Rapid-scanning electrochemical detector with micro working electrode for micro high-performance liquid chromatography

Summary A voltammetric system controlled by a personal computer was developed using a PC-8801 mk II computer (NEC, Tokyo, Japan) for rapid-scanning voltammetry. The system was combined with an ultra-small-volume electrolytic cell with a single-carbon-fiber (7-μm diameter and 3-mm length) working electrode and used as a new detector for micro high-performance liquid chromatography. The system's capability for the detection of catecholamines and their related compounds was investigated. Three-dimensional voltamo-chromatograms were obtained by adding the potential axis of the electrode to the conventional current and time axes, leading to a dramatic improvement in the identification of each chroamtographic peak.     

Berberine adsorption at a mercury electrode

The adsorption of berberine and canadine was measured by a.c. polarography. For berberine three different superficial structures were observed: the ions lying flat, the mixed layer of flat and perpendicular ions and the condensed film of perpendicular ions which incorporates anions of the supporting electrolyte. The boundaries between these structures are diffuse. The condensation is an instantaneous nucleation and two-dimensional film growth process. The adsorption of canadine is similar, but its condensed film is very stable and the exchange rate between the adsorbed canadine and the dissolved berberinium ions is very low. Different structures of the adsorbed berberine result in different faradaic square-wave responses.     

Residual photocurrent at a mercury electrode

The residual photocurrent observed at negative potentials with mercury in contact with slightly alkaline 1 M KCl solution has been reinvestigated and found to be caused partly by the reaction of e^?_aq with traces of unidentified electron acceptors and partly by reaction with the solvent. Such a solution can be freed from electron-accepting impurities by irradiation with light of shorter wavelength than 270 nm if SO^2?₃ is present at low concentration. The time constant for the homogeneous decay of e^?_aq at pH ca. 8.8 is then raised to about 400 μs. This slow decay is due to reaction with the solvent as the decay time constant can be increased considerably by raising the pH of the solution. In unsteady state experiments with purified 1 M KCl solutions of high pH containing no added scavenger for e^?_aq the current connected with electron emission and e^?_aq diffusion towards the electrode and the bulk of the solution, is cancelled out at ca. ?1.4 V vs. SCE by a thermal non-faradaic current component when 360 nm light is employed. When CO₂ is present at high concentration a similar cancellation is observed at lower pH at ca. ?1.1 V vs. SCE, suggesting a value for the diffusion coefficient of the radical-ion CO^?₂ about 3.3 times smaller than that of e^?_aq.     

Studies on mixed complexes of Cd(II) with propylenediamine and trimethylenediamine at the dropping mercury electrode

Propylenediamine and trimethylenediamine have been found to form mixed complexes with Cd(II) in aqueous medium with stoichiometries 1:1:1 and 1:1:2. The overall formation constants of these mixed complexes have been evaluated usingSchaap andMcMaster's method. The tendency of the complexes (simple and mixed) to add and substitute another ligand is discussed.

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Polarographische Untersuchungen an gemischten Komplexen von Cd(II) mit Propylendiamin und Trimethylendiamin

Zusammenfassung Propylendiamin und Trimethylendiamin bilden in wäßr. Lösung gemischte Komplexe mit Cd(II) in den stöchiometrischen Zusammensetzungen 1:1:1, 1:1:2 und 1:2:1., Die Bildungskonstanten dieser Komplexe wurden mit der Methode nachSchaap undMcMaster bestimmt. Die Tendenz der (einfachen und gemischten) Komplexe, mit anderen Liganden Addition oder Substitution einzugehen, wird diskutiert.

     

氨苄青霉素降解产物在汞膜电极上的伏安法研究

考察了氨苄青霉素在酸、碱条件下的降解产物于玻碳汞膜电极上的伏安行为 ,降解产物在醋酸缓冲溶液 ( p H5.5)的底液中均产生了灵敏的阴极还原峰 ,以碱降解条件为好。已将方法应用于模拟样品的分析。     

Anodic stripping voltammetry with mercury electrode potential-step and current-step methods

Prof. N. Tanaka (Sendai): (1) What is the difference of this method and the method proposed by Dr. Hickling? (2) How can this method be applied when two substances are present in the solution?Prof.P. Delahay: (1) If I recall correctly hickling in his work recently published [Anal. Chim. Acta, 14 (1956) 297] carries out the anodic oxidation with a stirred amalgam 1 would say that method is quite different from that of hickling. (2) The situation is similar to that in polarography. Both metals are deposited, and the potential first switched at a value at which only the nobler metal is oxidized. A second current-time curve is recorded at a potential at which both metals are oxidized. The concentration of the less noble metal is obtained by difference.Prof. H.A. Laitinen (Urbana): (1) What are the relative sensitivity limits of the potential step and current step methods? (2) Would Prof. Delahay care to comment on the application of these methods with solid microelectrodes? Here no diffusion can occur within the electrode and diffusion in the solution is ill-defined.Prof. P. Delahay: (i) Sensitivities are quite comparable although the potential-step method is more sensitive than the current-step method. (2) These methods can be applied to solid electrodes but a coulometric determination must then be made. Direct evaluation of the concentration from the current-time caracterstics (potential-time curves in the current-step method) is only possible when less than a monolayer is involved (or perhaps a few layers for a non-uniform deposit). See the recent work of nicholson published in J.Am.Chem.Soc.Dr. H. Gerischer (Stuttgart): (t) Wenn ich Sie recht verstanden habe, beruht die grosse Empfindlichkeit der Methode darauf, days eine elektrolitische Trennung und Anerkennung der Untersuchungssubstanz in dem kleinen Hg-Tropfen Erfolgt. (2) Wie wird die Auswertung des anodischen Stromstosses vorgenommen; durch einfache graphische Integration? (3) Glauben Sie nicht, dass die starke Konvektion in der Lösung auch eine Konvektion im Amalgam zur Folge hat, was zu einem weitgehenden Konzentrationsausgleich im Amalgam führen sollte wenn t relativ gross ist?Prof. P. Delahay: (1) The gain in sensitivity is indeed due to the extraction of the metal from the solution into a thin layer of mercury. The metal is deposited into mercury at a rate larger than that at which it can diffuse away. (2) There was no current integration but only direct measurement on the recorded current-time curve. (3) Convection obviously interferes but reproducible conditions can be achieved in analytical applications.Dr. Mechelynck (Mol, Belgique):Dans le cas ou les temps d'électrolyse sont longs, le gain expérimental est supérieur an gain théorique. Je pense qu'au point de vue analytique c'est un avantage, puisque ce phénomène, qui est dû à la sph$\text{'}\text{?}$ericité; de 1'électrode, est parfaitement reproductible si les électrolyses sont effectuées pendant (des temps identiques. Est-ce effectivement le cas?Prof. P. Delahay; Tout à fait d'accord. On est toutefois limité par la solubilité du métal dans le mercure.Prof. W. Kemula (Warsaw): Which substances have you studied? Have you met difficulties with purification of basic electrolytes?Prof. P. Delahay :Our study was limited to the development of some fundamental ideas and their verification and not with the details of application. We studied primarily cadmium. I might add that other metals were studied by nikei.ly and cooke in the work they reported at the Lisbon IUPAC meeting (see recent publication in Anal. Chem.).The purification of the supporting electrolyte is a serious problem. Another problem is the necessity of equilibrating the solution with its container. However, these difficulties are often common to many trace analysis methods.     

Studies of the process of catalytic hydrogen evolution on the epoxy-resin-impregnated graphite-based mercury film electrode

The phenomenon of catalytic hydrogen evolution in the presence of platinum salts on the epoxy-resin-impregnated graphite-based mercury film electrode was studied. Two measurements were performed: (i) the deposition of catalyst on the IGE surface from HCl solution containing H₂PtCl₆ and HgCl₂; (ii) the voltammetric curve of a solution of HCL + KCl was recorded. The effect of some parameters on the catalytic hydrogen evolution peak potential was studied. The investigations performed have shown that the four following parameters have a decisive effect on the form of the CHE curve: (1) Hg²⁺ concentration in the deposition step; (2) deposition potential value; (3) deposition time; (4) Pt(IV) concentration during the deposition step. It was established that, with increasing amount of Pt deposited on the IGE, a considerable shift of the CHE peak potential towards positive values occurred. A linear potential dependence of the CHE peak on log c_Pt was obtained when the Pt(IV) concentration was changed from 4×10⁻⁹M to 2×10⁻⁶M. That dependence can be employed well for analytical purposes.     

Electrochemical reduction of dihydroquercetin on mercury electrode

The methods of classical and ac polarography were used to study the electrochemical behavior of dihydroquercetin on a mercury dropping electrode. This process occurs only in protonic media. The protonated and nonprotonated forms of dihydroquercetin undergo electroreduction. The amount of electrons and protons participating in electroreduction of the studied flavonoid is determined. The nature of currents measured in the processes of electrochemical reduction of dihydroquercetin is studied.     

Voltammetry of resazurin at a mercury electrode

Electrochemical behavior of resazurin on HMDE in Britton-Robinson (B-R) buffers (pH 2.0–10.0) was studied using the square-wave voltammetry (SWV), square-wave adsorptive stripping voltammetry (SWAdSV), and cyclic voltammetry (CV) techniques. The voltammogram of resazurin in B-R buffer at pH < 4.0 exhibited two cathodic reduction peaks. The voltammetric peaks were obtained at −0.144 V (reversible) and −1.250 V (irreversible) at pH 3.2, and correspond to the reduction of resorufin to dihydroresorufin and to the catalytic hydrogen wave, respectively. At pH > 4.0, a new irreversible cathodic reduction peak, assigned to the protonation of N-oxide on the phenoxazin ring, was observed. Electrochemical parameters (I _p/E _p, I _p/v, I _p/pH, I _p/t _acc) of the compound were determined. From the voltammetric data, electrochemical reduction mechanisms for all peaks have been suggested. Maximum peak current for the reversible peak was obtained at pH 4.1. A linear relationship between the current and concentration was determined, and also the lowest detection limit was found as 3.25 × 10⁻⁸ mol L⁻¹ and 1.98 × 10⁻¹⁰ mol L⁻¹ for SWV and SWAdSV, respectively.     

苯亚甲基丙二腈在汞阴极上的还原反应

本文应用了多种电化学方法(循环伏安法,微分脉冲极谱法等)及UV光谱,GC-MS手段研究了苯亚甲基丙二腈(BDMN)在汞阴极上的还原过程.在0.03mol.L^-1四乙基溴化铵(TEAB),50%二氧六环-水介质中BDMN还原的微分脉冲极谱图上有两个峰,第一个峰是此分子中的双键还原,生成苄基丙二腈引起的,峰电位在-0.98V(vs.SCE).第二个峰的峰电位在-1.55V(vs.SCE),此峰是BDMN的水解产物苯甲醛还原为苯甲醇产生的.BDMN的整个电极反应为ECEC过程.此处,对BDMN及二氧六环在汞电极上的吸附现象也进行了研究.     

Reduction mechanism of benzoin on a mercury electrode

The electrochemical reduction of benzoin was studied in acid media at DME. Convolution potential sweep voltammetry was applied to obtain the charge transfer rate constants. The overall reduction corresponds formally to an irreversible two-electron, two-proton transfer. Deoxybenzoin was identified as the final product of the benzoin reduction. Low-temperature bulk electrolysis experiments showed the presence of an unstable enol intermediate. This compound undergoes an enol-keto transformation, whose acid catalyzed rate constants were determined by chronoamperometry. The kinetic results indicate a stabilization of the intermediate carbocation in the presence of perchlorate.     

Convolutive forecasting at a hanging mercury drop electrode

Convolutive forecasting is a procedure which converts a non-steady-state voltammogram into a steady-state voltammogram. The procedure was found to be successful on a range of sphericities generated by varying the size of a hanging mercury drop and the potential sweep rate in a series of cyclic voltammetric experiments. Both theoretical and experimental analyses on a reversible electrochemical system were considered. An expression establishing the relationship between the radius of a spherical electrode and the potential sweep rate was developed to define the limits of the convolutive forecasting algorithm which was used.     

Silica-modified electrode for the selective detection of mercury

Pure silica particles were dispersed within carbon paste and the resulting modified electrode was applied to the selective voltammetric detection of mercury(II) species after their accumulation at open circuit. The remarkable selectivity observed between pH 4 and 7 was attributed to the intrinsic adsorption mechanism which involves a condensation reaction between mercury(II) hydroxide and hydroxyl groups on the silica surface, leading to the formation of an inner-sphere-type surface complex. After optimization with respect to the electrode composition, the detection medium, and the voltammetric scan mode, a linear response was obtained in the concentration range between 2 × 10⁻⁷ M to 1 × 10⁻⁵ M, by applying anodic stripping square wave voltammetry. Various silica samples were used and their sorption behavior was discussed in relation to their specific surface area and porosity. The effect of chloride and pH on the accumulation of mercury(II) on silica was also investigated. Received: 4 September 1999 / Accepted: 5 January 2000     

The adsorption of acetylacetone at the mercury electrode

Adsorption of acetylacetone at the mercury electrode was studied in a wide pH range. Adsorption from acidic solutions is much stronger than the adsorption from basic solutions. The dependence of the differential capacity of the mercury electrode on the pH resembles the titration curve of acetylacetone. From such curves the pK of acetylacetone was calculated.     

Thin-layer cell with a rapidly dropping mercury electrode for liquid chromatographic detection

Summary A thin-layer cell with a horizontally positioned, rapidly dropping mercury electrode (TL-RDME) was developed for use in electrochemical detection in liquid chromatography. A specific application to electrosorptive detection based on the measurement of differential doublelayer capacitance, C_d, is detailed. The construction is of both Kel-F and acrylic materials, the latter chosen for ease of viewing. Interchangeable capillaries of varying internal diameter combined with gasket-determined channel thickness allows for a variation in drop times from below 10 ms to more than 200 ms. A drain solution flowing past the reference electrode, which is positioned upstream of the working electrode, provides a convenient electrolyte make-up stream. Typical C_d responses are compared to those obtained using a largevolume wall-jet cell/electrode geometry.     

Anodic stripping coulometry at a thin-film mercury electrode

Primary coulometric versions of d.c. anodic stripping voltammetry, performed in a microcell at a mercury-plated rotating glassy carbon electrode, are applied to the determination of cadmium and lead. With exhaustive preelectrolysis of the sample solution, 5–100 ng of the metal can be determined from the charge contained in the stripping signal; precision and accuracy are in the 5% range. An extrapolation procedure that requires only partial metal deposition is evaluated.     

General characteristics of the copper-based mercury film electrode

The optimum conditions for the preparation, storage, conditioning and renewal of copper-based mercury film electrodes (CBMFEs) are given. The voltammetric results obtained at these electrodes are compared with the predictions of the theory of cyclic and stripping voltammetry at the mercury film electrode, as well as with the results obtained at the silver-based and the platinum-based mercury film electrodes. The advantage of a CBMFE is prolonged life-time, whereas the disadvantages ar the decreased range of usable positive potentials and the possibility of interfering reactions of the electrodeposited metals with the copper substrate or copper dissolved in the mercury phase. The presence of copper has no essential influence on the behaviour of lead and thallium; it affects the behaviour of zinc markedly and that of cadmium and indium slightly. The conditions allowing the minimization of the harmful action of copper on the behaviour of cadmium and indium have been found.     

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.