STM and STS Studies of One-Dimensional Row Growth: Ag on Si(5 5 12)

We have studied the growth of Ag on Si(5 5 12) using scanning tunneling microscopy and spectroscopy (STM/STS). At metal coverages as low as 0.05 monolayer (ML), Ag forms well-ordered overlayer rows, or one-dimensional clusters, on the underlying silicon surface. To produce these ordered structures, it is necessary to anneal the surface to 450°C. As the coverage is increased above 0.05 ML, the rows grow in length and number until the surface forms a periodic array of such structures at 0.25 ML. A statistical analysis of the rows reveals a linear increase in median row length as a function of coverage. With regard to their electronic behavior, STS measurements show a significantly narrower band gap along the Ag rows than is found on the underlying silicon structures. Therefore, the deposited Ag atoms do retain some metallic behavior.     

Formation of YAG from coprecipitated yttrium aluminium hydroxides

Thermal decomposition of the amorphous coprecipitate of yttrium and aluminium hydroxides forming yttrium aluminium garnet has been investigated employing thermal analyses, X-ray diffraction and IR spectroscopy. On heating, the coprecipitate progressively loses water forming a stable but highly disordered hydroxy garnet which crystallizes at 1180 K and decomposes to YAG at 1290 K. Nucleation of the crystalline phase appears to begin at 800 K.The authors thank Dr. P. V. Ravindran of the Analytical Chemistry and Shri N. K. Kulkarni of the Fuel Chemistry Divisions of BARC for their assistance in the thermoanalytical experiments.     

Reduction of Eu3 + to Eu2 + in Al-Codoped Silica Glasses Fabricated by the Sol-Gel Technique and CO2-Laser Processing

The spectroscopic properties of europium in aluminium codoped silica glasses produced by the sol-gel technique have been studied with respect to the dopant concentrations and the thermal processing applied to the samples. After thermal annealing at temperatures up to 950_°C the bright red fluorescence around 613 nm characteristic for the trivalent europium ions (Eu^{3 +}) has been observed. The lifetime was measured to be 0.1–2.4 ms depending on dopant concentrations and thermal treatment. Subsequent CO₂-laser processing in air (short time remelting) gave rise to a bright blue fluorescence consisting of two broad bands, lying around 450 and 490 nm, with their peak position depending on the ratio between the aluminium and europium concentrations. The fluorescence lifetimes were found to be shorter than 1 s. This blue fluorescence is attributed to the divalent europium ion (Eu^{2 +}), leading to the conclusion that the CO₂-laser processing of europium doped alumina-silica glasses resulted in the reduction of the trivalent to the divalent europium ion. Laser processing could therefore be a valid alternative to conventional thermal annealing for the generation of Eu^{2 +} in alumina-silica glasses.     

Multielement radiochemical neutron activation analysis of high purity aluminium

Summary A radiochemical neutron activation analysis technique for the determination of 51 elements in high purity aluminium via medium- and long-lived indicator radionuclides has been developed. It is based on a separation procedure involving the removal of ²⁴Na, produced via the reaction ²⁷Al(n,), on a hydrated antimony pentoxide column and further separation of the radionuclide mixture into 11 groups on two Dowex 1X8 columns and one Dowex 50X8 column from HCl/HF and HCl media as well as the extraction of copper with APDTC in chloroform. The indicator radionuclides for the significant impurities thorium and uranium, ²³³Pa and ²³⁹Np, were separated with satisfactory selectivity in one fraction. For 43 elements, the limits of detection are <10 ng/g, for thorium and uranium, they are 50 pg/g. The method was applied to the analysis of different high purity aluminium samples. For a number of elements, the results of this technique are compared with those of other techniques.

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Radiochemische Multielement-Neutronenaktivierungsanalyse von hochreinem Aluminium

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Dedicated to Prof. Dr. W. Fresenius on the occasion of his 75th birthday     

Synthesis and thermal stability of hydrotalcites based upon gallium

Hydrotalcites based upon gallium as a replacement for aluminium in hydrotalcite over a Mg/Al ratio of 2:1 to 4:1 were synthesised. The d(003) spacing varied from 7.83 Å for the 2:1 hydrotalcite to 8.15 Å for the 3:1 gallium containing hydrotalcite. A comparison is made with the Mg/Al hydrotalcite in which the d(003) spacing for the Mg/Al hydrotalcite varied from 7.62 Å for the 2:1 Mg hydrotalcite to 7.98 Å for the 4:1 hydrotalcite. The thermal stability of the gallium containing hydrotalcite was determined using thermogravimetric analysis. Four mass loss steps at 77, 263–280, 485 and 828 °C with mass losses of 10.23, 21.55, 5.20 and 7.58% are attributed to dehydration, dehydroxylation and decarbonation. The thermal stability of the gallium containing hydrotalcite is slightly less than the aluminium hydrotalcite.     

ESR-spectrum of silver atoms stabilized in gamma-irradiated aluminium silicate modified by nickel

ESR technique was used to study the effect of conditions chosen to prepare samples to obtain and stabilize silver atoms forming under the action of⁶⁰Co -irradiation at 77 K in aluminium silicate modified by nickel and containing silver ions. The use of¹⁰⁹Ag isotope made it possible to detect two types of silver atoms Ag_I and Ag_II in -irradiated aluminium silicate modified by nickel and containing silver ions introduced by various techniques. The parameters of the ESR-spectrum of the silver particles under study have been determined and their thermal stability has been investigated.     

Some properties of a microwave boosted glow discharge source using neon as the operating gas

The use of neon as the operating gas for the analysis of aluminium samples with the microwave boosted glow discharge source has been studied. A new type of anode tube allowed the gas to enter the source near the sample surface so that more material was transported into the discharge. Erosion rates have been measured under conditions optimised for high line-to-background ratios and found to be lower than with argon (9 and 21 n/s, respectively). Despite the lower erosion rate the detection limits measured for a number of elements in aluminium are in the range 0.02–1 g/g and comparable to those obtained with argon as the operating gas.     

Flow injection-mini-column technique with ICP-AES detection for the isolation and preconcentration of the fast reactive aluminium fraction in waters

A flow injection mini-column system based on short reaction times with 8-hydroxyquinoline (oxine) with ICP-AES detection is described for the isolation and preconcentration of the fast reactive or toxic aluminium fraction in water samples. Using a 3 s reaction time with oxine (5 × 10^–4 mol/l) at pH 5.0, the fast reactive aluminium fraction is shown not to include the non-toxic AlF²⁺ species at low F^–: Al³⁺ molar ratios (0.3 : 1). The complexed aluminium is isolated in a stable and recoverable form on mini-columns of Amberlite XAD-2 (0.3 cm × 5.0 cm, resin particle size range 0.08 mm–0.16 mm)). The retained aluminium is recovered by back-flushing the analytical column with 1 mol/l HCl for final element specific detection by ICP-AES. Detection limits (after preconcentration) of 2 g/l, a linear range of 0–500 g/l, and possible preconcentration factors of up to 18 times are demonstrated with the present system. Implications for the possible solution of sample stability problems encountered with labile aluminium species analysis and the development of a field sampling technique are discussed, where the desired Al fraction is quantitatively retained in a stable form on mini-columns.     

Analysis of Toxic Aluminium Species in Natural Waters

Aluminium is known to be toxic to a wide range of aquatic organisms under certain conditions. Monomeric hydroxy ions have been found to be primarily responsible for aluminium aquatic toxicity.A survey of aluminium toxicity and a brief discussion of speciation schemes are presented. The fast reaction of Al³⁺ with pyrocatechol violet (PCV) followed by spectrophotometric analysis is a frequently used method for aluminium speciation. By using a flow system, one obtains fairly exact and reproducible control of the reaction time, and as a result it provides a direct method of analysis for free aluminium (including inorganic monomeric aluminium).The PCV-method has been adapted for the determination of aluminium in carbonate-rich natural waters using an improved buffering system. Thus it is possible to monitor aluminium concentrations in lake water as well as in pore water of the sediments of eutrophicated hardwater lakes that has been treated with aluminium salts as a restoration measure.     

INAA of trace impurities in aluminium clads

As a contribution to nondestructive neutron assay of reactor grade aluminium, a number of elements have been investigated qualitatively and quantiatively using a vertical channel in the IRT-5000 reactor with a thermal neutron flux of 7.6·10¹²n·cm^–2·s^–1. The -ray spectra of irradiated samples were analysed with a 30 cm³ Ge(Li) detector connected to an HP-computer and a 4096 channel analyser. The following impurities have been determined: Sc, Ca, Cr, Fe, Ni, Co, Zn, As, Sb, W, Au, Th and U, while Lu and Hf have been determined qualitatively only.     

On-line trace preconcentration / matrix separation by high-performance flow atomic spectrometry (HPF-FLAME AAS)

Summary In HPF-atomic spectrometry a high-performance flow / hydraulic high-pressure nebulization (HPF / HHPN) system is used for sample introduction and aerosol generation. By employment of techniques common in HPLC or ion chromatography, on-line trace element preconcentration / matrix separation and atomic spectrometric trace determinations can be carried out. Preconcentration of trace elements in samples of drinking water allows determinations within the lower g/L region by using flame AAS. On-line trace element preconcentration / matrix separation from aluminium leads to detection limits of approx. 0.1 to 1 g/g within less than 3 min of total analysis time. Dependent on concentration and the element involved, the relative standard deviation amounts to approx. 2 to 4% (2.5 to 25 g/g traces/aluminium).Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday     

Separation of chlorophyll a and metalloporphyrins by HPLC in normal-phase mode using a diol-phase

The isolation of high-purity chlorophyll a was achieved by HPLC using a micro-preparative column with a diol-phase (Polyol, Si 100, 5 m) in normal-phase mode. The procedure included the extraction of photosynthetic pigments from the leaves of stinging nettles and their partial purification as dioxane-water-chlorophyll-aggregates prior to the isolation by micro-preparative HPLC. Freeze-drying resulted in a flaky chlorophyll a, which was free of water. Each step of isolation was controlled by using an analytical HPLC-column with a diol-phase and a photodiode array detector. Synthesized metalloporphyrins (metal(II)-ion = Mg²⁺, Zn²⁺, Cu²⁺, Ni²⁺) with structures analogous to chlorophyll were also isolated by means of the micro-preparative HPLC-column containing a diol-phase. The purity of the obtained porphyrins was monitored with an analytical HPLC-column.Awarded poster prize     

Deep investigation on inorganic fraction of atmospheric PM in Mediterranean area by neutron and photon activation analysis

Background

Anthropogenic activities introduce materials increasing levels of many dangerous substances for the environmental quality and being hazardous to human health. Major attention has been given to those elements able to alter the environment and endanger human health.The airborne particulate matter pollutant is considered one of the most difficult task in environmental chemistry for its complex composition and implications complicating notably the behavior comprehension. So, for investigating deeply the elemental composition we used two nuclear techniques, Neutron Activation Analysis and Photon Activation Analysis, characterized by high sensitivity, precision and accuracy. An important task has been devoted to the investigation of Quality Control (QC) and Quality Assurance (QA) of the methodology used in this study.This study was therefore extended as far back as possible in time (from 1965 until 2000) in order to analyze the trend of airborne concentration of pollutant elements in connection with the industrial and lifestyle growth during the entire period.

Results

Almost all the elements may be attributed to long-range transport phenomena from other natural and/or anthropogenic sources: this behavior is common to all the periods studied even if a very light decreasing trend can be evidenced from 1970 to 2002. Finally, in order to investigate a retrospective study of elements in PM10 and their evolution in relationship with the natural or anthropogenic origins, we have investigated the Enrichment Factors. The study shows the EF trends for some elements in PM10 during four decades.

Conclusions

The two nuclear techniques have allowed to reach elevated sensibility/accuracy levels for determining elements at very low concentrations (trace and ultra-trace levels). The element concentrations determined in this study do not basically show a significant level of attention from a toxicological point of view.

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The correlations among various elements in neutron activation analysis — The evaluation of R-matrix and κ-matrix

The production rates (numbers of atoms per gram of the respective elements per second) of 40 radioactive nuclides of 34 elements by neutron capture reactions in a reactor were determined from about 130 photopeaks of the -ray spectra. The ratios of these production rates were called R-matrix elements. These production rates and the respective thermal neutron capture cross sections were used to calculate the respective apparent neutron fluxes at the position of irradiation and the -matrix elements which were the ratios of these apparent neutron fluxes. These matrix elements express clearly the correlations among various elements and thus may be used in the mono-standard or small-number-standards method in neutron activation analysis.     

Preparation of carrier-free group tracers from a gold target irradiated by high-energy heavy ions

A rapid radiochemical separation procedure has been finished by using of radiochemical separation and -spectrum measurement methods. In this isolation procedure, tantalum nuclides produced by the irradiation of natural tungsten targets with 14 MeV neutrons were extracted by methyl isobutyl ketone (MIBK) in the system of HF-HNO₃ mixed solutions. It has been found that tantalum was rapidly and efficiently separated in the isolation procedure and the decontamination factors of tungsten were more than 10⁴. The procedure is simultaneously used to separate tantalum from tungsten bombarded with intermediate energy (47 MeV/u)¹⁸O beams. The measured -spectra showed that tantalum can be separated from most elements produced in the nuclear reactions except for a few elements.     

β-Substituted cyclopropyl rádicals grafted to silica surface. Methods of preparation,structure, and properties

Cyclopropyl type radicals were identified on a surface of activated silica by ESR. The possibility of their preparation by two methods,i.e., by photocyclization of substituted allyl radicals and by interaction of cyclopropane molecules with particular surface defects, was demonstrated. The effects of -substituents on radiospectroscopic characteristics and thermal stabilities of the radicals were studied. Rate constants and activation energies of decomposition of substituted cyclopropyl radicals were estimated. The experimental data obtained were compared with the results of quantum-chemical calculations of model systems. The mechanism of photocyclization of allyl radicals was discussed. The transition states of decomposition of cyclopropyl and -fluorine-substituted cyclopropyl radicals were calculated. Electronegative substitution was found to result in a considerable decrease in the activation energy of the cyclopropane ring opening.The authors are grateful to E. G. Baskir for calling their attention to this publication.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2409–2421, December, 1995.The work was carried out with financial support of the Russian Foundation for Basic Research (Project code 94-03-09137) and the International Science Foundation (Grant No. MK9000).     

Comparison of supercritical fluid extraction and liquid-liquid extraction for isolation of selected pesticides stored in freeze-dried water samples

Summary The stability of freeze-dried water samples spiked with eight agrochemicals (atrazine, simazine, linuron, carbaryl, propanil, fenitrothion, parathion and fenamiphos) were examined to evaluate their suitability as candidate reference materials for their determination in water samples. In addition, two different extraction procedures, liquid-liquid and supercritical fluid extraction, were compared for the isolation and trace enrichment of target analytes from freeze-dried water samples. Final analytical determinations were by gas chromatography-nitrogen phosphorus detection and electronic impact mass spectrometry, and by liquid chromatography-diode array detection. The whole methodology developed in this paper permitted the determination of pesticides spiked in water at levels varying from 0.03 to 6.9 g L^–1.     

Flicker-Noise Spectroscopy for Analyzing Chaotic Time Series of Dynamic Variables: Problem of Signal-to-Noise Relation

The underlying principles of flicker-noise spectroscopy (FNS), which is a new method for obtaining information contained in chaotic time signals, are presented. The method's essence is the imparting of an information value to sequences of distinguishable irregularities (spikes, jumps, discontinuities of derivatives of various orders) in dynamic variables that characterize systems under test. The information distinguishability of the types of irregularities introduced here follows from the fact that the passport data that characterize the aggregate properties of the irregularities are distinguishably extracted when analyzing power spectra and difference momenta (structural functions) of different orders. Shown are potentialities of the FNS method associated with the introduction of quantitative criteria for the nonstationarity of time series produced by real systems. Revealing such criteria is of interest in connection with the all-pervading character of the nonstationarity phenomenon and with the search for harbingers of most dramatic changes in the state of evolving systems. The FNS method may be used for extracting phenomenological properties when analyzing time series that characterize evolution of complex systems. As an example, the dynamics of the electric potential fluctuation in an electromembrane system with a cation-exchange membrane in the region of the overlimiting current is considered. The introduced techniques of parametrization of a chaotic signal may effectively be used for the signal isolation against a noisy background even in the case where in fact one has to solve the problem of the isolation of two informatively-valuable signals if the information concerning parameters of the target signal is prespecified.