Evaluation of a carbon-rod atomizer for routine determination of trace metals by atomic-absorption spectroscopy applications to analysis of lubricating oil and crude oil

A carbon-rod atomizer (CRA) fitted with a 'mini-Massmann' carbon rod was evaluated for routine analysis of petroleum and petroleum products for trace metal content by atomic-absorption spectroscopy. Aspects investigated included sensitivity, detection limit, effect of solvent type, and interferences. The results of analysis of oil samples with this technique were compared with those obtained by other techniques. Metals studied were silver, copper, iron, nickel, and lead. Sensitivity and detection limit values obtained with the CRA were similar to those obtained with the carbon-filament atomizer. Strong 'solvent effects' were observed as well as interference by cations. On the basis of this study, design changes for the CRA are suggested, with the object of minimizing 'solvent effects' and interferences, increasing the atomization efficiency, and increasing the residence time of the atomic vapour in the optical path of the instrumental system.     

Single-phase and gram-scale routes toward nearly monodisperse Au and other noble metal nanocrystals

Single-phase approaches are introduced for the synthesis of nearly monodisperse Au and other noble metal nanocrystals. The new approaches possess all the advantages of the popular Brust method. With weak ligands or surfactants for the metal ions, the control of the size and size distribution of the nanocrystals in synthesis in the size range between 1 and 15 nm was achieved via maintaining balanced nucleation and growth by tuning the activities of the metal precursors and reducing reagents. Because only weak ligands are employed in the new synthetic schemes, surface modification and functionalization of the resulting nanocrystals can be readily carried out.     

Characteristics of vinyl iodide microwave plasma etching of GaAs/AlGaAs and InP/InGaAs heterostructures

Vinyl iodide (C₂H₃I) microwave discharges with additions of H₂ and Ar are found to provide faster etch rates than conventional CH₄/H₂/Ar discharges for InP, InGaAs, GaAs, and AlGaAs. This is a result of the relatively high volatilities of indium, gallium, and aluminum iodide species. The etched features are smooth and anisotropic over a wide range of do self-biases (–150 to –350 V), process pressures (1–20mTorr), and microwave powers (150–500 W). The polymer that forms on the mask during the plasma exposure can be readily removed in O₂ discharges. Electron spectroscopy for chemical analysis (ESCA) showed that the etched surfaces are slightly deficient in the group V elements under most conditions, but changes to the optical properties of the semiconductors are minimal. No defects are visible by transmission electron microscopy (TEM) in GaAs or InP samples etched at dc biases –250 V.     

Physically based molecular device model in a transient circuit simulator

Two efficient, physically based models for the real-time simulation of molecular device characteristics of single molecules are developed. These models assume that through-molecule tunnelling creates a steady-state Lorentzian distribution of excess electron density on the molecule and provides for smooth transitions for the electronic degrees of freedom between the tunnelling, molecular-excitation, and charge-hopping transport regimes. They are implemented in the fREEDA™ transient circuit simulator to allow for the full integration of nanoscopic molecular devices in standard packages that simulate entire devices including CMOS circuitry. Methods are presented to estimate the parameters used in the models via either direct experimental measurement or density-functional calculations. The models require 6–8 orders of magnitude less computer time than do full a priori simulations of the properties of molecular components. Consequently, molecular components can be efficiently implemented in circuit simulators. The molecular-component models are tested by comparison with experimental results reported for 1,4-benzenedithiol.     

Enhanced glucose production from cellulose using coimmobilized cellulase and β-glucosidase

β-Glucosidase was covalently immobilized alone and coimmobilized with cellulase using a hydrophilic polyurethane foam (Hypol®FHP 2002). Immobilization improved the functional properties of the enzymes. When immobilized alone, the K_m for cellobiose of β-glucosidase was decreased by 33% and the pH optimum shifted to a slightly more basic value, compared to the free enzyme. Immobilized β-glucosidase was extremely stable (95% of activity remained after 1000 h of continuous use). Coimmobilization of cellulase and β-glucosidase produced a cellulose-hydrolyzing complex with a 2.5-fold greater rate of glucose production for soluble cellulose and a four-fold greater increase for insoluble cellulose, compared to immobilized cellulase alone. The immobilized enzymes showed a broader acceptance of various types of insoluble cellulose substrates than did the free enzymes and showed a long-term (at least 24 h) linear rate of glucose production from microcrystalline cellulose. The pH optimum for the coimmobilized enzymes was 6.0. This method for enzyme immobilization is fast, irreversible, and does not require harsh conditions. The enhanced glucose yields obtained indicate that this method may prove useful for commercial cellulose hydrolysis.     

Capacitive-discharge-heated anisotropic pyrolytic graphite furnace used for atomic absorption spectrometry Part 2. Experimental Verification of Temperature Distribution of the Furnace Surface and of the Gas Phase

The predictions of a theoretical model, embodied in a computer program, describing the heating characteristics of the furnace surface and the gas phase of an anisotropic pyrolytic graphite furnace heated by the capacitive discharge technique are compared with the experimental results obtained by optical pyrometry and by two-line atomic absorption spectrometry, respectively. The surface temperature gradient around the circumference of the type 1 furnace and along the optical axis of the type 3 furnace are calculated and compared with the measured temperatures. The weighted-average of the theoretically predicted gas temperature is in reasonable agreement with the effective vapour temperature measured by two-line atomic absorption method. The heating rate of the furnace does not have a significant effect on the temperature distribution of either the furnace surface or the gas phase. The effect of the difference in the temperature distribution of the type 1 and type 3 furnaces on the atomic absorption signals is also discussed.     

Aggregation-fragmentation in a model of DNA-mediated colloidal assembly

We present results from an off-lattice Monte Carlo simulation of DNA-mediated colloidal assembly. In this simulation, the aggregation-fragmentation of a binary mixture of DNA-coated colloidal particles is studied through a simplified model of base-pair hybridization. Bonding between monomers is modeled as a simple temperature-sensitive A/B-type interaction, where type A and B monomers can bond to only the opposite type (no A/A or B/B attachments are allowed). The actual chemistry of base-pair hybridization is not included in the model. The morphological structures of the clusters formed as well as the kinetics of growth are analyzed in our 2D simulations. The fractal dimension and kinetic growth exponents for clusters formed near the DNA "melting" temperature agree with those seen previously for 2D diffusion-limited cluster aggregation (DLCA) models. The clusters appear more compact, exhibiting signs of local order at intermediate temperature values. At higher temperatures, the formation of large clusters is not favorable under the action of temperature-dependent fragmentation, and the system eventually reaches a steady state as a collection of small aggregates. The temperature profile for this dissolution of the colloidal assembly is sharp, indicating that the selective hybridization process provides a highly sensitive measurement tool. At high temperatures, we analyze the steady-state behavior of the average cluster size in terms of an aggregation-fragmentation model.     

On the adsorption and diffusion of Methylene Blue in glass fibers

The adsorption and diffusion phenomena of Methylene Blue dye on glass fiber have been explored. Both isotherm and kinetics have been studied. Effect of process parameters such as pH, adsorbent loading, and initial dye concentration was investigated. Temperature had a little effect on adsorption. A diffusion model was developed to estimate the diffusivity of the dye in glass fiber. The diffusivity was found to be varying exponentially with the dye concentration.     

Computer simulation of selective aggregation in binary colloids

The morphology of clusters formed by selective aggregation of binary colloids is studied in a two-dimensional Monte Carlo simulation for a large range of number fractions (200:1, 100:1, 10:1, 2:1). We find remarkable similarity in morphology to those observed in experiments, from the formation of closed "micelles" to large branched clusters. Quantitative studies of the fractal dimension, kinetics, and cluster size distribution are also carried out and compared with diffusion-limited cluster aggregation and reaction-limited cluster aggregation models.