摘要:

AbstractA composite electrode has a surface that consists of an ordered arrangement (array) or a random arrangement (ensemble) of conductor regions, typically micrometers in dimension, separated from one another by an insulator. These active regions (or microelectrodes) may themselves be of regular geometry and confined primarily to the surface, as with most array electrodes, or of irregular geometry and permeate the bulk of the material, as with most ensemble electrodes. In this review, we classify composite electrodes according to the distribution of a conductor and insulator within the material (or on the surface), survey a number of the fabrication schemes used to construct composite electrodes, discuss percolation theory as it applies to certain composite electrodes, examine the surface morphology of selected composite materials, discuss the origins of the enhancement in current density observed at many composite electrodes in both quiescent and flowing solutions, and provide a few examples illustrating the applications of composite electrodes to real analytical problems.

ISSN:1040-0397    

DOI:10.1002/elan.1140020702

出版商:VCH Verlagsgesellschaft mbH    

年代:1990

数据来源: WILEY

摘要:

AbstractA flow‐injection analysis (FIA) system for the determination of hydrogen peroxide was developed by utilizing a pineapple‐tissue‐based modified carbon paste electrode. Pineapple stem core tissue contains peroxidase, which is not available commercially in the pure isolated form. Hydrogen peroxide concentrations were determined in the presence ofO‐phenylenediamine as a hydrogen donor at a potential of −200 mV. The detection limit (S/N = 3) for hydrogen peroxide using this tissue electrode is 2.1 μM with a relative standard deviation of 0.44% (n= 12). The tissue‐modified carbon paste electrode is stable for up to 20 days with no significant acti

ISSN:1040-0397    

DOI:10.1002/elan.1140020703

出版商:VCH Verlagsgesellschaft mbH    

年代:1990

数据来源: WILEY

摘要:

AbstractSubstrate specificity, the linear measurement range, and the lifetime of monoenzyme electrodes were determined using L‐glutamate oxidase, L‐lysine oxidase, and tyrosinase immobilized in gelatin. These sensors were applied to the analysis of protein hydrolyzates. By coimmobilizing the three enzymes, an electrode responding to a group of amino acids was obtained. Its analytical performance was compared with that of an L‐amino acid oxidase sensor. The net charge of the immobilization matrix influenced the relative sensitivities of the amino

ISSN:1040-0397    

DOI:10.1002/elan.1140020704

出版商:VCH Verlagsgesellschaft mbH    

年代:1990

数据来源: WILEY

摘要:

AbstractVarious flow‐injection analysis arrangements employing immobilized oxidase and peroxidase enzymes in conjunction with fluoride ion‐selective electrode detection are described. Selective measurement of substrates is based on the oxidase‐catalyzed production of hydrogen peroxide. The liberated peroxide is detected via a coupled peroxidase reaction involving a fluoroaromatic reagent added to the carrier stream to yield free fluoride ions. Three specific arrangements are evaluated: (1) both enzymes coimmobilized on a single membrane covering the fluoride electrode detector, (2) both enzymes immobilized on controlled‐pore glass within a flow‐through reactor place before the potentiometric detector, and (3) two separate enzyme reactors placed in series upstream from the electrode. Optimum sensitivity toward model analyte glucose using glucose oxidase is achieved with arrangements based on the immobilized enzyme reactors. With these systems, detection of glucose at concentrations ranging from 0.1 to 10 mM is possible at throughputs of more than 30

ISSN:1040-0397    

DOI:10.1002/elan.1140020705

出版商:VCH Verlagsgesellschaft mbH    

年代:1990

数据来源: WILEY

摘要:

AbstractDielectric spectroscopy was employed to study the dielectric behavior of typical ISE membrane plasticizers and their compounds with PVC. Distinctly different patterns are reported with PVC compounds of polar and apolar plasticizers.

ISSN:1040-0397    

DOI:10.1002/elan.1140020706

出版商:VCH Verlagsgesellschaft mbH    

年代:1990

数据来源: WILEY

摘要:

AbstractThe electrochemical oxidation behavior of trimethoprim has been investigated at a pyrolytic graphite electrode over a wide pH range (1.35–10.71). Below pH 3.2, trimethoprim is electrooxidized in two well‐defined steps, each involving 4e, 4H+. Only one well‐defined oxidation peak was observed above pH 3.5 however. Products corresponding to each peak have been separated and characterized by thin‐layer chromatography (TLC), infra red (IR), and mass spectra. A tentative mechanism associated with the voltammetric oxidation has been proposed to account for the observed experimental behavior. The same product has also been observed during the metabolic oxidation of trimethoprim in albi

ISSN:1040-0397    

DOI:10.1002/elan.1140020707

出版商:VCH Verlagsgesellschaft mbH    

年代:1990

数据来源: WILEY

摘要:

AbstractThis work develops the theoretical equations for a single potential step—open circuit relaxation chronoabsorptometry for use in cases when the other species also displays absorption at the wavelength being observed for oxidation. A data analysis method is proposed to determine the rate constant of the catalytic reaction. The rate constants of ferrocyanide—aniline and ferrocyanide—antipyrine systems are determined by an optically transparent spectroelectrochemical method. The result coincides with the value determined by the electrochemical m

ISSN:1040-0397    

DOI:10.1002/elan.1140020708

出版商:VCH Verlagsgesellschaft mbH    

年代:1990

数据来源: WILEY

摘要:

AbstractControlled adsorptive accumulation of the food additives (azo dyes) ponceau 4R, Allura Red, and carmoisine at a hanging mercury drop electrode (HMDE) provided the basis for their determination by stripping voltammetry at nanomolar concentration levels. The adsorptive stripping response was evaluated with respect to experimental parameters such as accumulation potential, preconcentration time, pH, and bulk concentration. Best results were obtained using an ammonia buffer as the supporting electrolyte; depending on the dye, detection limits of 0.071, 0.076, and 0.043 nM were, respectively, achieved in pure dye solutions. The simultaneous determination was carried out by means of the generalized standard addition method.

ISSN:1040-0397    

DOI:10.1002/elan.1140020709

出版商:VCH Verlagsgesellschaft mbH    

年代:1990

数据来源: WILEY

摘要:

AbstractA novel amperometric method for biochemical substrates consuming oxygen in their enzyme reactions is proposed. This method is based on the measurement of the current peak observed when the sample solution is dropped on the thin membrane dipped in the enzyme solution, which is placed on the oxygen gas‐selective permeable membrane. In order to illustrate the present concept, uric acid was chosen as a measurable species and the response characteristics of the sensor were investigated. The peak current obtained by sample addition was proportional to the concentration of uric acid in the dropped solution, and the method was applied to measure uric acid in human urine. The results obtained were in fairly good agreement with those obtained using a conventional spectrophotometric metho

ISSN:1040-0397    

DOI:10.1002/elan.1140020710

出版商:VCH Verlagsgesellschaft mbH    

年代:1990

数据来源: WILEY

摘要:

AbstractAn amperometric flow injection system with an immobilized enzyme reactor that gives responses amplified by substrate recycling was employed for the highly sensitive determination of L‐glutamate. An L‐glutamate oxidase/glutamic‐pyruvic transaminase coimmobilized reactor was incorporated at fixed positions in a flow system. In the presence of sufficient amounts of L‐alanine and oxygen in the carrier solution, L‐glutamate was repeatedly converted to 2‐oxoglutarate by L‐glutamate oxidase, and the 2‐oxoglutarate produced was subsequently converted back to L‐glutamate by glutamic‐pyruvic transaminase during the passage through the enzyme reactor. As a result of this recycling reaction, a large amount of hydrogen peroxide was produced in the reactor and could be detected amperometrically at a downstream platinum detector. L‐Glutamate was determined with a 20‐ to 30‐fold increase in sensitivity compared to the unamplified responses. The detection limit

ISSN:1040-0397    

DOI:10.1002/elan.1140020711

出版商:VCH Verlagsgesellschaft mbH    

年代:1990

数据来源: WILEY