The hydrodynamics of the amperometric detector flow cell with a rotating disk electrode

Series of photographs of the sample flow pattern in the flow cell with a stationary as well as a rotating disk electrode (RDE) were taken with a motor-driven camera. With the stationary electrode, the flow pattern in the cell was mushroom-like. Rotating the electrode generated a secondary fluid motion in the flow cell which manifested itself as vertical circulation of the solution present in the flow cell. A qualitative hydrodynamic explanation of the observed flow patterns is given. Peak broadening effects induced by the RDE in the flow cell were observed only at very fast rotation speeds and high nozzle heights. The response surface of the amperometric detector flow cell with the RDE as a function of the rotation speed and the nozzle height was measured by applying the detector in combination with high-performance liquid chromatography, flow injection analysis and continuous flow analysis. Model curve-fitting calculations indicate that the flow pattern in the flow cell can be laminar or turbulent, depending on the exact cell geometry, rotation speed and nozzle height.     

Large-volume wall-jet cells as electrochemical detectors for high-performance liquid chromatography

The wall-jet electrode is an attractive configuration for electrochemical detectors for high-performance liquid chromatography (h.p.l.c.) on account of its high convective mass-transfer characteristics. Another important, though less recognized, feature is its small effective cell volume, which is shown to be almost independent of the geometric cell volume. The effective volume is less than the volume of the hydrodynamic boundary layer, or only a few microlitres. The practical use of the wall-jet detector in both normal-phase and reverse-phase h.p.l.c. is discussed with particular reference to the distance between the jet and the electrode. A new cell design is proposed.     

Evaluation of kel-f—graphite electrodes as detectors for continuous flow systems

A new electrode material, fabricated from Kel-F powder and graphite powder by pressing at 250°C and 1000 p.s.i., is applied as a detector in a thin-layer, flow-through cell, for applications to continuous flow streams. The detector is useful in a wide range of solvents for a wide range of species. Applications are described for 1,1'-bis(hydroxymethyl) ferrocene (BHMF), ascorbic acid, ferrocene, phenol, and hydroquinone, using various aqueous and methanolic solvent/electrolyte compositions. Detection limits range from 800 pg for BHMF to less than 20 pg for phenol. Background current fluctuations are used to estimate flow rate stability, which is the dominant factor in setting detection limits.     

Flow injection determination of bromide ion in a developer using bromide ion-selective electrode detector

A potentiometric flow injection determination method for bromide ion in a developer was proposed, by utilizing a flow-through type bromide ion-selective electrode detector. The sensing membrane of the electrode was Ag(2)S-AgBr membrane. The response of the electrode detector as a peak-shape signal was obtained for injected bromide ion in a developer. A linear relationship was found to exist between peak height and the concentration of the bromide ion in a developer in a concentration range from 1.0x10(-3) to 1.0x10(-2) mol l(-1). The relative standard deviation for 10 injections of a 6x10(-3) mol l(-1) bromide ion in a developer was 1.3% and the sampling rate was ca 17-20 samples h(-1). The present method was free from the interference of an organic reducing reagent, an organic substance in a developer sample solution for the determination of bromide ion in a developer.     

An electrochemical detector with a dropping mercury electrode for high-performance liquid chromatography

The electrochemical flow-through cell described has an active volume of less than 1 μl, and incorporates a dropping mercury electrode with drop times of about 0.05 s. Its performance as a detector for high-performance liquid chromatography is assessed for p-nitrophenol and nitrobenzene. The detection limits are 4–5 ng; the relative standard deviation of the peak height is better than 5% for the range 5–150 ng. The dependence of the response on flow rate, mercury pressure and drop time is described.     

Evaluation of high-performance liquid chromatography with a dual working-electrode electrochemical detector for the determination of catecholamines in human cerebrospinal fluid

A procedure is described for quantifying norepinephrine [1(3,4-dihydroxyphenyl)-2-aminoethanol] and dopamine (4-(2-aminoethyl)-1,2-benzenediol] in cerebrospinal fluid by high-performance liquid chromatography (h.p.l.c.) with dual working-electrode electrochemical detection. An ion-pairing reversed-phase h.p.l.c. technique is used after clean-up and preconcentration of the fluid on alumina. The dual working-electrode electrochemical detector used provides amplification in the electrochemical response per unit concentration of material passing through the detector. This is achieved by locating two working electrodes on the opposite walls of a thin-layer cell, holding them close together and at potentials that will induce a sequence of oxidation/reductions. The greater the number of oxidation/reduction cycles, the greater the amplification of the electrochemical response. Peak height ratios are linear over the range expected and detection limits are 3 pg for norepinephrine and 5 pg for dopamine. The concentrations of norepinephrine and dopamine in human cerebrospinal fluid were found to be 141 pg ml^?1 and 262 pg ml^?1 with relative standard deviations of 6.8 and 5.8%, respectively.     

Development of a tubular fluoride potentiometric detector for flow analysis: evaluation and analytical applications

In this work a construction procedure for tubular fluoride electrode to be used in flow systems is outlined. The electrode was constructed from a commercially available, LaF₃ single crystal. Principal advantages of the flow detector presented include simplicity of construction, robustness, durability, low cost and easy coupling into any point of a flow manifold.Evaluation of the intrinsic working characteristics of the potentiometric detector in a low dispersion manifold is presented with respect to analytical and dynamic parameters. The constructed detector has similar working characteristics to those of the conventional fluoride electrodes, namely the detection limit, lower limit of linear response and operational pH range.The analytical usefulness of the constructed device was assessed in a flow system developed for fluoride determination in toothpaste, tablet, collutory and water samples for which the reference procedures suggest the determination of fluoride ion with a conventional ion selective electrode.     

Flow injection analyses: Part I. A new concept of fast continuous flow analysis

The concept of a new continuous flow analyser system is described. Based on instant discrete sampling by injection into a carrier stream, the system allows continuous flow analysis to be performed in a fast, much simplified way. As the continuous flowing stream is characterized by a turbulent rather than a laminar flow, the discrete instant sampling creates geometrically well-defined segments of sample solution within the flowing stream. Because of the absence of lag phase, an unprecedented sampling rate for continuous flow analysis of well over 200 samples per hour can be achieved; and even manual injection of the samples allows a very high degree of accuracy and precision to be obtained ( ? ± 1%). Uses of the system in various analytical procedures are described and discussed. A potentiometric sensor (the air-gap electrode used in a flow-through unit) and a spectrophotometric arrangement with a flow-through cell have been used as detector units.     

Thin layer flow cell with a rotating disk electrode

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.     

Detection of phenolic compounds in flow systems based on tyrosinase-modified reticulated vitreous carbon electrodes

The fabrication and performance of a reticulated vitreous carbon (RVC)-based tyrosinase flow-through electrode, in which the enzyme was covalently immobilized, is reported. The bioelectrode was tested as an amperometric detector for phenolic compounds. Variables affecting the construction of the enzyme flow-through electrode such as the RVC chemical pretreatment procedure, the enzyme immobilization method in the RVC matrix, the enzyme loading and the pH value of the buffer solution used, were optimized by flow-injection with amperometric detection. A good immobilization of the enzyme in the RVC matrix, in spite of the hydrodynamic conditions, was found. The same tyrosinase-RVC electrode could be used with no significant loss of the amperometric response for around 20 days, and reproducible responses could be achieved with different electrodes constructed in the same manner. Moreover, the operational stability of the bioelectrode was tested under continuous monitorization conditions. Calibration plots by flow injection with amperometric detection at -0.20 V were obtained for phenol, 2,4-dimethylphenol; 3-chlorophenol; 4-chlorophenol; 4-chloro-3-methylphenol and 2-aminophenol, with detection limits ranging from 2 mug l(-1) (4-chloro-3-methylphenol) to 2 mg l(-1).     

Which Parameters Affect the Response of the Channel Biosensor?

The important parameters in defining the response of the portable channel biosensor described previously are explored by connecting the portable flow cell to a gravity feed flow system and using a highly defined enzyme immobilization protocol which ensures the enzyme reaction is a surface reaction. The enzyme glucose oxidase (GOD) was immobilized by covalent attachment to a self‐assembled monolayer modified gold surface. As a glucose solution flowed down the rectangular duct defined by the flow cell, it passed over the enzyme layer where the enzyme reaction produced hydrogen peroxide. The hydrogen peroxide was swept further downstream to the detector electrode. The response of such an enzyme electrode was shown to be limited by mass transport of the cosubstrate oxygen to the enzyme layer. Increasing the amount of oxygen in the sample meant the response of the biosensor became limited by the enzyme kinetics. The influence of parameters such as flow rate, height of the channel, enzyme layer length and the gap between the enzyme layer and the detector electrode were explored.     

Limiting diffusion currents in hydrodynamic voltammetry: Part IV. Stationary disk and ring electrodes in a rotational flow produced by a rotating disk

A new electrolysis cell for the use in hydrodynamic voltammetry is devised, in which a stationary disk or ring electrode is immersed coaxially in a rotational flow produced by a uniformly rotating disk (rotor) and works as an indicator electrode. Theoretical equations of the limiting diffusion currents at such a disk or ring electrode are derived. The theoretical predictions for the dependence of the limiting diffusion current upon various experimental variables, i.e., the rotation speed of the rotor, the bulk concentration of depolarizer, the geometrical parameter of the electrode and the viscosity of the solution, are verified experimentally for the reduction of ferricyanide ions.     

The behaviour of an electrochemical detector used in liquid chromatography and continuous flow voltammetry: Part 2. Evaluation of low-temperature isotropic carbon for use as an electrode material

Unalloyed Pyrolyte, a low-temperature isotropic carbon (LTIC) material is evaluated electrochemically for use as an electrode material in h.p.l.c. thin-layer electrochemical detection cell systems. LTIC is compared to carbon paste electrode material in all systems studied, and to glassy carbon impregnated with ceresin wax in the hexacyanoferrate(II)/(III) redox system. Electrode areas, residual currents, and potential windows are described. Quantitative kinetic studies are reported for the hexacyanoferrate(II)/(III) redox pair in 2.0 M KCl, and rate constants are given. Quantitative rate constant studies are also reported for o-dianisidine in 1.0 M H₂SO₄. Finally, qualitative rate comparisons are made by cyclic voltammetric techniques between carbon paste and LTIC electrode materials for maetanephrine, normetanephrine, adrenaline, noradrenaline, dopamine, 3,4-dihydroxyphenylalanine, 5-hydroxytryptophan, and chlorpromazine in chemical systems reflecting previous applications of h.p.l.c. with electrochemical detection.     

Use of ion-selective electrodes in industrial flow analysis

Two cells with ion-selective electrodes are described for industrial flow analysis, involving the flow-injection technique and continuous monitoring. The dependence of the electrode signal on the experimental parameters is discussed using the determination of cyanide as a model. It is shown that sensitive and reproducible measurements are possible even if the industrial application requires that the measuring cell be large, provided that the flow rate of the carrier liquid is sufficiently high. Large cel volumes are advantageous in continuous monitoring, as the signal approaches that obtained under steady-state conditions. Experiments with a calcium ion-selective electrode and with an ammonia gas probe show that the slow response of the sensor severely limits its applicability in flow analysis.     

Determination of sulfite in beer samples using an amperometric fill and flow channel biosensor employing sulfite oxidase

A simple method is described to determine sulfite in beer samples using a fill and flow channel biosensor. A droplet of sample is placed into the inlet of a rectangular flow cell and begins to flow through the channel by capillarity. The flow is maintained and controlled by a porous outlet plug of defined porosity. In a rectangular flow cell, the sample solution flows through three consecutive zones: over a predictor electrode, an enzyme layer and a detector electrode. Together these three zones enable the differentiation between current due to sulfite and current due to other electroactive species in the sample. The predictor electrode is located upstream, and on the opposite channel wall to the enzyme layer and detector electrode, and is poised at the same potential (+0.65 V versus Ag/AgCl) as the detector electrode. On this electrode, the current contribution from all species in the sample solution that are oxidized at that potential is determined. The enzyme layer contains sulfite oxidase, which, in the process of oxidizing sulfite, produces hydrogen peroxide, which itself is reduced by excess sulfite. The current at the downstream detector electrode is therefore different from that at the predictor electrode as a result of the enzyme reaction and the difference of the currents, corrected for the dimensions of the electrodes, is proportional to the concentration of sulfite. The method enables a straightforward correction of the interfering current at the detector electrode and a determination of the analyte concentration. The effect of interferences from ascorbic acid, ethanol, sorbic acid and tartaric acid in the detection of sulfite is efficiently removed. The concentration of sulfite in a sample of beer measured by the biosensor is equivalent to that measured using a reference method based on the AOAC-recommended Monier-Williams method.     

Single-point titrations: Part 4. Determination of acids and bases with flow injection analysis

A single-point titrimetric system for acids and bases based on the flow injection principle is reported. The sample (30 μl) is introduced into a water stream with a pneumatic injector; this stream reacts with a linear acidic or basic buffer solution in a merging stream, and the peak height is recorded potentiometrically with a glass electrode in a flowthrough cell. The peak maxima are a linear function of the acid or base concentration in the range 0.01–0.1 M. At a sampling rate of 180 samples per hour, the relative standard deviation is less than 1%. The method can be used at sampling rates as high as 720 samples per hour.     

High flow-rate cells for continuous monitoring of low concentrations of electroactive species by polarography and stripping voltammetry at the static mercury drop electrode

A high-capacity flow-through cell which can be used at a maximum flow rate of 300 ml min^-1 has been developed for continuous monitoring of electroactive substances. The cell is compatible with the recently developed static mercury drop electrode. Comparative studies with a cell employing a conventional dropping mercury electrode are described. A wide range of polarographic techniques is applied, and it is demonstrated that the static mercury drop electrode improves the limits of detection, that laminar flow conditions are essential for low noise levels of operation, and that solution flow through a sulphite bed is a more effective method of oxygen removal than nitrogen bubbling. The combination of a microprocessor-controlled polarographic system, static mercury drop electrode and high-volume flow cell is very versatile for the determination of trace levels of electroactive species in flow streams. Preliminary results on anodic stripping voltammetry in flow streams are reported.     

Determination of peroxides by capillary zone electrophoresis with amperometric detection

The combination of cathodic amperometric detection with capillary zone electrophoresis is demonstrated to be a versatile method for the quantification of organic and inorganic peroxides. A gold microelectrode, polarized at -600 mV against an Ag/AgCl reference electrode, is placed at the end of the capillary. Since the electroosmotic flow purges the detector electrode from oxygen, no degassing of the detector cell or the sample is necessary. With an injection volume of ca. 1 nl, hydrogen peroxide, peroxosulfate, peroxy alkanoic acids and the hydroperoxides of linoleic acid can be detected down to 10 micromol/l. Separation of the isomeric hydroperoxides of the unsaturated fatty acids is achieved by addition of beta-cyclodextrin to the electrolyte.     

Amperometric detection of amines and amino acids in flow injection systems with a nickel oxide electrode

The use and characteristics of a nickel oxide electrode as a detector for amines in a flow injection system are described. The anodic electrode reaction mechanism involves a higher oxidation state of nickel maintained by the applied potential (+0.49 V vs. SCE). The electroanalytical parameters are investigated and the currents for a series of amines and amino acids are compared. Two electrode configurations are compared. The flow injection technique is shown to be suitable for buffered 25-μl samples of pH as low as 3. The linear range for glycine is 10^-6–10^-3 M with detection limits of a few nanograms.     

Identification of barbiturates from extracts of urine,stomach fluid,liver and kidney by high-performance liquid chromatography and field desorption mass spectrometry1,2

The characteristic behaviour of a series of important barbiturates in field desorption mass spectrometry (f.d.m.s.) is described. Detection limits are reported for three standard barbiturates by f.d.m.s. and high-performance liquid chromatography (h.p.l.c.). The off-line combination of h.p.l.c. and f.d.m.s. is used in forensic investigations to identify these drugs unequivocally in body fluids such as urine and stomach fluids and in extracts from human liver and kidney tissues. Eighteen barbiturates were tested. The use of f.d.m.s. as an off-line h.p.l.c. detector provides detection limits (S/N =3:1) ranging from 1 ng g^-1 to 10 ng g^-1, depending on the compound and on the preliminary clean-up used.