Determination of kinetic and thermodynamic parameters of surface confined species through ac voltammetry and a nonstationary signal processing technique: the Hilbert transform

Data analysis of voltammetric responses has usually been done through application of the fast Fourier transform although it is widely accepted that electrochemical signals are intrinsically nonlinear and nonstationary. In this work, we present a time-series analysis based on the Hilbert transform (HT), a nonstationary signal processing technique, as an alternative tool that can overcome many of the difficulties associated with Fourier techniques. We use the HT to study the behavior of thin-film processes when the excitation perturbation is ac voltammetry. From the analysis of simulated data, we propose simple relations that enable species-specific kinetic and thermodynamic parameters to be estimated, without prior utilization of baseline subtraction even when double layer capacitance significantly influences the current response. We also propose a method to determine whether the characteristics of the applied voltage perturbation are adequate for the accurate estimation of these parameters. The methodology developed here will be applied to previously published experimental time series data (Guo, S. X.; Zhang, J.; Elton, D. M.; Bond, A. M. Anal. Chem. 2004, 76, 166-177.) obtained with ac voltammetry to show how a number of physical parameters can be directly extracted from the processed data.     

A simultaneous hit finding and timing method for pulse shape analysis of drift chamber signals

An algorithm for the analysis of the digitized signal waveform of drift chamber pulses is described which yields a good multihit resolution and an accurate drift time determination with little processing time. The method has been tested and evaluated with measured pulse shapes from the full size prototype of the OPAL central detector which were digitized by 100 MHz FADCs.     

Network flow procedures for the analysis of cellular manufacturing systems

A three-phase network-flow-based procedure is developed for minimizing intercellular part moves in machine-part grouping problems. The unique feature of this methodology is its consideration of several variations related to the number of cells, the number of machines in each cell, and the part family size. The first phase computes a functional relationship between machines on the basis of either a machine-part matrix or actual operation sequences for the parts being considered. The final purpose of this phase is a network modeling of the problem. The second phase partitions the network according to mutually exclusive sets of nodes that represent manufacturing cells. A 0-1 integer programming model and a 0-1 quadratic programming model are discussed and network-flow-based solution procedures are developed. Finally, the third phase identifies the part families. A 0-1 integer programming model is formulated and the solution of this model is again performed through a network approach that allows the identification of a feasible assignment of parts to machine cells. Computational results indicate that the proposed approach is appropriate for solving large-scale industrial problems efficiently.     

Determination of monomethylcadmium in the environment by differential pulse anodic stripping voltammetry

A differential pulse anodic stripping voltammetric (DPASV) method was used to differentiate between the cadmium species Cd(2+) and MeCd(+) (Me = methyl) in aquatic systems. These two species show peaks in the DPASV voltammogram which differ by 112 mV. In model experiments, it was demonstrated that monomethylcadmium is not stable at pH 2, but under higher pH conditions, normally found in fresh and ocean water samples, the identity of MeCd(+) was verified by different investigations, including cyclic voltammetry, selective extraction of a complex of diethyldithiocarbamate with MeCd(+) into n-hexane, and photochemical dissociation of MeCd(+) by UV irradiation. It was also shown that humic acids do not influence the voltammetric determination of monomethylcadmium. For the first time, it was possible to analyze MeCd(+) in environmental samples. During different expeditions with the German research vessel Polarstern, monomethylcadmium could be determined above the detection limit of 470 pg L(-1) in nearly all surface water samples of the South Atlantic with spot concentrations of up to about 700 pg L(-1), whereas in the North Atlantic only 15-30% of the total samples showed MeCd(+) concentrations above this limit. The existence of MeCd(+) in the remote area of the South Atlantic, as well as positive correlations with the local bioactivity in the ocean, indicates biomethylation as the most probable formation process for this methylated cadmium species. This assumption is supported by the simultaneous occurrence of other methylated heavy metal compounds, such as Me(3)Pb(+). Up to 48% of the total cadmium was found to be monomethylcadmium in some Arctic meltwater ponds.     

On a progressive simultaneous iteration method for vibration analysis of gyroscopic systems

This paper presents details of an eigenproblem solution procedure for the efficient and accurate free vibration analysis of gyroscopic systems such as spinning structures. The procedure enables computation of the first few roots and vectors without having to compute any other. While structural discretization is achieved by the standard finite element method, eigenproblem solution of the resulting equations is effected by a newly developed progressive simultaneous iterative (PSI) method. Numerical solution of relevant example problems re presented which are compared to those obtained by ther existing analysis techniques, testifying to substantial gains in the efficacy of the newly developed solution algorithm. Copyright © 2001 John Wiley & Sons, Ltd.     

Electrochemical behaviour of surface zirconium oxides in the presence of some electroactive species

The open circuit potential, capacitance and resistance techniques were used to investigate the behaviour of two types of surface zirconium oxides in pure 0.5 N H₂SO₄ and in the presence of additives of K₂CrO₄, KI, FeSO₄ and HCOOH.

The air-formed preimmersion oxide film grows in all solutions following a logarithmic rate law. Depending on the nature of the additive the oxide grows directly by oxidation of the surface by the oxidizing additive (K₂CrO₄) or indirectly by mediating the oxidation by atmospheric oxygen in the presence of reducing additives.

Anodically formed zirconium oxide films dissolve in all solutions. The results of impedance measurements indicate that adsorption of HCOOH activates the oxide surface owing, possibly, to its hydrophilic nature while iodide adsorption diminishes the reactivity of active sites (probably oxygen vacanies) where the dissolution process is initiated.     

Separation of electron-transfer and coupled chemical reaction components of biocatalytic processes using Fourier transform ac voltammetry

The underlying electron-transfer and coupled chemical processes associated with biologically important catalytic reactions can be resolved using a combination of Fourier transform ac voltammetry with an analysis of the separated dc and ac components. This outcome can be achieved because the response associated with generation of the catalytic current is essentially confined to the steady-state dc component, whereas the electron-transfer step is dominant in the fundamental and higher harmonics. For the mediated oxidation of glucose with glucose oxidase, it was found that the underlying reversible redox chemistry of the mediator, ferrocenemonocarboxylic acid, as detected in the third and higher harmonics, was totally unaffected by introduction of the catalytic process. In contrast, for the catalytic reduction of molecular oxygen by cytochrome P450, slight changes in the P450 redox process were detected when the catalytic reaction was present. Simulations of a simple catalytic reaction scheme support the fidelity of this novel FT ac voltammetric approach for examining mechanistic nuances of catalytic forms of electrochemical reaction schemes.     

A thermodynamic model based on exergy flow for analysis and optimization of pulse tube refrigerators

A thermodynamic model based on exergy flow through pulse tube refrigerators (PTRs) is developed. An exergetic efficiency parameter representing the losses in the pulse tube itself is proposed. The effects of control parameters representing a general phase shifter and their effect on the system performance are discussed. Analytical solutions representing important parameters in the design of PTRs such as the load curve, cooling power and efficiency in terms of basic system input parameters are developed. It is shown that the analytical model is powerful and convenient for optimization of PTRs and in quantifying its operational bound and important losses. Results indicating a compromise between cooling power and efficiency in PTRs under certain conditions are presented and discussed.     

Differential pulse voltammetry of toxic metal ions at the boron-doped CVD diamond electrode

Boron-doped polycrystalline diamond films were grown over a molybdenum substrate by a microwave plasma CVD process using a methane and hydrogen gas mixture at a pressure of 35 ± 1 Torr. Boron doping of diamond was achieved in situ by using a solid boron source while growing diamond in the CVD processxu. We have observed a negligible background current (l) for diamond by differential pulse voltammetry in 0.5 M NaCl, 0.5 M H₂SO₄, and 0.5 M HNO₃ solutions over a wide potential range. Therefore, diamond will certainly have a use as an electrode material in electroanalytical applications to detect trace toxic/nontoxic metal ions such as cadmium, lead, copper, and silver. Differential pulse voltammetry was used to detect and evaluate the presence of lead ions in 0.5 M NaCl and cadmium ions in 0.5 M H₂SO₄ supporting electrolyte solution using highly conducting boron-doped diamond coated molybdenum electrode material. Furthermore, reverse differential pulse voltammetry was used to evaluate the presence of copper and silver ions in 0.5 M H₂SO₄ and 0.5 M HNO₃ solution, respectively. Diamond electrode has been used in this study to detect metallic ions in the solution over a wide potential range that covers + 0.8 V to –0.4 V vs., SHE.     

Multi-stage simultaneous lot-sizing and scheduling for flow line production

This paper addresses the simultaneous lot-sizing and scheduling of several products in multi-stage flow line production systems consisting of heterogeneous parallel production lines per stage. The limited capacity of the production lines may be further reduced by sequence dependent setup times. Deterministic, dynamic demand of standard products has to be met without backlogging with the objective of minimizing sequence dependent setup, holding and production costs as well as costs for external purchase, overtime, and standby. Different mixed-integer programming (MIP) model formulations are proposed and tested using a standard MIP-solver. Furthermore, construction heuristics like LP-and-Fix and Relax-and-Fix are designed and applied. The solution quality and computational performance of these approaches are examined in several test scenarios.     

热声系统内交变流动特性的理论分析

从流动特性角度分析了热声系统中速度分布的一些特征和形态。对动量方程的数值解的分析表明:当rh/vδ<0.6或Reω<3(Reω为交变流动的雷诺数)时,流动可视为准稳态振荡,压力振荡和速度振荡的相位差趋向于零,声场接近于行波声场;当0.63.5或Reω>100流动发展为完全湍流状态,流动的大部分区域的压力振荡和速度振荡的相位差趋向于π/2,声场接近于驻波声场。     

声振系统中高频能量流分析法研究进展

能量流分析法是预测声振系统中高频动力响应的一种有效手段,该方法引入的假设条件较少,并可基于声振系统的几何模型计算随空间变化的稳态动力响应,应用上具有很大的优点。本文首先回顾了不同结构件（包括杆、梁、板及复合结构）各种波场的能量微分方程及其理论基础的发展过程,然后,针对不同类型的声振系统耦合形式,包括结构/结构耦合,结构/流体耦合及其它耦合情况,说明了基于能量流分析的耦合问题处理方法的发展,最后,说明了计算随机激励下声振系统响应的随机能量流分析法的相关研究,以及其与经典统计能量分析法间理论关系研究的进展情况。     

Determination of selenium in natural waters by adsorptive differential pulse cathodic stripping voltammetry

In this work bovine albumin was used innovatively as a medium for adsorptive accumulation of Se–I₂ on thin mercury film electrode. Se–I₂ was formed by reaction between Se(IV) and iodide in HCl media. The adsorbed Se–I₂ was stripped in 0.05 M HCl by differential pulse cathodic potential scan. The proposed method was successfully applied to analysis of Se(IV) and Se(VI) in natural waters sampled from some lagoons south of Caspian Sea. The optimum reaction conditions and other analytical parameters and influence of cations and anions were studied. The detection limit was 0.37 ng mL⁻¹. The obtained results were compared with the results of DPCSV after electrochemical preconcentration, HG-AAS and ICP-AES.     

Digital holographic microscopy with reduced spatial coherence for three-dimensional particle flow analysis

We investigate the use of a digital holographic microscope working in partially coherent illumination to study in three dimensions a micrometer-size particle flow. The phenomenon under investigation rapidly varies in such a way that it is necessary to record, for every camera frame, the complete holographic information for further processing. For this purpose, we implement the Fourier-transform method for optical amplitude extraction. The suspension of particles is flowing in a split-flow lateral-transport thin separation cell that is usually used to separate the species by their sizes. Details of the optical implementation are provided. Examples of reconstructed images of different particle sizes are shown, and a particle-velocity measurement technique that is based on the blurred holographic image is exploited.     

Enthalpy flow rate of a pulse tube in pulse tube refrigerator

An integration formula of enthalpy flow rate along a pulse tube in pulse tube refrigerators is described on the assumption of sinusoidal mass flow rate and sinusoidal pressure fluctuation. For ideal double inlet and ideal orifice pulse tube with helium as working medium, it is simplified to a polynomial formula. Polynomial formulas for roughly evaluating the volume of the pulse tube in ideal double inlet and ideal orifice pulse tube refrigerators are also given.     

Determination of diffusion coefficients of electroactive species in time-of-flight experiments using a microdispenser and microelectrodes

Two novel methods for the determination of diffusion coefficients of redox species combining the special properties of microdispensing devices and microelectrodes are presented. Both are based on the local application of tiny volumes of the redox-active species by means of a dispenser nozzle at a defined distance from the surface of a microelectrode. The microelectrode, which is inserted through the bottom into an electrochemical cell, is held at a constant potential sufficient to oxidize or reduce the electro-active species under diffusional control. The dispenser, which is filled with the electro-active species, can be positioned by means of micrometer screws over the microelectrode. After dispensing a defined number of droplets near the microelectrode surface, the current through the microelectrode is recorded, usually yielding a peak-shaped curve having a defined time delay between the shooting of the droplets and the maximum current. The time that is necessary to attain maximum current, together with the known distance between two dispensing points, can be used to determine the diffusion coefficient of the electroactive species without knowledge of any system parameters, such as concentration of the redox species, diameter of the electroactive surface or number of transferred electrons. A similar method for the determination of diffusion coefficient of redox species involves a second redox species for calibration purposes. A mixture of both species is shot close to the microelectrode surface. Due to the different formal potentials of the redox species that are used, they can be distinguished in sequential experiments by variation of the potentials that are applied to the microelectrode, and it is thus possible to determine the individual transit times of the redox species independently. The difference in the transit times, together with the known diffusion coefficient of one of the redox species, can be used to calculate the unknown diffusion coefficient of the second one.     

Volt-ampere characteristics of an ac arc in a transverse gas flow

An analytical dependence of the volt-ampere characteristic of an arc on the velocity of the incoming transverse gas flow is obtained. The stability threshold of arc combustion is determined.Notation h enthalpy - density - t time - v velocity - , thermal conductivity and electrical conductivity - E electric field strength - U potential at the arc - Q radiant flux - T period of oscillation of the current - r radius - l length of electric arc - d, L diameter and length of electrode - temperature - kinematic viscosity - Pr Prandtl number - current function Indices 0 gas parameters far from arc - 1 parameters at arc boundary - x projection on the axis 0x - y projection on the axis 0y - E effective value - e characteristic value Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 47, No. 1, pp. 133–138, July, 1984.     

Scanning electrochemical cell microscopy: theory and experiment for quantitative high resolution spatially-resolved voltammetry and simultaneous ion-conductance measurements

Scanning electrochemical cell microscopy (SECCM) is a high resolution electrochemical scanning probe technique that employs a dual-barrel theta pipet probe containing electrolyte solution and quasi-reference counter electrodes (QRCE) in each barrel. A thin layer of electrolyte protruding from the tip of the pipet ensures that a gentle meniscus contact is made with a substrate surface, which defines the active surface area of an electrochemical cell. The substrate can be an electrical conductor, semiconductor, or insulator. The main focus here is on the general case where the substrate is a working electrode, and both ion-conductance measurements between the QRCEs in the two barrels and voltammetric/amperometric measurements at the substrate can be made simultaneously. In usual practice, a small perpendicular oscillation of the probe with respect to the substrate is employed, so that an alternating conductance current (ac) develops, due to the change in the dimensions of the electrolyte contact (and hence resistance), as well as the direct conductance current (dc). It is shown that the dc current can be predicted for a fixed probe by solving the Nernst-Planck equation and that the ac response can also be derived from this response. Both responses are shown to agree well with experiment. It is found that the pipet geometry plays an important role in controlling the dc conductance current and that this is easily measured by microscopy. A key feature of SECCM is that mass transport to the substrate surface is by diffusion and, for charged analytes, ion migration which can be controlled and varied quantifiably via the bias between the two QRCEs. For a working electrode substrate this means that charged redox-active analytes can be transported to the electrode/solution interface in a well-defined and controllable manner and that relatively fast heterogeneous electron transfer kinetics can be studied. The factors controlling the voltammetric response are determined by both simulation and experiment. Experiments demonstrate the realization of simultaneous quantitative voltammetric and ion conductance measurements and also identify a general rule of thumb that the surface contacted by electrolyte is of the order of the pipet probe dimensions.