Au-Induced Faceting of Si(5 5 12)

The growth of Au on the stable, high-index Si(5512) surface has been studied using scanning tunneling microscopy (STM). At very low coverages and moderate annealing temperatures (0.1ML, 400–500°C), Au appears to decorate the underlying Si rows and form an array of rows that maintains the underlying (5512) periodicity of 5.4nm. For higher annealing temperatures and coverages, however, Au causes faceting to a number of nearby planes. The two primary facets formed at lower (0.15ML) and higher (0.5–2ML) coverages are the (337) and (225) planes, which are tilted 0.7° down [towards (111)] and 1.1° up from (5512), respectively. Both orientations are in fact subunits of the (5512) unit cell, so their presence is not surprising. In addition to these facets, two types of sawtooth morphologies composed of planes oriented further from (5512) are found at very high annealing temperatures (800–900°C). These include (113)+(7715) planes at very low coverage (0.05ml) and (113)+(5511) planes at higher coverage (1ML), where (113) is tilted up by 5.3° and (7715) and (5511) are tilted down by 2.9° and 2.2°, respectively. Au adsorption on Si(5512) therefore results in the formation of five possible facet planes: (113), (225), (337), (5511), and (7715).     

Electrochemical Determination of 4-Nitrophenol Using a Single-Wall Carbon Nanotube Film-Coated Glassy Carbon Electrode

A single-wall carbon nanotubes (SWNT) film coated glassy carbon electrode (GCE) was fabricated for the direct determination of 4-nitrophenol (4-NP). The electrochemical behaviors of 4-NP at the SWNT-film coated GCE were examined. In 0.1M phosphate buffer with a pH of 5.0, 4-NP yields a very sensitive and well-defined reduction peak at the SWNT-modified GCE. It is found that the SWNT film exhibits obvious electrocatalytic activity towards the reduction of 4-NP since it not only increases the reduction peak current but also lowers the reduction overpotential. Based on this, an electrochemical method was proposed for the direct determination of 4-NP. The reduction peak current varies linearly with the concentration of 4-NP ranging from 1×10^–8 to 5×10^–6M, and the detection limit is 2.5×10^–9M after 3min of open-circuit accumulation. The relative standard deviation at 2×10^–7M 4-NP was about 6% (n=10), suggesting excellent reproducibility. This new method was successfully employed to determine 4-NP in several lake water samples.     

Determination of Caffeine Using a Simple Graphite Pencil Electrode with Square-Wave Anodic Stripping Voltammetry

A simple commercial graphite pencil electrode (GPE) was utilized for monitoring caffeine using the square-wave anodic stripping voltammetry (SWASV) method. This method was applied to determine the caffeine levels in several tea samples, which yielded a relative error of 1% in the concentrations. Caffeine was deposited at 0.0V (vs. Ag/AgCl), then reduced at +1.40V to strip it on the GPE. Optimal experimental conditions for the analysis were found to be as follows: pH value of 9 for the medium; deposition potential of 0.0V; deposition time of 120s; SW frequency of 25Hz; SW amplitude of 45mV, and step potential of 6mV. Given these optimum conditions, a linear range was observed within the concentration of 0500mgL^–1. At caffeine concentrations of 50.0, 250.0, and 500.0mgL^–1, the relative standard deviations in measured concentrations (n=12) were 0.19, 0.09, and 0.11%, respectively. The detection limit was found to be 9.2mgL^–1, which is comparable with the result obtained using a carbon paste electrode, equivalent to 8.2mgL^–1.     

Fluorescence Quenching Studies of Curcumin by Hydrogen Peroxide in Acetonitrile Solution

Summary. Steady state quenching studies of curcumin, 1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione, fluorescence by hydrogen peroxide were conducted in acetonitrile solution. A quenching rate constant, k_q, of 1.05×10¹⁰M^–1·s^–1 was obtained with a short fluorescence lifetime of 347ps. The reaction rate constant, which is within the diffusion-limited regime, is activation-controlled. The rate constant of deactivation of the thermally excited curcumin was 1.2 orders of magnitude more nonradiative (2.67×10⁹s^–1) than radiative (2.16×10⁸s^–1). The reaction was exothermic with a G° of –1.97eV and solvent reorganization energy of 1.37eV. These values indicate that the electron transfer reaction is solvent-mediated with electron transfer rate constant, k_ET, of 2.16×10¹⁰s^–1.     

Determination of Trace Proteins by Rayleigh Light Scattering Technique with Indophenol Blue

The interaction of indophenol blue (IPB) with proteins in aqueous solution has been studied by optical absorption and Rayleigh light scattering (RLS) spectroscopy. At pH 3.8, the weak RLS of IPB is enhanced by proteins. Based on this phenomenon, a novel method for the determination of proteins at nanogram levels using the RLS technique is developed. The method is simple, practical and sensitive. The linear range is 0.25–20.9µgmL^–1 for BSA, and 0.25–17.6µgmL^–1 for HSA. The detection limits (S/N=3) are 23ngmL^–1 for BSA and 22ngmL^–1 for HAS. The results for the determination of proteins in human serum samples are very close to those obtained by an established clinical method. There is very little interference from amino acids, metal ions or other coexisting compounds.     

Selective Spectrophotometric Determination of Aluminium in the Presence of Beryllium and Lanthanide Cations

A spectrophotometric method for the selective determination of Al(III) in the presence of Be and Ln (lanthanide) cations is proposed. It is based on the selective reaction of SXO (Semi-Xylenol Orange) with Al(III) at pH 2.6. The presence of 8%v/v of 1,2-ethanediol serves to stabilize the chelate formed by heating at 100°C for 5min. 0.5mg each of Be(II), Ce(III), La(III), 4mg of Mn(II), 1.2mg of Pb(II), 1mg of Tl(I), 40mg each of Ca(II) and Mg(II) and 1.9mg of sodium dihydrogen phosphate are tolerable. A ligand buffer of HEDTA-Pb is incorporated to further enhance the selectivity of the color reaction. Under the specified conditions SXO reacts with Al(III) to form a 1:1 chelate. Its molar absorptivity at 526nm was found to be 3.3×10⁴Lmol^–1cm^–1. The linear regression equation for 2–20µg of Al(III) is A=0.04458C+0.0112 (where C stands for the concentration of Al(III), µg per 25mL) and correlation coefficient =0.9988. The RSD at the level of 10µg (n=10) and LOQ were found to be 3.5% and 2µg, respectively.     

Determination of the Antibacterial Drug Enrofloxacin by Solid-Phase Spectrofluorimetry

A method for the determination of trace amounts of enrofloxacin based on solid-phase spectrofluorimetry has been developed. The relative fluorescence intensity of enrofloxacin fixed on Sephadex SP C-25 gel was measured directly after packing the gel beads in a 1-mm silica cell, using a solid-phase attachment. The wavelengths of excitation and emission were 277 and 448nm, respectively. The linear concentration range of application was 0.2–4.0ngmL^–1 of enrofloxacin, with a relative standard deviation of 1.2% (for a level of 2.0ngmL^–1) and a detection limit of 0.04ngmL^–1. The method was applied to the determination of enrofloxacin in commercial pharmaceutical formulations and spiked canine serum samples. It was validated using HPLC as a reference method and applying the standard addition methodology. Recovery levels of the method reached 100% in all cases.     

Preconcentration of Cd(II) and Pb(II) Using Humic Substances and Flow Systems Coupled to Flame Atomic Absorption Spectrometry

This paper demonstrates the potential of two natural adsorbents, vermicompost and humic acid, for preconcentration of cadmium(II) and lead(II) using flow systems coupled to flame atomic absorption spectrometry (FAAS). The procedure involves the adsorption of Cd(II) or Pb(II) on these materials (using columns containing 25mg) and subsequent elution for determination by FAAS. Cadmium(II) was preconcentrated for 4min (flow rate of 4.0mLmin^–1) and eluted with 220µL of 3.0molL^–1 HNO₃. Under these conditions, preconcentration factors of 46 and 27 were obtained for vermicompost and humic acid, respectively. Except when using 1.0molL^–1 nitric acid (for humic acid), all conditions for lead(II) preconcentration were identical to those for cadmium(II), and preconcentration factors of 62 and 83 were obtained when vermicompost and humic acid, respectively, were used. The systems were stable with only slight variations in the slopes of the analytical curves (ca. 5% after 8h working period). The long-term stability shows that a minimum of 120 and 100 cycles, respectively, can be run using the same masses of vermicompost or humic acid. The detection limits for Cd(II) were 0.4 and 0.8µgL^–1 for vermicompost and humic acid respectively, while the detection limits for Pb(II) were 8.8 and 12.1µgL^–1, also for vermicompost and humic acid. The accuracy of the methods was checked by using spiked and real (certified and reference) samples. Due to the concomitant sorption of other metals leading to variable slopes for lead and cadmium determination, it was necessary to adopt the standard addition method for calibration purposes.     

Photoluminescence of osmocene and decamethylosmocene

Osmocene (and decamethylosmocene) is characterized by a lowest-energy ligand field triplet which occurs at _max=372nm (374nm) in absorption and 567nm (572nm) in emission. This orange–yellow phosphorescence is rather intense at 77K but is also visible at r.t.     

Determination of Trace Iron in KH2PO4 Crystal by Derivative Adsorption Chronopotentiometry

A sensitive derivative adsorption chronopotentiometric method was developed to determine trace amounts of Fe³⁺. The influence of preconcentration time, pH of solution and constant reducing current of the peak height is discussed. The detection limit and the linear range are 5.0×10^–10 and 2.0×10^–93.0×10^–6molL^–1, respectively. The method was used to determine trace Fe³⁺ in KH₂PO₄ crystals (KDP).     

Preparation of a Novel Agarose-Salen Adsorbent,and its Use for Efficient Column Preconcentration and Flame AAS Determination of Lead in Water

A cross-linked agarose (Novarose) support was chemically modified with a novel salen-type Schiffs base; 2,2-[1,2-ethandiyl bis(nitrilimethylidyne)] bis(orthocresol) and was used to develop an efficient, simple and low-cost method for matrix separation and preconcentration of lead in water. The binding capacity of the adsorbent, packed in a 6.5mm i.d. glass column, was tested for different metal ions, and a capacity of 4973 (±95) µg per mL of packed adsorbent for triplicate measurements was obtained for lead at pH 6.0.The effects of pH, ionic strength, buffer concentration, sample volume, eluent concentration and volume, and matrix ions on the column recovery of Pb²⁺ were carefully studied. Quantitative recoveries were obtained for the analyte in a pH range of 5.5 to 7.0. Enrichment volumes up to 300mL and ionic strengths up to 0.5molL^–1 had no significant effect on the recovery of Pb²⁺. The method also tolerated relatively high concentrations of matrix ions such as Ca²⁺, Mg²⁺, Fe³⁺, Cu²⁺, Zn²⁺, Mg²⁺ and Cd²⁺ with no considerable effect on the analytes signal. A few mL of 0.4molL^–1 hydrochloric acid was sufficient for quantitative elution of the analyte from a 0.5mL column before its flame AAS determination. Preconcentration factors up to 60 and a detection limit of 2.5µgL^–1 were obtained for the method. The precision, expressed as relative standard deviation, was 2.9% (at 100µgL^–1) calculated from six replicate measurements. The method was successfully applied to the determination of Pb²⁺ in some river and spiked river water samples with good precision.     

Multi-Syringe Flow Injection System with In-Line Pre-Concentration for the Determination of Total Phenolic Compounds

A multi-syringe flow injection system for the determination of total phenolics based on the 4-aminoantipyrine reaction is proposed. In-line preconcentration using solid-phase extraction was implemented, offering an environmentally friendly alternative to organic solvent extraction performed in the batch procedure and in previously described flow methodologies. Therefore, phenolic compounds were retained in Amberlite XAD4 resin using acidic conditions. The elution was carried out using 0.10molL^–1 NaOH solution, promoting the ionisation of the retained phenolic compounds, and thus their desorption.Since different elution profiles were obtained for several phenol derivatives, the calibration curves were established using the peak area as analytical signal. Moreover, a dynamic calibration range was applied by using a variable sample volume (0.500–6.00mL) and calibration curves based on the mass of phenol retained in the pre-concentration column. A methodology based on a fixed sample volume (24.0mL) was also proposed for determination at concentration levels below 50µgL^–1. A limit of detection of 52ng of phenol (9 or 2µgL^–1) was achieved with a sampling frequency of 10–16h^–1 for the variable volume method and 4h^–1 for the fixed volume method. The application of the proposed methodology to reference material provided results that were within the certified acceptance limits.     

Kinetics of formation,dissociation and equilibrium constants of pentacyanoaquoruthenate(II) with heterocycles

The kinetics of formation and dissociation reactions of [Ru(CN)₅L]^3– with a series of heterocyclic ligands were studied in aqueous media. In this presence of an excess of heterocycle, the observed second order rate constants were calculated from the k_obs versus [ligand] plot at =0.100m NaClO₄. Activation parameters for the formation reactions (H=28±7kJmol^–1 and S=140±35JK^–1mol^–1) are comparable for all systems, indicating a common mechanism. The kinetics of exchange of coordinated heterocycles for 1,3,5-triazine yielded a rate saturation typical of a limiting dissociative mechanism. Activation parameters of the limiting first order specific rate of dissociation reactions were H=85±7kJmol^–1 and S=18±4JK^–1mol^–1. Equilibrium constants were calculated from the second order rates of formation and pseudo-first order rates of dissociation reaction.     

Polarographic Determination of Lovastatin in the Presence of H2O2 in Pharmaceuticals, Human Urine and Serum

The polarographic reduction and catalytic behavior of lovastatin are studied by polarography and cyclic voltammetry. The reduction wave of lovastatin appears at ca. –1.49V (vs. SCE) in 0.16molL^–1 Na₂B₄O₇–KH₂PO₄ (pH=7.4) supporting electrolyte containing 20% ethanol. It is ascribed to a 2e^–, 2H⁺ addition to the carbonyl group on lactone ring. If H₂O₂ is present, the reduction wave is catalyzed to produce a polarographic catalytic wave. Based on the catalytic wave, a novel method for the determination of lovastatin is proposed. A rectilinear calibration curve of the catalytic wave was obtained for lovastatin concentration in the range 1.5×10^–8 to 1.0×10^–6molL^–1. The peak current of the catalytic wave is ca. 12 times higher than that of the corresponding reduction wave. The detection limit is 8.0×10^–9molL^–1. The proposed method is simpler, faster and more sensitive than the known methods for lovastatin analysis, and can be applied to the direct determination of lovastatin in pharmaceuticals, urine and serum without preliminary separation.     

Exciton Chirality. (A) Origins of and (B) Applications from Strongly Fluorescent Dipyrrinone Chromophores

Summary. (A) The origin of exciton interaction and examples of its application to organic stereochemistry are presented. (B) N,N-Carbonyl-bridged dipyrrinones constitute a new class of highly fluorescent chromophores suitable for investigations of stereochemistry and absolute configuration. N,N-Carbonylxanthobilirubic acid esters are strongly fluorescent, with a fluorescence quantum yield (_F) 0.8, but produce only weak exciton CD from the trans-1,2-cyclohexanediol template. The ester of an analog with benzoic acid replacing propionic, N,N-carbonyl-8-(4-carboxyphenyl)-3-ethyl-2,7,9-trimethyl-(10H)-dipyrrin-1-one, exhibits strong fluorescence (_F=0.68, _em=493nm, _ex=422nm in CHCl₃) and UV-Vis absorption (21000 at 424nm) in organic solvents. Its diester with (1S,2S)-cyclohexanediol is fluorescent and exhibits exciton circular dichroism (=+15dm³·mol^–1·cm^–1, =432nm; =–4dm³·mol^–1cm^–1, =380nm) that correlates with the Exciton Chirality Rule.     

The Fluorescent Reaction Between Quinaldine Red and Nucleic Acids and its Application to Fluorescent Assay of DNA and RNA

The reaction between quinaldine red (QR) and nucleic acids was studied. The free QR alone has no fluorescence in solution. However, it becomes fluorescent after binding to nucleic acids, giving maximum emission at 607nm with maximum excitation at 557nm. Maximum fluorescence intensity is produced in the pH range of 3.2–3.6. Based on the fluorescent reactions, a novel fluorometric method was developed for rapid determination of nucleic acids using QR as the fluorescent probe. Under optimal conditions, the calibration graphs were linear in the range of 0–30.0µgmL^–1 for CT DNA and 0–20.0µgmL^–1 for yeast RNA. The limits of detection were 38ngmL^–1 for CT DNA and 142ngmL^–1 for yeast RNA. Four synthetic samples were determined with satisfaction.Received December 20, 2002; accepted March 27, 2003 Published online August 8, 2003     

Thermal decomposition of halo-bis(piperidyldithio-carbamates) of As(III), Sb(III) and Bi(III)

Simultaneous TG/DTG/DTA studies under non-isothermal conditions have been carried out in air and nitrogen on some halo-dithiocarbamates of the general formula XM[S₂CN(CH₂)₅]₂ (X=Cl, Br and I; andM=As, Sb and Bi).E* values for the 1st stage of decomposition were determined by graphical methods and the TTN temperatures were calculated from the TG profiles. A possible mechanism of the decomposition reaction is suggested, based on the thermoanalytical and pyrolysis results and the mass spectral data. The kinetic analysis data on five of the above dithiocarbamates and nine complexes of the general formula M[S₂CN<]₃ (M=As, Sb and Bi; and N<=NEt₂, N(CH₂)₅ and N(CH₂)₄O) were studied by the QIA (quasi-isothermal analysis) technique in air atmosphere. An example of kinetic parameter (k andn) estimation for the first decomposition stage is given for Bi[S₂CN(CH₂)₅]₃, with the assumption of different kinetic equations.

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Zusammenfassung Einige helo-Dithiocarbamate der allgemeinen Formal XM[S₂CN(CH₂)₅]₂ (X=Cl, Br und I;M=As, Sb und Bi) wurden mittels simultaner TG/DTG/DTA unter nichtisothermen Bedingungen in Luft und Stickstoff untersucht. Für den ersten Zersetzungsschritt wurden E^*-Werte durch graphische Methoden bestimmt und die TIN-Temperaturen aus den TG-Profilen berechnet. Ein auf den Ergebnissen der Thermogravimetrie und Pyrolyse sowie auf massenspektroskopischen Daten beruhender möglicher Mechanismus wird vorgeschlagen. Fünf der angeführten Dithiocarbamate und 9 Komplexe der allgemeinen Formel M[S₂CN<]₃] (M=As, Sb und Bi;N=NEt₂, N(CH₂)₅ und N(CH₂)₄O) wurden mittels QIA (quasi-isotherme Analyse) in Luft untersucht. Als Beispiel ist die Bestimmung der kinetischen Parameter (k undn) für den ersten Schritt der Zersetzung von Bi[S₂CN(CH₂)₅]₃ unter Annahme verschiedener kinetischer Gleichungen angegeben.

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, - XM[S₂CN(CH₂)₅]₂, =Cl, r l, a M=As, Sb Bi. ^a , -. - . M[S₂CN<]₃, M=As, Sb Bi, a N<=NEt₂, N(CH₂)₅ N(CH₂)₄O. k ³[S₂N(₂)₅]₃.

     

In-Situ Separation and Determination of Palladium from Platinum Based on Different Vaporization Temperatures by Electrothermal Vaporization Inductively Coupled Plasma Optical Emission Spectrometry with YPA4 Resin Acting Both as Adsorption Material and Chemical Modifier

A new method has been developed for in-situ separation of Pd from a Pt matrix and determination of trace Pd based on different vaporization temperatures by electrothermal vaporization (ETV) inductively coupled plasma optical emission spectrometry (ICP-OES) with the use of polythioether backbone modified with a diaminoisopropylmercaptane chelating group (YPA₄), both as solid phase extractant and chemical modifier. In 0.5M HNO₃, Pd and Pt can be adsorbed by YPA₄. The resin loaded with Pd and Pt was then prepared to form a slurry that can be directly introduced into the graphite furnace without any pretreatment. The factors affecting in-situ separation of Pd from the Pt matrix were investigated in detail. It was found that, in the presence of YPA₄, Pd could be quantitatively vaporized at lower vaporization temperatures (1800°C–1900°C), while Pt could not be vaporized from the graphite furnace at the same temperature, its quantitative vaporization temperature being 2600°C. Based on the different vaporization temperatures, in-situ separation of Pd from the Pt matrix and determination of trace Pd by ETV-ICP-OES was achieved in the temperature range of 1800°C–1900°C. Under the optimized conditions, the detection limit (3) of Pd is 60pg, and the relative standard deviation (RSD) is 5.6% (n=9, C=0.2µgmL^–1). The method has been applied to the determination of trace Pd in anode slime and Auto Catalyst NIST SRM 2557 reference material, and the determined values coincide with the certified values.     

Differential determination of arsenic (III) and total arsenic with L-cysteine as prereductant using a flow injection non-dispersive atomic absorption device

Speciation of arsenic in environmental samples gains increasingly importance, as the toxic effects of arsenic are related to its oxidation state. A method was developed for the determination of trace amounts of arsenic(III) and total arsenic by flow injection hydride generation coupled with an in-house made non-dispersive AAS device. The total arsenic is determined after prereduction of arsenic(V) to arsenic(III) with L-cysteine in a low concentration of hydrochloric, acetic or nitric acid. The conditions for the prereduction, hydride generation and atomization were systematically investigated. A quartz tube temperature of 800°C was found to be optimum in view of peak shape and baseline stability. Pb(II), Ni(II), Fe(III), Cu(II), Ag(I), Al(III), Ga(II), Se(IV), Bi(III) were checked for interfering with the 2g/L As(V) signal. A serious signal depression was only observed for Se(IV) and Bi(III) at a 150-fold excess. With the above system, arsenic was determined at a sampling frequency of about 1/min with a detection limit (3) of 0.01g/L using a 0.5mL sample. The reagent blank was 0.001±0.0003 absorbance units and the standard deviation of 10 measurements of the 2 g/l As signal was found to be 1.2%. Results obtained for standard reference materials and water samples are in good agreement with the certified values and those obtained by ICP-MS     

The Dissociation Constants of 1,1′-Bis (pyridinium-4-aldoxime)trimethylene dibromide

Summary. The dissociation constants of the two oxime groups of 1,1-bis(pyridinium-4-aldoxime)trimethylene dibromide (TMB-4) were determined using spectrophotometric data. Two numerical methods were applied to treat the overlapping equilibria. The results obtained by both agreed with each other and their mean values at 25°C corrected for the ionic strength of 0.05moldm^–3 are pK_a1=7.49±0.11 and pK_a2=8.96±0.09. These values were discussed in terms of the pK_as of 1,1-bis(pyridinium-4-aldoxime)oxydimethylene dichloride (Toxogonin), a similar dioxime, which were derived by extrapolation of literature data.