Andrzej S. Baranski

End-column electrochemical detection for inorganic and organic species in high-voltage capillary electrophoresis

The electronics and construction for an end-column ultramicroelectrode (3–10 μm) detection system that permits the use of medium-sized capillaries (25 μm I.D.) without appreciable effects from the potential field at the end of the capillary. Normal peak-to-peak noise over 10 s was 0.01–0.1 pA. The background noise observed for a 200 × μm carbon-fiber electrode placed either 180 μm within a 25-μm capillary or at a point 500 μm away from the capillary was essentially the same. A study of detector response as a function of the position of the electrode has shown that accurate location of the electrode is important for sensitive and reproducible detection. These studies also showed that differences between the density of the electrolyte existing the capillary and the electrolyte in the detection cell could cause anomalous electrode response depending on the location of the electrode relative to the end of the capillary. Application of a carbon fiber or an Hg film electrode gave detection limits (twice the peak-to-peak noise over 10 s) of 2 · 10−8 mol/l for Pb2+, 1 · 10− 5 mol/l for NO2− and 5 · 10−10 mol/l for catechol.

On possible systematic errors in determinations of charge transfer kinetics at very small electrodes

This work may signal a breakthrough in studies of electrode kinetics; however, the proposed methodology should be carefully scrutinized to avoid potential errors in the interpretation of data

Fourier transform square-wave voltammetry

Abstract In this technique a potential excitation waveform is obtained by a superposition of a small amplitude square-wave modulation on a staircase function. The square-wave cycle is repeated several times (usually 16) at each potential step. The electrode response is sampled at even time intervals 2 n (where n = 2, 3, 4…) times per square-wave cycle, and the fast Fourier transform of the electrode response at each potential step is performed. This allows the electrode admittance to be calculated for various odd harmonic frequencies and plotted as a function of the staircase potential. In addition, the average current at each potential step as well as the even harmonic response generated by non-linearity of the studied system can be obtained from collected data. The admittance data are almost identical with those obtained from traditional ac voltammetric experiments, but the equipment is less expensive and the experiments are run much faster and provide more information. The technique was tested by studying kinetics of Cd 2+ reduction from NH 4 + | NH 3 (aq) buffer on the static mercury electrode. Experimental results were compared with those obtained by digital simulation. Diagrams of electronic circuits used in the experiments are also presented.

Dielectrophoretic and electrothermal effects at alternating current heated disk microelectrodes.

When a disk microelectrode is polarized with an alternating potential of very high frequency (0.1-2 GHz) and a high amplitude (up to 2.8 V rms), the electrode is heated up, and at the same time, a very intense electric field is created around the electrode (>10(6) V/m for electrodes 1 microm in radius). This strong electric field gives rise to positive or negative dielectrophoretic effects. Positive dielectrophoretic effects can be used to assemble nanowires from nanoparticles at the electrode edge. On the other hand, a negative dielectrophoretic effect is probably responsible for jet boiling observed at overheated microelectrodes. In addition, a combination of a high temperature gradient and a high potential gradient generates an intense electrothermal flow of solution which very strongly enhances the mass transport and is responsible for intense convection in such systems. The electrothermal flow and dielectrophoretic forces can be generated directly on a microelectrode employed in electrochemical detection because the high frequency ac polarization of the electrode does not interfere with the acquisition of analytical signals.

OPTIMIZATION OF AC VOLTAMMETRIC MEASUREMENTS AT ULTRAMICROELECTRODES

Abstract The precision of activation resistance measurements under ac voltammetric conditions was theoretically examined. In particular effects of the electron transfer rate constant, solution conductivity, double layer capacitance, stray capacitance of the circuit, electrode size, reactant concentration, frequency and electrode potential were analyzed. The results allow selection of optimal conditions for the determination of charge transfer kinetics from ac impedance experiments carried out with ultramicroelectrodes. It has been shown that in some systems the determination of rate constants up to 100 cm/s may be possible.

Electrochemical studies of η5-cyclopentadienyliron hexafluorophosphates of η6-substituted arenes and η6-heterocycles

Abstract Polarographic half-wave potentials for two reduction steps of 49 cyclopentadienyliron complexes of substituted arenes or heterocycles in dimethylformamide were determined. A fast cyclic voltammetry (10–40 V/s) was used to study the electron transfer kinetics of some of these complexes. After correction for the double layer effects, the rate constants of all the complexes studied show that the transfer of electrons in the second reduction step occurs significantly faster than in the first one. This was interpreted as a result of greater delocalization of electrons in the 20-electron complexes of iron compared to the 19-electron complexes.

Kinetics and thermodynamics of thioglycol adsorption on mercury ultramicroelectrodes

Abstract A method for determination of the thermodynamic and kinetic parameters of electrosorption from fast sweep rate cyclic voltammograms was developed and tested using a simulation program. The method was then used for determination of the thioglycol adsorption kinetics on Hg in KOH and H 2 SO 4 aqueous solutions. Cyclic voltammetric experiments were carried out with a hemispherical Hg ultramicroelectrode (5 μm radius) at sweep rates ranging from 1000 to 100 000 V s −1 . The experimental data fit relatively well to a theoretical model based on the Frumkin adsorption isotherm, despite the fact that the current model assumes independence of the electrosorption valency and the interaction parameter of the potential and the electrode coverage, and that it neglects possible reorientation of adsorbed molecules on the surface with a change in electrode coverage and/or potential. It was found that the electrosorption valency for thioglycol in both solutions studied is similar and close to unity; the standard energies of adsorption are 2.93 and −46.6 kJ mol −1 in 1 M H 2 SO 4 and 1 M KOH respectively; the interaction parameters are −3670 J mol −1 (in 1 M H 2 SO 4 ) and 2450 J mol −1 (in 1 M KOH); the rates of adsorption and desorption steps extrapolated to the zero charge potential are k a 0 = 4.7 × 1O 5 s −1 , k d 0 = 9.8 × 10 6 s −1 (in 1 M H 2 SO 4 ) and k a 0 = 1.7 × 10 12 s −1 , k d 0 = 2.12 × 10 4 s −1 (in 1 M KOH). The interaction parameter for the activated complex in both studied solutions is 3000 J mol −1 larger than the corresponding interaction parameter of the product. The results are discussed in terms of participation of protons in the adsorption-desorption process and the effect of potential on the orientation of adsorbed molecules.

Experimental factors in pulsed electrochemical detection in capillary electrophoresis

Abstract A number of experimental approaches that should offer improved S/N in electrochemical detection were evaluated and compared. In addition, to evaluate and optimize the electrochemical response behavior of analytes under actual CE conditions, an on-line cyclic voltammetry (CV) system was developed. The experimental parameters examined included waveform shape, waveform frequency and various signal treatments, using a lock-in amplifier and Fourier analysis. A multiple-step pulse waveform provided the maximum S/N enhancement [up to 10-fold relative to pulse amperometric detection (PAD)], with detection limits in the range of 2·10−8 to 2·10−7 mol/l. The use of the second harmonic from Fourier analysis offered the best improvement in baseline stability. Other approaches such as high pulse frequency (100 to 200 Hz), data collection over selected time windows, digital filters and average smoothing also enhanced S/N 2- to 5-fold relative to PAD. On-line CV studies showed that the adsorption of organic electrolytes on electrode surfaces can inhibit O2 reactions, and thus give low and stable background currents without O2 removal. The CV studies also showed that detection of Pb2+, Cu2+ and Ag+ affected subsequent electrode response, and that H+ evolution contributed to the cathodic signals of the analytes Ni2+, Co2+ and Zn2+.

AC voltammetric measurements of fast charge-transfer processes at ultramicroelectrodes

The methodology for carrying out ac voitammetric measurements with ultramicroelectrodes is discussed. Electron-transfer rate constants for the oxidation of ferrocene (FeCp2) at a Pt ultramicroelectrode and cobaltocene (CoCp2) at Hg, Au and Pt ultramicroelectrodes were measured at frequencies from 2 to 50 kHz. It was shown that the method can provide accurate information about fast interfacial charge-transfer kinetics in organic solvents. The rate of CoCp2 oxidation on Hg or Au electrodes was found to increase with a decrease in the supporting electrolyte concentration, but in the case of the Pt electrode no effect of the supporting electrolyte concentration was observed for oxidation of either CoCp2 or FeCp2. The state of the Pt—solution interface was examined by studying the H+ adsorption—desorption process in aqueous H2SO4 and the double-layer capacitance in nonaqueous media.

NUMERICAL SIMULATION OF KINETICALLY CONTROLLED ELECTROSORPTION PROCESSES UNDER CYCLIC VOLTAMMETRIC CONDITIONS

Abstract The computations described in the paper are based on the Frumkin adsorption isotherm. Variation of the electrosorption valency with potential is taken into account. The rate-determining step can be either diffusion or interfacial kinetics. The kinetics of charge transfer associated with the formation of a chemical bond between an adsorbate and an electrode is described in terms of the Butler-Volmer equation. The effect of lateral interactions between adsorbed molecules and the activated complex is also considered. The presented dependences of the peak width and peak potential on the electrode coverage and the sweep rate allow the determination of various kinetic and thermodynamic parameters of the adsorption process. Computations were carried out for both linear and spherical symmetry of diffusion.