Wall-jet Cell Research Articles

Highly sensitive and selective determination of riboflavin by flow injection analysis using parallel dual-cylinder microelectrodes

A novel wall-jet cell with parallel dual cylinder (PDC) microelectrodes was constructed and used for flow injection analysis (FLA). The detector takes the advantages of “redox recycling” between bipotentiostated microcylinder electrodes (−0.4 V/SCE and 0.0 V/SCE respectively) and shows high sensitivity and selectivity for riboflavin which is electrochemically reversible. In 0.1 mol l −1 NaHSO 3 media and at a flow rate of 1.0 ml l −1, 56.8% collection efficiency was found at a PDC device with cylinder diameter of 20 μm and an interelectrode gap of 2 μm. With this device, the FIA peak current at collector electrode had a linear relationship with riboflavin concentration over a wide range (from 70 ng to 35 pg), and the detection limit was 35 pg (signal-to-noise ratio of 3). For 10 successive replicate injections of 35 ng and 3.5 ng of the analyte, relative standard deviations were 1.7% and 2.8% respectively. This method was applied to the direct determination of riboflavin (vitamin B 2) in multivitamin tablets without the usual liquid chromatographic separation procedures and good results were obtained. The method proposed is simple, rapid and economic.

Discontinuous-flow potentiometric determination of chloride in a large-volume wall-jet cell using an ion-selective electrode based on a silver chloride film chemically deposited on a silver iodide support

A robust and sensitive chloride ion-selective electrode can be prepared by modifying the surface of an iodide-selective electrode using the chemical reaction with mercuric chloride in an oxidizing medium containing excess chloride. A thin film of silver chloride is thus formed ensuring a rapid and reproducible response to chloride. The analytical parameters of this electrode are similar to those of commercial silver chloride ion-selective electrodes, but its electrical impedance and signal noise are substantially lower and the response somewhat faster. Its sensitivity toward surfactants is somewhat suppressed. The electrode was used for discontinuous flow potentiometric (DFP) determinations in a large-volume wall-jet cell in which the electrode surface can be continuously reactivated by a cleaning solution contained in the cell. The method was applied to determination of chloride in ground waters from an industrial waste dumping site. The limit of determination is low 9 μg Cl −/l (2.6 × 10 −7 M), the precision good (the relative standard deviation varies from 0.6 to 3.0% for chloride contents from 2.90 to 0.15 mg/l, respectively) and the method correlates satisfactorily with the results of an indirect AAS determination of chloride. The sample throughput is high—90 measurements can be carried out per hour, corresponding to 30–40 determinations per hour.

Batch and flow determination of uranium (VI) by adsorptive stripping voltammetry on mercury-film electrodes

Uranium(VI) was determined by square wave adsorptive stripping voltammetry on a rotating-disc mercury-film electrode using 8-hdyroxyquinoline as a complexing agent (batch mode), or, alternatively, on a wall jet mercury-film electrode with cupferron as complexing agent (flow-through mode). For the flow-through configuration, the combination of a novel wall jet cell with an automated flow system and a commercial computer-controlled potentiostat allowed complete keyboard control of the sequence of operations. Both continuous flow and flow injection (FI) schemes for preconcentration were assessed, and various instrumental parameters are discussed. Linearity held typically for a concentration range of two orders of magnitude (for 60 s preconcentration). It was found that, whereas the batch and continuous-flow modes are more suitable for the determination of lower concentrations of UVI( 10–7 mol l–1). The batch and flow methods were, finally, applied to the determination of UVI in a reference sea-water sample with accuracies of 7.2 and 4.2%, respectively, and relative standard deviations of 7.8 and 8.3%, respectively.

Small-volume batch-injection analyser

The performance characteristics of a small-volume wall-jet cell for batch-injection analysis (BIA) are evaluated. It is shown that a large dilution is not required for reliable BIA/amperometric measurements, as the cell solution is not active under the wall-jet hydrodynamics. Hence, the analyte accumulated in the cell over a prolonged operation is not contributing to the response. In contrast, BIA/potentiometric measurements, carried out in the small cell, are subject to memory effects that can be addressed via proper attention to the cell chemistry. The cell design (40 ml capacity) permits easy and fast exchange from amperometric and potentiometric sensing surfaces and maintains the attractive features of large-volume BIA cells (speed, precision, microlitre samples, etc.). Such a cell should facilitate the development of portable and compact BIA systems and would minimize the disposal of large-volume cell solutions. The study also offers a better understanding of the factors influencing BIA operations.

Electrochemical detection for flow analysis and liquid chromatography: present status and some roads to the future

The field of flow electroanalysis is briefly surveyed, emphasizing the common hydrodynamic basis of continuous monitoring, continuous flow and flow-injection analysis and liquid chromatography (LC). Examples of recent developments are given for potentiometric and voltammetric detection. Special attention is paid to the use of ion-selective microelectrodes and large-volume wall-jet cells in potentiometric detection, the use of voltammetric microelectrodes for detection in open-tubular column LC, physical, electrochemical and biochemical modification of solid voltammetric electrodes and application of cells with solid polymer electrolytes to amperometric detection in liquids with very low electrical conductivity.

Flow monitoring of glutamate and aspartate in foods and pharmaceutical products with immobilized bienzyme electrochemical cells

AbstractElectrochemical enzyme cells for glutamate and aspartate analysis have been assembled and used in continous‐flow and in flow‐injection analysis. The enzymes, glutamate oxidase (GOD) and aspartate amino‐transferase (AST), were co‐immobilized on an Immobilon Immuno‐affinity membrane, which was placed into a three‐electrode, flow‐through cell. Additional protective membranes were used to reduce the electrochemical interferences and to protect the immobilized enzymes. Thin‐layer and walljet cells were used and compared. Calibration curves for glutamate and aspartate showed a linearity range of 1–500 μmol/L and 10–500 μmol/L respectively. The sensitivity was higher using the flow‐through system with a response time of 2 min. Better reproducibility was achieved using the FIA procedure with a response time of less than 1 min. Glutamate in foodstuffs and asparatate in some pharmaceutical products were analyzed. Results were compared and correlated well with an HPLC procedure.

Determination of penicillin-V in standard solution and in fermentation broth by flow-injection analysis using fast-responding enzyme glass electrodes in different detection cells

An enzyme immobilization technique to measure penicillin concentration was developed. Penicillinase is immobilized by cross-linking with a very fine film of glutaraldehyde, which is sprayed on to the sensitive ends of two different pH glass electrodes. An extremely short response time of less than 10 s is guaranteed with electrodes made by this technique when measurements of penicillin are conducted in batch. The electrodes show good stability for 12 days when the sensor remains at room temperature. As the response time of the sensor is very short, an immediate determination of penicillin-V is possible by incorporating this enzyme electrode in a continuous-flow system, thereby dispensing with the need for an on-line enzyme reactor. The penicillin concentration is calculated as the difference in the potentiometric signal obtained as a peak when the sample flows past the sensor. Sample throughputs up to a maximum of 200 h −1 can be achieved. Samples are injected into a carrier stream consisting of a buffer of constant pH. Three different configurations of detection cells were designed and tested with the penicillinase electrode. First the performance of a magnetically stirred flow cell was studied. Subsequently a modified flow-through cell was tested, where the sensor was placed perpendicular to the direction of flow. A wall-jet cell was used to test the electrode where the flow direction coincided with the electrode axis. Finally, the magnetically stirred flow cell was used to detect penicillin-V in fermentation broth samples.

Unsegmented flow analysis with ion-selective electrodes by use of a large-volume wall-jet cell Continuous electrode reactivation in the determination of fluoride and chloride

A simple, large-volume wall-jet cell was designed for unsegmented flow analysis. The working electrode is immersed in a solution that reactivates the electrode surface. During the sample measurement, the working electrode is screened from the reactivation solution by the streaming sample solution; between the individual samples, air is pumped through the jet and agitates the reactivating solution at the electrode surface. The properties of the cell were investigated with the fluoride and chloride ion-selective electrodes and the method was applied to determination of fluoride and chloride in steel corrosion products and in a reference sample of the fly dust from electric-arc furnaces. The method permits about 90 measurements per hour, the results are reproducible and the limits of determination (6.3 × 10 −8 and 2.3 × 10 −7 M for fluoride and chloride, respectively) are substantially lower than the values commonly obtained in batch experiments. At least 150 measurements can be carried out without significant changes in the reliability of determination and the reactivation solution can then be replaced.

Chemically modified ion-sensitive field-effect transistors: application in flow-injection analysis cells without polymeric encapsulation and wire bonding

New wall jet and flow-through cells have been developed which can be used for flow-injection analysis using chemically modified ion-sensitive field-effect transistors (CHEMFETs). In these designs traditional bonding and encapsulation with epoxy resin have become superfluous. This ISFET chip has been designed to meet the requirements of these two cell types. Measurements with plasticized PVC-valinomycin-modified CHEMFETs in these flow cells are presented. The effects of flow-rate and sample size on the recorded peak were investigated. The operational performance of the wall jet and flow-through cells were compared; the recorded peaks with the wall jet cell are less symmetrical than those with the flow-through cell.

Design and properties of a flow-injection analysis cell using potassium-selective ion-sensitive field-effect transistors as detection elements

The combination of flow-injection analysis (FIA) and chemically modified ion-sensitive field-effect transistors (CHEMFETs) is described. In a wall-jet cell, two identical potassium-selective CHEMFETs were used for a differential measurement using a platinum (pseudo-)reference electrode. Silicone-rubber membrane materials, chemically bound to the SiO 2 gate oxide, were used with valinomycin as the ionophore. The optimized FIA system showed a linear response of 56 mV per decade for potassium concentrations above 5 × 10 −5 M. Preliminary results of potassium determinations in human serum and urine samples are presented.

Amperometric detection at a carbon-fibre array ring/glassy-carbon disk electrode in a wall-jet cell

A wall-jet cell incorporating a carbon fibre array ring/glassy-carbon disk electrode has been constructed, and characterized by the cyclic voltammetry and flow-injection techniques. The ring (composed of several microdisks) and glassy-carbon disk electrode, can be used separately for different purposes, e.g., detection in solution without a supporting electrolyte, collection/shielding detection with dual-electrode and voltammetric/amperometric detection with series dual-electrode. The electrode shows better collection and shielding effects than the usual ring—disk electrode in quiescent solution and the series dual-electrode in a thin-layer flow-through cell. The detection limit at the ring electrode is comparable with that at a conventional-size electrode, and has been used in the mobile phase without a supporting electrolyte, proving to be a promising detector for normal-phase liquid chromatography.

Liquid chromatography-electrochemical detection of inorganic arsenic using a wall jet cell with conventional and microsized platinum disk electrodes

A critical evaluation of a highly sensitive and specific liquid chromatography amperometric detection method (wall jet cell) for the determination of arsenic has been undertaken with platinum disk electrodes of variable size. The method for detection of arsenic involves a surface-based oxidation process in which As(III) is converted to As(V). This work suggests that advantages and disadvantages offered by using microelectrodes as an alternative to conventionally sized electrodes are critically dependent on the mechanism of the electrode process

The kinetics of colloidal deposition under conditions of controlled potential

The kinetics of the deposition of colloidal graphite particles onto an indium tin oxide electrode have been studied under conditions of controlled mass transport in a wall jet cell. The rate of deposition is followed by measuring the intensity of the light from the evanescent wave scattered by deposited particles. The effects of varying the potential on the deposition kinetics have been studied. For potentials more positive than −400 mV, the deposition into the primary minimum is always mass transport controlled, showing a radial dependence on r − 5 4 and a dependence on the volume flow rate of V 3 4 f. At more negative potentials, rather variable results are found. For some surfaces there is no deposition while for others the deposition is also mass transport controlled. Intermediate cases can be found. The intensity of the scattered light is less for particles deposited under these conditions and is less the lower the electrolyte concentration. These results are explained quantitatively by assuming that the particles are being held in the secondary minimum. The lower the electrolyte concentration, the further the secondary minimum is from the electrode surface and the lower is the intensity of the evanescent wave. For the intermediate cases kinetic analysis gives the depth of the secondary minimum. The variability of the results is discussed in terms of the roughness of the surface and the interaction of the particles with the radial velocity component.

Conducting organic salt amperometric glucose sensor in continuous-flow monitoring using a wall-jet cell

A simplified method for preparing tetrathiafulvalene−7,7,8,8-tetracyanoquinodimethane-based amperometric enzyme electrodes is described. The electrode is shown to be suitable for the routine monitoring of blood glucose levels. Using the wall-jet cell geometry, the steady-state mode gave more consistent results than the flow-injection mode because of variability in the blood matrix from patient to patient. The results show that, for the wall-jet cell geometry, due consideration must be given to the orientation of the enzyme electrode with respect to the jet inlet. High precision (<2% r.s.d.) and accuracy are then feasible.

Comparative study of first-, second- and third-generation amperometric glucose enzyme electrodes in continuous-flow analysis of undiluted whole blood

First-, second- and third-generation amperometric glucose enzyme electrodes were compared under flow-injection and steady-state conditions for the monitoring of undiluted whole blood. First-generation electrodes, based on the detection of hydrogen peroxide at a platinum electrode, are generally unsuitable because of the eventual poisoning of the electrode and because of their susceptibility to oxygen variation. Second-generation electrodes in which a mediator is used for the reoxidation of glucose oxidase are more suitable for the analysis of whole blood under both steady-state and flow-injection conditions. However, the choice of mediator is important. The best results with regard to linear range and stability were obtained with tetrathiafulvalene, whereas dimethylferrocene required considerable pretreatment before use. A third-generation electrode based on tetrathiafulvalene-tetracyanoquinodimethane where direct oxidation of glucose oxidase occurs also proved useful but showed lower stability and a smaller dynamic range compared with the second-generation devices. Flow-injection and steady-state studies were carried out using wall-jet cell geometry.

Design and development of a flow-through glucose reactor

A flow-through mini-reactor containing immobilized glucose oxidase (GOD) was designed, developed and evaluated. The immobilization of glucose oxidase directly on an electrode surface gives rise to very high losses in its activity whereas immobilization on a nylon support was found to be much more satisfactory. The mini-reactor, containing the covalently immobilized GOD, was coupled to the inlet of a wall—jet cell electrochemical detector, where the hydrogen peroxide produced by the enzymatic reaction was detected. This glucose sensor, mini-reactor plus wall—jet cell electrochemical detector, showed a detection limit of 5 μmol dm −3, good linearity and a very short response time and enabled us to study the kinetics of the enzyme immobilized on a porous support. The covalently immobilized enzyme has a lifetime of around one month. Application to the determination of glucose in blood plasma is described.

Design and development of a flow-through glucose reactor

A flow-through mini-reactor containing immobilized glucose oxidase (GOD) was designed, developed and evaluated. The immobilization of glucose oxidase directly on an electrode surface gives rise to very high losses in its activity whereas immobilization on a nylon support was found to be much more satisfactory. The mini-reactor, containing the covalently immobilized GOD, was coupled to the inlet of a wall-jet cell electrochemical detector, where the hydrogen peroxide produced by the enzymatic reaction was detected. This glucose sensor, mini-reactor plus wall-jet cell electrochemical detector, showed a detection limit of 5 μmol dm −3, good linearity and a very short response time and enabled us to study the kinetics of the enzyme immobilized on a porous support. The covalently immobilized enzyme has a lifetime of around one month. Application to the determination of glucose in blood plasma is described.

Dimetridazole Residues in Pork Tissue. I. Assay by Liquid Chromatography with Electrochemical Detector

A liquid chromatographic (LC) method with electrochemical detection in the reductive mode was developed for the quantitative determination of dimetridazole (DMZ) and its major metabolite (HMMNI) at residue levels in pork tissue. For blood plasma, a sample is precipitated with 2 volumes of acetonitrile and centrifuged, and a diluted aliquot of the supernatant liquid is chromatographed. For muscle, a 10 g sample is extracted 3 times with dichloromethane. After evaporation of the combined extracts, the residue is redissolved in a mixture of hexane and mobile phase (0.3% TEA in 0.6M ammonium acetate pH 5.0 and acetonitrile, 85 + 15) and centrifuged, and an aliquot of the lower phase is chromatographed. Chromatography is accomplished using valve switching with 2 liquid circuits, employing the same mobile phase for both. The sample is deaerated by sparging with helium under slight positive pressure to prevent rediffusion of the oxygen. The sample is first loaded into a deoxygenator and the flow is stopped for complete deoxygenation. The flow is then resumed to transfer the sample into the first, low back-pressure column (ODS, 10 microns, 4.6 x 200 mm). Switching the valve at this point removes the deoxygenator from the circuit and connects the first column to a second one (ODS, 5 microns, 4.6 x 150 mm) in tandem. After the effluent is passed through a second deoxygenator to reduce the residual oxygen in the mobile phase, it is monitored by an electrochemical detector with a screened wall jet cell and a gold mercury electrode, set at -1.2 V.(ABSTRACT TRUNCATED AT 250 WORDS)

Computerized flow injection potentiometric stripping analysis with large-volume wall-jet cell

A microcomputer system for flow injection potentiometric stripping analysis is presented permitting fast digital recording of potential time-curves. A large-volume wall-jet cell simple in design has been used as detector cell. Investigations about the influence of electrode pretreatment and solution delivery mode (i.e. gravity flow, pumping) reveal the strong influence of these parameters on the magnitude and precision of stripping times. The optimized system was applied to the simultaneous determination of cadmium and lead in geological samples.

Band broadening in a wall-jet type electrochemical detector for liquid chromatography

Band broadening in a large volume wall-jet cell electrochemical detector for liquid chromatography has been studied. Two regimes of stable operation were found which correlated well with the formation of smooth flow patterns. Smooth flow is also found to be important for avoiding erroneous results caused by backmixing.