Determination of yttrium and rare-earth elements in rocks by graphite-furnace atomic-absorption spectrometry

With use of synthetic solutions and several international standard reference materials a method has been developed for determining traces of Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu in rocks by electrothermal atomization in a pyrolytically-coated graphite furnace. Depending on the element, the sensitivity is of the order of 10(-9)-10(-12) g at 2500 degrees . To avoid matrix interferences the lanthanides are separated from the common elements by co-precipitation with calcium and iron as carriers. The data for Canadian reference rock SY-2 (syenite), U.S.G.S. reference rocks W-2 (diabase), DNC-1 (diabase) and BIR-1 (basalt), and South African reference rock NIM-18/69 (carbonatite) obtained by graphite-furnace atomization are compared with the values obtained by flame atomic-absorption. The results are in good agreement with literature values.     

The determination of lead in 13 USGS standard rocks

Lead was determined in 13 U.S. Geological Survey standard rocks by graphite furnace atomization and atomic-absorption spectrometry after extraction of lead with diethylammonium diethyldithiocarbamic acid. An analysis of variance of the results obtained from a random sampling of three different bottles of each standard rock showed no heterogeneity among bottles, at the F(0.95) level, for a 100-mg sample-size. The relative error of the method, based on the standard deviation of the mean within bottles, was generally less than 10%.     

Electronic structure and prediction of atomization energy of naked homoleptic uranium hexacarbonyl U(CO)6

Our ab initio all-electron fully relativistic Dirac-Fock and nonrelativistic Hartree-Fock self-consistent field (SCF) calculations predict the octahedral (Oh) uranium hexacarbonyl U(CO)6 to be bound with the calculated atomization energy of 49.84 and 48.76 eV at the predicted U-C bond lengths (assuming the C-O bond distance fixed at 1.17 A) of 2.53 and 2.63 A, respectively. Moreover, our all-electron fully relativistic Dirac-Fock SCF calculations predict U(CO)6 to be lower in energy by 3.90 eV with respect to dissociation into U plus six CO ligands. We predict U(CO)6 (Oh) to be very stable in view of our predicted large atomization energy (approximately 49 eV) and stability (approximately 4 eV) with respect to dissociation into U plus six CO molecules. Innovative techniques should be devised for the synthesis of uranium hexacarbonyl since the usual synthetic methods have failed so far for this naked actinide hexacarbonyl.     

Determination of nanogram amounts of bismuth in rocks by atomic absorption spectrometry with electrothermal atomization

Bismuth concentrations as low as 10 ng g^-1 in 100-mg samples of geological materials can be determined by atomic absorption spectrometry with electrothermal atomization. After HF—HClO₄ decomposition of the sample, bismuth is extracted as the iodide into methyl isobutyl ketone and is then stripped with ethylenediaminetetraacetic acid into the aqueous phase. Aliquots of this solution are pipetted into the graphite furnace and dried, charred, and atomized in an automated sequence. Atomic absorbance at the Bi 223.1-nm line provides a measure of the amount of bismuth present. Results are presented for 14 U.S. Geological Survey standard rocks.     

Structures and bonding in K0.91U1.79S6 and KU2Se6

The compounds K0.91U1.79S6 and KU2Se6, members of the AAn2Q6 actinide family (A = alkali metal or Tl; An = Th or U; Q = S, Se, or Te), have been synthesized from US2, K2S, and S at 1273 K and U, K2Se, and Se at 1173 K, respectively. KU2Se6 shows Curie-Weiss behavior above 30 K and no magnetic ordering down to 5 K. The value of mu(eff) is 2.95(1) mu(B)/U. Its electronic spectrum shows the peaks characteristic of 5f-5f transitions. It is a semiconductor with an activation energy of 0.27 eV for electrical conduction. Both K0.91U1.79S6 and KU2Se6 crystallize in space group Immm of the orthorhombic system and are of the KTh2Se6 structure type. Both contain infinite one-dimensional linear Q-Q chains characteristic of the AAn2Q6 family. Typical of the known AAn2Q6 compounds, in KU2Se6, there are two alternating Se-Se distances of 2.703(2) and 2.855(2) A, both much longer than an Se-Se single bond. In contrast, in K0.91U1.79S6, the first sulfide of this family to be characterized structurally, there are alternating normal S2(2-) pairs 2.097(5) A in length. In K0.91U1.79S6, the formal oxidation state of U is 4+.     

Investigation of interference mechanism of cobalt chloride on the determination of bismuth by electrothermal atomic absorption spectrometry

The interferences of cobalt chloride on the determination of bismuth by electrothermal atomic absorption spectrometry (ETAAS) were examined using a dual cavity platform (DCP), which allows the gas-phase and condensed phase interferences to be distinguished. Effects of pyrolysis temperature, pyrolysis time, atomization temperature, heating rate in the atomization step, gas-flow rate in the pyrolysis and atomization steps, interferent mass and atomization from wall on sensitivity as well as atomization signals were studied to explain the interference mechanisms. The mechanism proposed for each experiment was verified with other subsequent sets of experiments. Finally, modifiers pipetted on the thermally treated sample+interferent mixture and pyrolyzed at different temperatures provided very useful information for the existence of volatilization losses of analyte before the atomization step. All experiments confirmed that when low pyrolysis temperatures are applied, the main interference mechanisms are the gas-phase reaction between bismuth and decomposition products of cobalt chloride in the atomization step. On the other hand, at elevated temperatures, the removal of a volatile compound formed between analyte and matrix constituents is responsible for some temperature-dependent interferences, although gas-phase interferences still continue. The experiments performed with colloidal palladium and nickel nitrate showed that the modifier behaves as both a matrix modifier and analyte modifier, possibly delaying the vaporization of either analyte or modifier or both of them.     

Simultaneous multielement atomic-absorption analysis of biological materials

A prototype multielement atomic-absorption spectrometer (SIMAAC) consisting of a continuum source and an echelle polychromator modified for wavelength modulation, has been used to determine several elements in a variety of biological materials. Analyses have been done with flame atomization as well as graphite furnace atomization. Compromise atomization conditions do not significantly limit the accuracy or precision. Analytical results for a variety of samples are reported.     

Bond Energy Scheme for Estimating Heats of Formation of Monomers and Polymers. VI. Sulfur Compounds

The bond energy scheme is extended to sulfur compounds and heats of formation and atomization energy terms derived from thermochemical data reviewed to 1977, for bonds of sulfur with carbon, hydrogen, halogens, and oxygen atoms. A precision of ± 1 kcal/mole was attainable for the covalent bonds of divalent sulfur in the lowest oxidation state S(± II). The higher valency states: S(IV) and S(VI) involve polar contributions depending upon the electrouegativity of the combining atom as well as (d^π -p^π) orbital promotion energies which are specific to the compound and transferable to other molecules only with a limited precision, no better than about ± 3 kcal/mole. The atomization energy terms (E_a ^25°C) of various bonds of sulfur a are found consistent with the experimental bond dissociation energies and bear a relationship with bond lengths and force constants as observed in the previous work. Heats of polymer-forming reactions and heats of formation of sulfur-containing monomers and polymers are estimated from the newly derived bond energy terms.     

Direct determination of lead in bovine liver by solid sampling with graphite furnace atomic absorption spectrometry—a comparison of tube wall atomization, platform atomization and probe atomization

A comparative study of three atomization techniques is presented: tube wall atomization, platform atomization and probe atomization, in graphite furnace atomic absorption spectrometry for direct determination of lead in bovine liver. Also presented is the effect of oxygen ashing on the lead atomic absorption signal and on the background absorption due to the organic matrix of bovine liver. Study of the lead atomic absorption pulses given by the three atomization techniques using a detection system having a short time constant relative to the atomization and the residence time of lead from Bovine Liver, SRM No. 1577, has provided a new insight into the atomization processes involved in the techniques. The importance of isothermal atomization in reducing or eliminating matrix interferences and the role of oxygen gas in the ashing cycle are presented and discussed.     

Vaporization and atomization of uranium in a graphite tube electrothermal vaporizer: a mechanistic study using electrothermal vaporization inductively coupled plasma mass spectrometry and graphite furnace atomic absorption spectrometry

The mechanism of vaporization and atomization of U in a graphite tube electrothermal vaporizer was studied using graphite furnace atomic absorption spectrometry (GFAAS) and electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS). Graphite furnace AAS studies indicate U atoms are formed at temperatures above 2400°C. Using ETV-ICP-MS, an appearance temperature of 1100°C was obtained indicating that some U vaporizes as U oxide. Although U carbides form at temperatures above 2000°C, ETV-ICP-MS studies show that they do not vaporize until 2600°C. In the temperature range between 2200°C and 2600°C, U atoms in GFAAS are likely formed by thermal dissociation of U oxide, whereas at higher temperatures, U atoms are formed via thermal dissociation of U carbide.The origin of U signal suppression in ETV-ICP-MS by NaCl was also investigated. At temperatures above 2000°C, signal suppression may be caused by the accelerated rate of formation of carbide species while at temperatures below 2000°C, the presence of NaCl may cause intercalation of the U in the graphite layers resulting in partial retention of U during the vaporization step. The use of 0.3% freon-23 (CHF₃) mixed with the argon carrier gas was effective in preventing the intercalation of U in graphite and U carbide formation at 2700°C.     

Some observations on the interdependence of cyanide concentration and atomization efficiency in the nitrous oxide-acetylene flame

A relationship is derived between CN concentration and atomization efficiency in the C₂H₂-N₂O flame. The relationship is examined under various experimental conditions and β values are estimated for a number of elements. The possibility of complex vapour phase oxide formation by U is considered.     

Atomic-absorption determination of beryllium in geological materials by use of electrothermal atomization

A method is described for the atomic-absorption determination of beryllium in geological materials, that utilizes electrothermal atomization after a separation by solvent extraction. Samples are decomposed with hydrofluoric acid and nitric acid in Teflon-lined pressure decomposition vessels. Beryllium is isolated by its extraction as beryllium acetylacetonate at pH 8 into xylene and back-extraction in 3M hydrochloric acid. The method has been successfully applied to the determination of beryllium in 14 U.S. Geological Survey standard rocks. Four subsamples from four bottles of each standard sample were analysed in random order. The mean beryllium contents (ppm) are: AGV-1, 1.98; PCC-1, 0.024; MAG-1, 2.84; BHVO-1, 0.90; DTS-1, 0.026; SCo-1, 1.74; SDC-1, 2.52; BCR-1, 1.44; GSP-1, 1.22; SGR-1, 0.86; QLO-1, 1.83; RGM-1, 2.21; STM-1, 8.75; G-2, 2.29. An analysis of variance shows that all the samples may be considered homogeneous at F(0.95) except AGV-1 and DTS-1 which may be considered homogeneous at F(0.99).     

Trace metal assay of U(3)O(8) powder by electrothermal AAS

Methods have been developed for the direct determination of Ag, Ca, K., Li, Mg, Na, Pb, Sn and Zn in U(3)O(8) powder samples by electrothermal AAS. Nanogram and lower amounts of these elements have been determined with a relative standard deviation of 6-16% in mg amounts of sample (either alone or mixed with an equal weight of graphite). The results for NBL reference samples were in reasonable agreement with the certified values. X-Ray diffraction studies on the residues left from the graphite mixtures after the atomization cycle, confirmed the formation of uranium carbide (UC(2)).     

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.     

Tautomeric equilibria of 2‐ and 4‐thiouracil in gas phase and in solvent: A density functional study

The relative stabilities of the five favored tautomers of 2‐ and 4‐thiouracil in gas phase and in water solution were determined by density functional theory employing the Becke, Lee, Yang, and Parr (B3LYP) exchange–correlation potential and the three 6‐31G(d,p), 6‐311++G(d,p), and triple‐zeta valence (TZVP) basis sets. Zero‐point vibrational corrections were also computed. Bulk solvent effects were studied in the framework of the self‐consistent reaction field approach by the polarizable continuum model. All calculations indicate that the most stable tautomer for both species, in the gas phase as well as in solution, has the oxo‐thione form, in full agreement with the previous ab initio and experimental studies. The tautomeric stability orders obtained in the aqueous solution are sensibly different from that in the gas phase. At B3LYP/6‐311++G(d,p) level in the gas phase, the following orders of stability for 2‐ and 4‐thiouracil tautomers were observed, respectively: S2U1>S2U2>S2U4>S2U5>S2U3 and S4U1>S4U2>S4U3>S4U4>S4U5. The corresponding trends in the aqueous phase are S2U1>S2U3>S2U2>S2U5>S2U4 and S4U1>S4U2>S4U3>S4U5>S4U4. On the basis of the computed energy differences we can hypothesize that only the oxo‐thione forms of 2‐ and 4‐thiouracil should exist in the gas phase and in water solution. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 44–52, 2001     

Role of metal matrix modifier in ashing and beginning of the atomization process in graphite furnace-atomic absorption spectrometry

Summary It has been shown that Pb, Sn and In form alloys with the Pd matrix modifier during the ashing and the beginning of the atomization process in graphite furnace atomic absorption spectrometry.Pb and Sn were chosen as analytes and Ag, Sb, Cu, Au, Pt, Pd, Cd, and Mg as co-existing elements or matrix modifiers. The activity coefficients of Pb in the alloys Pb-Ag and Pb-Sb are similar to the value of Pb alone (or about 1.0), and those in the alloys Pb-Au, Pb-Pt and Pb-Mg are lower than the value of 1.0; in particular the activity coefficients of Sn in the alloy Sn-Pd is extremely low. The activity coefficients of Pb in the alloys Pb-Cd and Pb-Cu are higher than 1.0.The movement of volatilization to higher effective temperatures in the atomization were studied; it was found that: 1) Where the activity coefficient of the analyte was lower than 1.0, intermetallic compounds were formed and the atomization shifted to higher temperatures. 2) Atomization was not altered (even though the activity coefficients were different from 1.0) if the modifier elements formed alloys with Pb, which had melting temperatures lower than the ashing and the initial temperatures of the atomization of Pb. 3) For metals such as Mg, which are neither reduced to metal nor form alloys with the analyte during the ashing and the atomization process, the role as matrix modifier is different, as has also been studied herein.     

The Application of Rapid Furnace Atomic Absorption Spectrometric Method in Water Analysis and Quality Control

In recent years, the principle of a modern atomization system generally called the stabilized temperature platform furnace (STPF) is able to satisfy a number of physical requirements when approaching isothermal conditions during the atomization process. Fast furnace programs, in which the drying and ashing times are minimized or the ashing step is eliminated, are attractive because they significantly shorten overall analytical time and increase analytical throughout.     

Atomization from a tantalum surface in graphite furnace atomic absorption spectrometry

The mechanism of atom formation of U, V, Mo, Ni, Mn, Cu and Mg atomized from pyrolytic graphite and tantalum metal surfaces has been studied. The mechanism of atom formation for U from a graphite tube atomizer is reported for the first time. The peak absorbance for U and Cu is increased by factors of 59.7 and 2.0, respectively, whereas that of V, Mo and Ni is reduced by several orders of magnitude when they are atomized from a tantalum metal surface. The peak absorbance of Mn and Mg is not appreciably affected by the material of the atomization surface. Interaction of Mn and Mg with the graphite surface and formation of their refractory carbides was found to be negligible. Uranium forms a refractory carbide when heated from a graphite surface.     

Application of a fractional factorial design for the determination of cadmium by ETA-AAS in different atomization systems

A multivariate study was applied to the investigation of the variables affecting the cadmium sensitivity in a graphite furnace, with and without platform atomization or matrix modifier. The results indicated that, depending on the atomization system used, the factors studied (drying, ashing, atomization, calibration curve) exert an influence on the analysis. The interactions between the factors were also analyzed.     

Studies on the sensitivity of yttrium by electrothermal atomization from metallic and metal-carbide surfaces of a heated graphite atomizer in atomic absorption spectrometry

Atomization of yttrium in tube-type electrothermal atomizers was studied using various atomization surfaces: pyrocoated graphite surface, carbidized graphite surface and tantalum or tungsten metal surfaces. Carbidizing of pyrocoated graphite tubes with other carbide-forming metals (Ta, Zr or La) produces refractory metal-carbide surfaces thereby preventing the carbide-forming yttrium to come in physical contact with the reactive graphite surface. The result is an enhancement in the analytical sensitivity (peak height absorbance) of yttrium. The atomization of Y from a metal surface (Ta or W) gives better analytical sensitivity, lower atomization temperature, and negligible memory effect compared with those from metal-carbidized surfaces.