Gold electrodes from recordable CDs for mercury quantification by flow injection analysis

The development of a new methodology for the construction of very efficient flow cells for mercury detection by potentiometric stripping analysis, employing the thin gold layer of recordable CDs as working ¶electrode is reported. This new source of electrodes (CDtrodes) show very attractive performance, similar to that obtained with commercial gold electrodes, with superior versatility. The low cost of this new source of “gold electrodes” allows a frequent replacement of the electrode, avoiding cumbersome clean-up treatments. Various experimental parameters have been optimized to yield low detection limits (0.25 ng/mL of mercury for 5 min deposition at 0.3 V) and good precision (standard deviation of 1.9% was obtained for 15 repetitive measurements using 10 ng/mL of mercury). Standard curves were found to be linear over the range of 0.5–100 μg L^–1 of mercury. The flow cells developed were used for the quantification of mercury in oceanic and tap water.     

Determination of Se(IV) by anodic stripping voltammetry using gold electrodes made from recordable CDs

This work presents a simple, reproducible and low cost method, employing differential pulse anodic stripping voltammetry, for determination of selenium(IV). A gold electrode obtained from recordable compact disks (CD-R) was used to evaluate the voltammetric behavior of the metallic ion in 0.1 mol L⁻¹ HClO₄. To evaluate the voltammetric behavior of Se(IV), parameters such as deposition potential and deposition time were optimized. A wide linear response range, from 0.5 to 291 ng mL⁻¹, was obtained using a 5.0 mm diameter gold electrode. Recovery tests for Se(IV) utilizing standard reference solutions provided values between 94 and 96%.     

Multi-enzyme electrodes for the determination of starch by flow injection

A tri-enzyme electrode has been developed for determining starch in a flowing stream based on amperometric monitoring of hydrogen peroxide at a potential of +600 mV versus a silver--silver chloride reference electrode. The nylon-based starch-sensing membranes (over a platinum electrode) were prepared from an enzyme cocktail containing various ratios of amyloglucosidase (AMG), mutarotase (MUT) and glucose oxidase (GO). The best starch-sensing membrane (to give the type A electrode) was made from an enzyme cocktail of AMG--MUT--GO (2000 + 100 + 100 U; where 1 U = 16.67 nanokatals), i.e., containing a high level of AMG. In this system, starch samples were first incubated with soluble alpha-amylase (alpha-AMY) (1000 U) for 1 h at room temperature prior to analysis with the tri-enzyme electrode. Attempts were also made to immobilize alpha-AMY on to nylon net, either alone or as a component of a four-membrane starch-enzyme electrode but starch signals were weak compared with those generated by starch pre-treated with soluble alpha-AMY. This system, associated with the type A starch electrode, not only exhibited a calibration of wide linear range (1 x 10(-4)-0.1% m/v starch) but also showed promising operational properties. It has excellent thermal stability over the range 30-70 degrees C.     

Fast analysis of terbutaline in pharmaceuticals using multi-walled nanotubes modified electrodes from recordable compact disc

In this study, homemade disposable gold electrodes made from recordable compact disks were modified with carbon nanotubes for amperometric quantification of terbutaline sulfate in pharmaceutical products. A flow cell using an impingent jet of solution on the electrode surface was build and used for amperometric detection, and a series of experiments were carried out to find the best experimental conditions for the new electrode in a specially designed cell. A linear response for terbutaline was obtained in the range from 3.0 × 10⁻⁶ to 5.0 × 10⁻⁴ mol L⁻¹ (at 0.63 V vs. Ag/AgCl). The limits of detection and quantification were calculated as 5.8 × 10⁻⁷ mol L⁻¹ (S/N = 3) and 1.9 × 10⁻⁶ mol L⁻¹ (S/N = 10), respectively. A frequency of 30 injections h⁻¹ was attained. The proposed method was successfully applied to the analyses of commercial syrup samples, and all results were in good agreement with those obtained by using high performance liquid chromatography and capillary electrophoresis-tandem mass spectrometry.     

On-line determination of mercury(II) by membrane separation flow injection analysis

A rapid and inexpensive gas-diffusion (GD) flow injection method for the on-line determination of Hg(II) in aqueous samples is described. The analytical procedure involves the injection of a Hg(II) sample into a 1.5 M H₂SO₄ carrier stream which is merged with a reagent stream containing 0.6% SnCl₂ and 1.5 M H₂SO₄. Under these conditions Hg(II) is reduced to metallic mercury which partially evaporates through a Teflon membrane into an acceptor stream containing 1.75×10⁻⁴ M KMnO₄ in 0.3 M H₂SO₄. The decrease in the absorbance of the acceptor stream at 528 nm corresponding to the absorption maximum of the permanganate anion can be related to the original concentration of Hg(II) in the sample. The method is characterized by a detection limit of 4 μg l⁻¹ and a sampling frequency of 8 h⁻¹. The flow system was successfully applied to the analysis of river samples spiked with Hg(II).     

Disposable barrel plating nickel electrodes for use in flow injection analysis of trivalent chromium

We report a disposable barrel plating nickel electrode (Ni-BPE) coupled with a specifically designed electrochemical cell for use in flow injection analysis for the determination of trivalent chromium (Cr^III). The response of the activated Ni-BPE was found very sensitive under alkaline condition with a systematic increase in current signal upon successive addition of Cr^III. Under optimized conditions, the calibration curve showed a linear range up to 1 mM with a detection limit (S/N = 3) of 0.30 μM. Continuous hydrodynamic flow of 0.1 M NaOH at regular Cr^III injections verify the reproducibility of the present system. The proposed method was successfully used to detect the amount of Cr^III in environmental water to show its potential use in real applications.     

Ultra-simple adaptor to convert batch cells with mercury drop electrodes in voltammetric detectors for flow analysis

A simple, robust and fast-responding flow adaptor for mercury drop electrodes (MDEs) is described. An L-shaped PTFE tube with an internal diameter of 0.5 mm is fixed with a silicone ring on the glass capillary of a MDE, in such a way as to direct the outcoming flow onto the mercury drop, from a distance of about 0.5 mm. Any commercial or laboratory-made batch cell, provided with an MDE, serves for the purpose. The level of supporting electrolyte in the cell is maintained constant through a siphon or a lateral draining orifice. The adaptor is compatible with all the different brands and operating modes of the MDEs (free dropping, controlled drop time, renewable static drop, hanging drop or sessile drop). Flow injection experiments were conducted with the following amperometric detection modes: sampled-DC, reverse pulse amperometry (RPA), and anodic stripping voltammetry (ASV). The FIA-RPA peaks presented a R.S.D.<0.8% for 1.0×10⁻⁵ mol l⁻¹ lead(II) (N=30, V_sample=100 μl). The response time (0-63% of the signal maximum) to a concentration step is 1.2 s for 500 μl injections of 0.1 mmol l⁻¹ ascorbic acid in acetate buffer at a flow rate of 1 ml min⁻¹, which corresponds to a response volume of 20 μl. As an example of practical application, copper(II) was determined in fertilizers by RPA using the standard addition method, at an analytical frequency of 90 injections per h.     

An electrochemical cell coupled with disposable screen-printed electrodes for use in flow injection analysis.

An electrochemical cell coupled with disposable screen-printed electrodes (SPEs) that is specifically designed for use in flow injection analysis (FIA) is described in this study. The cell is made of foldable polyoxymethylene (acetal) thick platelets with the bottom portion consisting of a cavity track to drag the SPEs in position and the top portion having predrilled T-like holes to arrange the Ag/AgCl reference electrode and stainless steel inlet & outlet. An "O ring" is suitably fixed on the top of the working electrode to form a thin-layer space where the electrochemical reaction can take place. Hydrodynamic characterization was validated by using a benchmark hexacyanoferrate redox couple. The results of practical analysis of glucose in human plasma clearly demonstrate the characteristics and applicability of the proposed wall-jet electrochemical cell in FIA.     

Selectivity enhancement by flow injection analysis

Flow injection analysis offers numerous possibilities for significantly increasing the selectivity of existing methods by utilizing knowledge of the chemistry of those methods. It also enables new selective methods to be created by utilizing kinetic methods and fast separation techniques such as gas diffusion, dialysis, extraction and ion-exchange columns. Selectivity enhancements and increased sensitivity can be achieved by incorporating the kinetic techniques of kinetic discrimination and/or kinetic enhancement into the timing of the system or the reagent concentrations and conditions for a given method. Methods have been developed for quantifying ozone, chlorine dioxide, chlorate ion, and chlorite and chlorate ions sequentially. A dual-phase gas diffusion system for hydride generation provides significant decreases in the interferences observed for transition metals.     

A Simple injection valve for flow injection analysis

A novel sample introduction valve, based on a modified glass syringe, is described for flow injection analysis. Sample volumes of 1.4 μl or more can be injected manually or automatically, by driving with a 12-V solenoid. The valve has advantages of freedom from maintenance and long lifetime.     

Citrate selective electrodes for the flow injection analysis of soft drinks, beers and pharmaceutical products

Aiming the establishment of simple and accurate readings of citric acid (CA) in complex samples, citrate (CIT) selective electrodes with tubular configuration and polymeric membranes plus a quaternary ammonium ion exchanger were constructed. Several selective membranes were prepared for this purpose, having distinct mediator solvents (with quite different polarities) and, in some cases, p-tert-octylphenol (TOP) as additive. The latter was used regarding a possible increase in selectivity. The general working characteristics of all prepared electrodes were evaluated in a low dispersion flow injection analysis (FIA) manifold by injecting 500 μl of citrate standard solutions into an ionic strength (IS) adjuster carrier (10⁻² mol l⁻¹) flowing at 3 ml min⁻¹. Good potentiometric response, with an average slope and a repeatability of 61.9 mV per decade and ±0.8%, respectively, resulted from selective membranes comprising additive and bis(2-ethylhexyl)sebacate (bEHS) as mediator solvent. The same membranes conducted as well to the best selectivity characteristics, assessed by the separated solutions method and for several chemical species, such as chloride, nitrate, ascorbate, glucose, fructose and sucrose. Pharmaceutical preparations, soft drinks and beers were analyzed under conditions that enabled simultaneous pH and ionic strength adjustment (pH=3.2; ionic strength=10⁻² mol l⁻¹), and the attained results agreed well with the used reference method (relative error<4%). The above experimental conditions promoted a significant increase in sensitivity of the potentiometric response, with a supra-Nernstian slope of 80.2 mV per decade, and allowed the analysis of about 90 samples per hour, with a relative standard deviation <1.0%.     

Cloud-point extraction/preconcentration on-line flow injection method for mercury determination

A cloud-point extraction/preconcentration (CPE/P) step is incorporated on-line into a flow injection system which is used to determine low levels of Hg(II) added to natural water samples. The analyte is complexed with dithizone. A solid reagent column (SRC) is used to prepare the reagent on-line by using 5% (v/v) Triton X-100 solution as solvent. The CPE/P is carried out by using the non-ionic surfactant Triton X-100. After obtaining the cloud-point on-line, the surfactant-rich phase containing the complex is collected in a mini column packed with cotton wool. Then, a hot water stream is passed through the column to elute the complex and the absorbance is measured at 500 nm.All the flow and chemical variables are optimized and the enhancement factor for the system is estimated. The calibration is linear over the range 0.05-0.5 μg ml⁻¹, the R.S.D. is 4.8%, the limit of detection (signal:noise = 3) is 0.014 μg ml⁻¹ and the sample throughput is 30 h⁻¹. An open/closed system is used to eliminate the interference of iron(III).     

Amperometric detection of hydrazine by cyclic voltammetry and flow injection analysis using ruthenium modified glassy carbon electrodes

Glassy carbon electrodes modified with (5-amino-1,10-phenanthroline)bis(bipyridine)ruthium(II) chloride hydrate, [(bpy)₂Ru(5-phenNH₂)]Cl₂·H₂O, are shown to oxidize hydrazine with excellent sensitivity. The presence of an amine group on the ruthenium complex facilitates electropolymerization onto the electrode surface. Using cyclic voltammetry, a large catalytic current is observed upon oxidation of hydrazine in phosphate buffer (pH 5.0), compared to the current obtained from the ruthenium-modified electrode with no hydrazine present. The sensitivity of cyclic voltammetry is sufficient for obtaining a linear calibration curve for hydrazine over the range of 10⁻⁵ to 10⁻² M. Hydrodynamic amperometry was used to determine the working potential for flow injection analysis. The limit of detection for hydrazine was determined to be 8.5 μM using FIA. The thickness of these films was shown to increase linearly with the number of electropolymerization cycles, in the range of 1000-2500 nm for 5-20 cycles, respectively, using Rutherford backscattering spectrometry (RBS). RBS analysis also suggests that the film is multilayered with the outermost layers containing a high ruthenium concentration, followed by layers where the concentration of ruthenium decreases linearly and approaches zero at the electrode surface.     

Spindle capillary for mercury drop electrodes

The spindle capillary fundamentally improves the performance of dropping mercury and stationary mercury drop electrodes in their various applications and activates further development of electrochemical, electroanalytical and interfacial research techniques.     

Integrated microconduits for flow injection analysis

A novel approach for constructing continuous flow systems for analysis s described and its versatility is demonstrated for a number of flow-injection systems with integrated potentiometric or optical detectors and with integrated gas-diffusion or ion-exchange units. Optimization of the miniaturized designs was achieved by means of scaling factors based on the theory of similarity.     

Industrial process control by flow injection analysis

The principle of physiochemical and chemical modulation in flow injection analysis is outlined. Advantageous properties of flow injection analyzers for chemical on-line process control are summarized. Three exampls of applications are presented. First, peak-width measurements enable chemical batch processes to be monitored over several orders of magnitude of analyte concentration whereas peak-height measurements are selected in the critical state of the process where very small changes of analyte concentration must be determined precisely. Secondly, exploitation of variable gradient dilution to match sample concentration to the needs of accurate analysis is combined with trapped-zone selective spectrophotometry. Finally, frequency-discriminated chemical analysis is feasible by combining sample gradient formation with reagent injection.