Bassam A. Freiha

Amperometric biosensing of organic peroxides with peroxidase-modified electrodes

Abstract An amperometric enzyme electrode for organic peroxides, based on the incorporation of horseradish peroxidase (HRP) into a carbon paste matrix, is reported. The electrode responds rapidly to low (micromolar) concentrations of such peroxides, in accordance to the following sensitivity trend: butanone peroxide > tert-butyl peroxibenzoate > cumene hydroperoxides > triphenylmethyl hydroperoxide > tert-butyl hydroperoxide > tert-amyl perbenzoate > tertbutyl peroxiacetate. The effect of various operational parameters is explored for optimum analytical performance. The dynamic properties of this electrode are exploited for detection in flow injection systems. Work in mixed nonaqueous-aqueous media allows the quantitation of highly hydrophobic peroxides and extension of the linear range. Similar organic peroxide detection is reported for the incorporation of HRP within a rigid graphite-epoxy matrix, and for a tissue (horseradish-root) modified electrode. Applicability of the HRP electrode for assays of drinking water is illustrated and prospects for its utility in environmental work are discussed. Analogous measurements using another (fungal) peroxidase are reported.

Differential pulse voltammetry with medium-exchange for analytes strongly adsorbed on electrode surfaces

Abstract Differential pulse voltammetry with medium-exchange is employed for selective and sensitive measurements of electroactive species strongly adsorbed on electrode surfaces. In order to minimize interferences resulting from macro solution constituents, the working electrode is transferred into a electrolyte solution possessing ideal background characteristics following the preconcentration/adsorption step. The increased selectivity obtained by making the measurement in the solution is demostrated by the determination of the adsorbable species in the presence of 10 3 –10 4 -fold excess of a solution species with similar redox potential. Illustrative voltammograms of chlorpromazine in blood and urine are presented.

Preconcentration of uric acid at a carbon paste electrode

Abstract Uric acid is shown to adsorb rapidly on a carbon paste electrode, with the surface species retaining its characteristic electroactivity. Using this phenomenon to preconcentrate uric acid, voltammetric measurements are improved with respect to sensitivity and selectivity. By simply immersing the electrode in the uric acid-containing sample for a given period of time, and then transferring the electrode to an electrolytic blank solution, a high degree of selectivity is achieved. Solution-phase species with similar redox potentials, e.g., ascorbic acid, do not interfere. The differential pulse response is evaluated with respect to concentration dependence, preconcentration period, reproducibility, rotation speed, pH, paste composition, and other variables. The method is exploited for selective detection of uric acid in a flow injection system. Illustrative voltammograms of uric acid in urine and plasma are presented.

Adsorptive/extractive stripping voltammetry of phenothiazine compounds at carbon paste electrodes

Abstract An investigation of the extractive/adsorptive accumulation behavior of various phenothiazine drugs at carbon paste electrodes is presented. These compounds exhibit different degrees of accumulation, depending on the side-chains of the molecule. The voltammetric experiments are accompanied by analogous solvent extraction studies. The extraction/adsorption process serves as an effective preconcentration step which improves the subsequent voltammetric measurement with respect to the sensitivity and selectivity. After 10 min preconcentration, a detection limit near 1.5 × 10−9M perphenazine is obtained. By making the measurement in an exchange (blank) solution, the accumulated phenothiazines can be measured in the presence of macro solution constituents with similar redox properties. Applicability to selective measurements in serum is illustrated.

Vitreous carbon-based composite electrode as an electro-chemical detector for liquid chromatography

Abstract The characteristics of an electrochemical detector for liquid chromatography with an array of vitreous carbon electrodes in a thin-layer channel are described. This array is fabricated by filling the pores of a reticulated vitreous carbon disk with non-conductive epoxy. The resulting flow-by electrode exhibits a flow-rate independent response due to its nonlinear mass-transport properties. For the same reason, the detector shows 3–5 improvements in signal-to-noise characteristics over commercial detectors with continuous glassy caron electrode, operated under otherwise identical conditions. Epinephrine, dopamine, acetaminophen and potassium ferrocyanide were used as test systems to give detection limits at the pg level. Repetitive injections yield peak heights with a relative standard deviation of 0.3%. The cell is easy to construct from inexpensive materials.

Evaluation of differential pulse voltammetry at carbon electrodes.

The background and analytical responses of differential pulse voltammetry at carbon electrodes have been evaluated. Transient currents associated with small potential steps have been examined and are attributed mainly to redox reactions of various groups on the electrode surface. Instrumental parameters have been thoroughly investigated and optimized for improving the signal-to-background response. Glassy-carbon and carbon-paste disk electrodes have been compared, and the latter found to yield better response owing to the lower background current. Dopamine, epinephrine, chlorpromazine, ascorbic acid, homovanillic acid and ferrocyanide were used as test systems.

Subtractive differential pulse voltammetry following adsorptive accumulation of organic compounds

Subtractive differential pulse voltammetry following adsorptive preconcentration of organic compounds at solid electrodes is described. Different preconcentration periods are used, and the difference between the oxidation (stripping) currents is recorded. Background currents which are independent of the preconcentration period cancel out. Combining the enhanced peak current, due to the preconcentration step, with the background current correction of the subtractive mode, gives improved sensitivity and/or allows the use of shorter preconcentration periods. Chlorpromazine and dopamine have been used as test systems. A detection limit of around 1 x 10(-9)M has been obtained for chlorpromazine with a 10-min preconcentration period. Applicability to clinical samples is illustrated by the determination of chlorpromazine in whole blood and urine.

Flow electrolysis at a porous tubular electrode with internal stirring

Abstract The design and characteristics of a porous tubular electrode in which effective mass transport is ensured by a stirrer rotating inside the tube are described. The electrode provides high analytical currents owing to the combination of the high surface area with high mass-transport rates. The dependence of the limiting current and the degree of conversion on stirring rate, flow rate, and electrode length are described. The completeness of the electrolytic process is as high as 100%. A new hydrodynamic modulation mode, based on measuring the current difference while switching the stirring on and off, is used to correct background contributions. Analytical utility of the cell is demonstrated by application to flow injection and continuous flow analyses. Caffeic acid, p-acetylaminophenol, dopamine, and hexacyanoferrate(II) were used as test systems to give detection limits at the nanomolar concentration level.

Evaluation and improvement of the resolution of voltammetric measurements

Two approaches for estimating and improving resolution in chromatography analyses can be applied successfully to voltammetric measurements. It is shown that the resolution of voltammetric procedures yielding symmetric (or nearly symmetric) current peaks can be described by R = 2DeltaE (p)1.699 (b(1,2,1) + b(1,2,1)) where DeltaE(p) is the difference between the peak potentials of the two analytes, and b(1,2,1) and b(1,2,2) are the peak half-widths. The window diagram approach is used to improve the resolution between neighbouring voltammetric peaks by optimization of the supporting electrolyte composition. The applicability of these approaches to differential-pulse anodic-stripping measurements at the mercury film electrode is demonstrated.

Thin layer flow cell with a rotating disk electrode

Abstract A channel-type electrochemical flow cell with a rotating disk electrode has been constructed and characterized. The work shows that the effect of rotation is to enhance the rate of mass transport. At rotation speeds higher than 900 rpm the response becomes independent of the flow rate and channel thickness. As a result, low flow rates can be used without sacrificing the sensitivity. Analytical advantages are demonstrated using liquid chromatography, stripping analysis, stopped rotation voltammetry and flow injection analysis.