter ion and mercury present. Negative shifts in the appearance temperature were noted for low levels of mercury salts, whilst the higher levels of mercury caused only slight increasing shifts in the appearance temperature. Peak-area absorbance increases with mercury mass, but a steady-state absorbance was reached above a certain mercury chloride concentration. If mercury is present, a thermal dissociation of a mercury selenium compound is the suggested Se atomization mechanism.     

Comparison of modifiers for determination of arsenic, antimony and selenium by atomic absorption spectrometry with atomization in graphite tube or hydride generation and in-situ preconcentration in graphite tube

The study was performed to compare the effect of magnesium modifier (magnesium nitrate) with that of other modifiers (palladium nitrate and nickel nitrate) in determination of arsenic, antimony and selenium by atomic absorption spectroscopy with atomization in a graphite tube, with generation of hydrides and in situ preconcentration in a graphite tube. The assumed criterion of a modifier performance was the magnitude of the analytical signal. It was found that in determinations with atomization in a graphite furnace the effects of all these modifiers were comparable, while in those with hydride generation and in situ preconcentration in a graphite tube the magnesium modifier showed poorer performance (25% decrease of the analytical signal). In determinations of arsenic and selenium the analytical signal obtained with magnesium salt as a modifier was comparable with those obtained in the presence of all other modifiers.     

Influence of volatile hydride-forming elements on antimony determination by atomic-absorption spectrometry with hydride-generation

A study was undertaken to determine the interfering effects of arsenic, bismuth, germanium, lead, selenium, tin and tellurium on trace determination of antimony by atomic-absorption spectrometry with hydride-generation. A 1% NaBH(4) solution was used as reductant and a small amount of oxygen was added to the hydrogen produced, to support the combustion and atomization of SbH(3). The interference from selenium in the determination of antimony is removed if potassium iodide-ascorbic acid solution or copper sulphate is added to the sample solution. The interference of tin and tellurium can also be avoided by adding potassium iodide-ascorbic acid solution. A possible interference mechanism is discussed.     

Mechanism of modification by palladium in graphite furnace atomic absorption spectrometry

The effects of palladium and mixtures containing palladium on the absorbance characteristics of lead, thallium, cadmium, selenium, manganese and cobalt are described. These data, together with results of scanning electron microscopy showing the distribution of palladium on the graphite surface, indicate that palladium has a physical mechanism of analyte modification. During furnace heating, the analyte dissolves in molten palladium and may combine with it chemically. However, the rate limiting step leading to atomization appears to be diffusion of the analyte from palladium. The addition of magnesium, molybdenum or powdered carbon increases the speed of diffusion by causing palladium to form smaller droplets, and hence produces sharper absorbance peaks. Palladium becomes less effective as the atomization temperature increases, because the rate of diffusion is higher. This accounts for palladium having only a small stabilizing effect on less volatile elements such as manganese and cobalt. The addition of ascorbic acid to palladium has no significant effect on its modifying properties in a dilute nitric acid matrix. Results of kinetic studies on the atomization of gold are consistent with analyte diffusion out of palladium as the rate-limiting step leading to atomization.     

Matrix modification with silver for the electrothermal atomization of arsenic and selenium

Silver as a matrix modifier is shown to improve the carbon-rod atomization of both arsenic and selenium for atomic absorption spectrometry. Compared to nickel, the efficiency of silver is greater for arsenic and about the same for selenium. Silver fulfils two functions in its reaction, namely stabilization during the ashing stage and enhancement of absorbance in the final atomization.     

The relation of double peaks, observed in quartz hydride atomizers, to the fate of free analyte atoms in the determination of arsenic and selenium by atomic absorption spectrometry

The mechanism at the origin of double peaks formation in quartz hydride atomizers were investigated by continuous flow hydride generation atomic absorption spectrometry. Arsenic and selenium were used as model analytes. The effect of atomization mode (flame-in-gas-shield (FIGS), miniature diffusion flame and double flame (DF)) and some experimental parameters as oxygen supply rate for microflame and the distance from atomization to free atoms detection point, were investigated on the shape of both analytical signals and calibration graphs. Rollover of calibration graphs and double peak formation are strictly related each to the other and could be observed only in FIGS atomizer mode under some particular conditions. A mechanism based on incomplete atomization of hydrides cannot explain the collected experimental evidences because the microflame of FIGS is able to produce quantitative atomization of large amount of hydrides even at supply rate of oxygen close to extinction threshold of microflame. The heterogeneous gas–solid reactions between finely dispersed particles, formed by free atom recombination, and the free atoms in the gaseous phase are at the origin of double peak formation.     

Investigation of phosphorus atomization using high-resolution continuum source electrothermal atomic absorption spectrometry

The atomization of phosphorus in electrothermal atomic absorption spectrometry has been investigated using a high-resolution continuum source atomic absorption spectrometer and atomization from a graphite platform as well as from a tantalum boat inserted in a graphite tube. A two-step atomization mechanism is proposed for phosphorus, where the first step is a thermal dissociation, resulting in a fast atomization signal early in the atomization stage, and the second step is a slow release of phosphorus atoms from the graphite tube surface following the adsorption of molecular phosphorus at active sites of the graphite surface. Depending on experimental conditions only one of the mechanisms or both might be active. In the absence of a modifier and with atomization from a graphite or tantalum platform the second mechanism appears to be dominant, whereas in the presence of sodium fluoride as a modifier both mechanisms are observed. Intercalation of phosphorus into the graphite platform in the condensed phase has also been observed; this phosphorus, however, appears to be permanently trapped in the structure of the graphite and does not contribute to the absorption signal.     

Kinetics of selenium atomization in electrothermal atomization atomic absorption spectrometry (ETA-AAS). Part 3: Chemical interference of sulphate using palladium modifiers

The interference caused by sulphate (as the sodium salt) in the electrothermal atomization atomic absorption analysis of selenium was investigated for prereduced and unreduced palladium nitrate modifiers. Kinetic parameters of the selenium atomization were calculated for both types of modifier with varying amounts of sulphate added. Prereduced palladium was a better modifier since it tolerated higher amounts of interferent. For high levels of interferent, the kinetic parameters approached that of selenium without modifier. It was postulated that the interference was caused by the formation of palladium sulphate which reduces the number of active palladium sites available for selenium stabilization. The poorer performance of the unreduced modifier was explained in that the formation of stabile palladium sulphate hindered the reduction of Pd(II) to palladium metal which was needed for the selenium stabilization. Sulphate only interfered on the high temperature stabilization process; the low temperature stabilization, linked to the formation of a [Pd,Se,O] compound, was unaffected. The results support earlier literature reports that selenium loss occurs by covolatilization with the matrix and gives a reason why palladium modifiers are rendered useless by the sulphate interferent.     

Simultaneous determination of Se and As by hydride generation atomic absorption spectrometry with analyte concentration in a graphite furnace coated with zirconium

Conditions for the simultaneous determination of selenium and arsenic at ng l⁻¹ level were developed. Simultaneous determination of these elements was possible through the multielement capabilities of hydride generation, with in situ trapping and atomization in a graphite tube coated with zirconium and measurements using a dual channel atomic absorption spectrophotometer. The zirconium coating employed in this work was relatively stable; once formed it could stand ≈80 firings without any significant change in the efficiency of hydride collection. This appears to be an advantage over the palladium coating, which is usually formed individually before each measurement because of its thermal instability during the atomization step of the furnace temperature programme. As a result, two determinations of the two elements could be performed in 2.5 min. Under the optimized conditions, concentration detection limits of 17 and 13 ng l⁻¹ for a 7.1 ml sample volume were obtained for selenium and arsenic, respectively (absolute detection limits 120 and 92 pg for Se and As).     

Study of the electrothermal atomization of selenium (IV) in transversely-heated graphite atomizers in the presence of nickel compounds as chemical modifiers

The electrothermal atomization of selenium, in transversely-heated graphite atomizers, was studied with regard to the effect of the addition of nickel nitrate and nickel chloride as chemical modifiers. Particular importance was given to the behavior of the analyte in aqueous standards and in sodium chloride containing solutions, after thermal reduction of the modifiers prior to the injection of the analyte solution into the atomizer. Thermal reduction of either nickel compound brings about an enhancement in the sensitivity of selenium measurements, as compared to those obtained by the injection of both the analyte and the modifier together into the atomizer. Additionally, thermal reduction of the modifiers permits the presence of chloride, as sodium chloride, to be tolerated in amounts as high as 500 μg Cl⁻ with sensitivity losses lower than 10%.     

Hydride atomization in a cool hydrogen—oxygen flame burning in a quartz tube atomizer

A method of selenium hydride atomization using a cool highly fuel rich-hydrogen—oxygen diffusion flame burning in the inlet part of a T-shaped quartz tube placed in the optical axis of an atomic absorption spectrometer is investigated. It is shown that hydride atomization is caused by free radicals (H, OH) generated in the flame. Atomization (proceeding probably with 100% efficiency) is completed before the gases reach the optical axis. The life-time of free radicals is considerably reduced by admixture of even small amounts of oxygen to the hydrogen flush gas.A mathematical function describing the dependence of the selenium peak areas on the gas flow is derived and values for the selenium atomic absorption coefficient and rate constant for selenium decay by reactions on the tube walls are obtained by fitting experimental data to this function. The atomic absorption coefficient agrees well with a theoretically estimated value. The described method seems promising for absolute analysis by AAS.     

Electrothermal atomization of palladium stabilized selenium in the presence of phosphate

The interference caused by phosphate (as Na₂HPO₄) in the electrothermal atomic absorption determination of selenium was investigated for reduced and unreduced palladium nitrate modifiers. An increase of the amount of phosphate in the sample was accompanied with increasing losses of selenium. Kinetic parameters of the selenium atomization were calculated for various amounts of phosphate interference. These results were compared with previous findings for palladium stabilized selenium and the sulfate interference observed for this system. The increasing chemical interference is due to phosphorous replacing the selenium bound by palladium. The phosphorus thus makes the palladium surface unavailable for the stabilization of selenium.     

Determination of selenium in technical sulphuric acid by flame atomic absorption spectrometry after electrochemical preconcentration

Electrochemical preconcentration on a platinum filament, followed by atomization in an argon—hydrogen flame with simultaneous electrothermal heating of the filament, is used for the determination of selenium in technical sulphuric acid. The effect of experimental parameters such as acid concentration, deposition potential and temperature are described, and the speciation of selenium in sulphuric acid is discussed. The technical acids were found to contain selenium in the range 30–60 μg l^-1.     

Mechanism of boron atomization in graphite furnace atomic absorption spectroscopy

The mechanism of the atomization of boron and the enhancement of sensitivity by matrix modifier Sr(NO₃)₂ in graphite furnace AAS were discussed. X-ray diffraction and thermodynamic calculation were applied to study the mechanism of boron atomization with and without matrix modifier Sr(NO₃)₂. The formation of boron atom is due to the sublimation of solid boron which derived from the reduction of B₂O₃ by carbon. The enhancement of boron signal in the presence of Sr(NO₃)₂ is due to the formation of SrB₆ before atomization, which decreased the volatization losses of B₂O₃ and retarded the formation of B₄C.     

Nickel and strontium nitrates as modifiers for the determination of selenium in wine by Zeeman electrothermal atomic absorption spectrometry

A mixed matrix modifier of nickel and strontium nitrates was used as a chemical modifier for the determination of selenium in wines by Zeeman electrothermal atomic absorption spectrometry. Wine samples were heated on a boiling water bath with small amounts of nitric acid and hydrogen peroxide. For complete elimination of interference, especially from sulfates and phosphates, selenium is complexed with ammonium pyrolidinedithiocarbamate (APDTC), extracted into methyl isobutyl ketone (MIBK), and measured by ETAAS. The graphite furnace temperature program was optimized for both aqueous and organic solutions. Pyrolysis temperatures of 1300 degrees C and 800 degrees C were chosen for aqueous and organic solutions, respectively; 2700 degrees C and 2100 degrees C were used as optimum atomization temperatures for aqueous and organic solutions, respectively. The optimum modifier mass established is markedly lower than those presented in the literature. The platform atomization ensures pretreatment stabilization up to 1100 degrees C and 1600 degrees C, respectively, for organic and aqueous selenium solutions. The procedure was verified by the method of standard addition. The investigated wine samples originated from the different regions of the Republic of Macedonia. The selenium concentration varied from not detectable to 0.93 microg L(-1).     

Graphite-furnace atomization of phosphorus

The slopes of the ln (absorbance) vs. T^?1 dependences for platform/graphite furnace atomization of phosphorus in the presence of Ni²⁺ or La³⁺ modifiers are measured, and found to be similar to the theoretical slope calculated for isothermal atomization. It is concluded that in both cases the atomization mechanism is the same and is expressed by P_2(gas) ? 2P_(gas); P_(gas) ? P^*_(gas). The atomic absorption signal for phosphorus appears and reaches its maximum later when the atomization is carried out from the platform in the presence of modifiers. Hence atomization takes place under nearly isothermal conditions and is much more efficient, thus providing the best conditions for the determination of phosphorus. Treatment of a deteriorated graphite surface with ZrOCl₂ solution repairs any defects and improves the sensitivity of phosphorus determination.     

Flameless Atomic Absorption Determination of Volatile Hydrides Using Cold Trap Collection

Abstract

The hydrides of arsenic, antimony, bismuth, and selenium are collected in a liquid nitrogen cold trap and then volatilized into either an argon-entrained air-hydrogen flame or into a Perkin-Elmer HGA-2000 Graphite Furnace for atomic absorption measurement. Flameless atomization results in approximately ten-fold lower detection limits. The sensitivities and detection limits in nanograms are, respectively, 1.0 and 0.2 for arsenic, 5.6 and 1 for antimony, 2.0 and 1 for bismuth, and 40 and 10 for selenium.