Contribution of migration current to the voltammetric deposition and stripping of lead with and without added supporting electrolyte at a mercury-free carbon fibre microdisc electrode

In order to assess the contribution and analytical significance of migration, electrochemical studies on the deposition and stripping of lead at a carbon fibre microelectrode (diameter of 10 μm) have been undertaken in aqueous solutions containing 1 mM lead ions with variable KNO₃ supporting electrolyte concentrations (10⁻¹ to 10⁻⁵ M), as well as in the total absence of deliberately added supporting electrolyte. The methodology involved the application of cyclic voltammetry to characterise the Pb²⁺ (solution)+2e⁻Pb (metal) process in both the reduction (Pb deposition) and stripping (Pb dissolution) directions. The use of a mercury-free carbon surfaces means that the lead stripping does not occur from the amalgam state, as is commonly the case in anodic stripping voltammetry. In the deposition step, the current rises sharply with potential in response to a lead nucleation-growth process and then reaches an almost potential independent limiting value. The stripping step, obtained on the reverse scan, exhibited oxidation peak currents resulting from the redissolution or stripping of the metal from the electrode surface. The influence of the electrolyte concentration and hence migration current at −0.8 V versus Ag/AgCl for the deposition process, as well on the redissolution peak current and the dependence of the voltammograms on scan rate (10–1000 mV) are discussed. Interestingly, neither deposition limiting nor stripping peak currents vary in a simple manner with added supporting electrolyte concentrations, with maximum values being observed at 10⁻⁵ M rather than zero concentration of added KNO₃. An important implication for the voltammetric determination of lead in low ionic strength media by the very sensitive technique of anodic stripping voltammetry is that use of the method of standard additions commonly employed to minimise unknown matrix problems, is prone to error when contributions to the process from migration current are important.     

Adsorptive Stripping Analysis of Riboflavin at Electrically Heated Graphite Cylindrical Electrodes

Electrically heated graphite cylindrical electrodes (HGCEs) made from ground pencil leads have been used to perform adsorptive stripping square wave voltammetry (SWV) measurements of trace riboflavin (RF). The SWV stripping peak current was significantly enhanced with increasing the electrode temperature only during preconcentration step. This enhancement was due to the forced thermal convection induced by heating the electrode rather than the bulk solution. It is the thermal convection that has the ability to improve mass transfer and facilitate adsorption thus enhance stripping responses. It was found that the detection limit of 5×10^?9 M (S/N=3) could be obtained at an electrode temperature of 72 °C during 5 min accumulation, more than one magnitude lower than that at 22 °C (room temperature), the sensitivity could be enhanced ca. eight or four folds for two different RF concentration ranges. So it is possible to develop a new highly sensitive method to determine riboflavin at HGCEs. Such HGCEs were also successfully used to determine RF in multivitamin tablets.     

Microwave-enhanced electro-deposition and stripping of palladium at boron-doped diamond electrodes

In situ microwave activation has been applied to the electro-deposition and stripping of palladium metal (which is widely used as a catalyst) at cavitation resistant boron-doped diamond electrodes. Focused microwave radiation leading to heating, boiling, and cavitation is explored as an option to improve the speed and sensitivity of the analytical detection procedure. The deposition and anodic stripping of palladium by linear sweep voltammetry in 0.1 M KCl (pH 2) solution and at boron-doped diamond electrodes is shown to be strongly enhanced by microwave activation due to both (i) the increase in mass transport and (ii) the increase in the kinetic rate of deposition and stripping.The temperature at the electrode surface is calibrated with the reversible redox couple Fe(CN)₆⁴⁻/Fe(CN)₆³⁻ and found to be reach 380 K. In the presence of microwave radiation, the potential of onset of the deposition of palladium is strongly shifted positive from −0.4 to +0.1 V versus SCE. The optimum potential for deposition in the presence of microwaves is −0.4 V versus SCE and the anodic stripping peak current is shown to increase linearly with deposition time. Under these conditions, the stripping peak current varies linearly with the palladium concentration down to ca. 2 μM. At concentration lower than this a logarithmic variation of the stripping peak current with concentration is observed down to ca. 0.1 μM (for 5 min pre-concentration in presence of microwave radiation).     

An iridium based mercury film electrode: Part II. Comparison of mercury-film electrode behaviors: theory versus reality

An experimental study of a 2 mm diameter iridium based mercury film rotating disk electrode (IrMFE) is presented. Using anodic stripping voltammetry and test ions of cadmium, lead and zinc, the effects of Hg film thickness on peak potential, half-width and peak height are compared with the predictions given by the De Vries and Van Dalen theory. The response of the Ir-MFE was found to be theoretical for Hg film thicknesses less than 1 μm. Application of this classical theory to real Hg film electrodes, on any type of substrate, is found to be inappropriate when the Hg film thickness begins to exceed 1 μm. It is also shown that the stripping peak characteristics at a semi-spherical film electrode can be estimated by assuming that it is composed of a large number of annular flat films. The individual responses of the flat films can be summed to give the overall semi-spherical response.     

Separation of double-stranded DNA in conventional and isoelectric buffers: studies on stability and separation performance

In the capillary electrophoresis of double-stranded DNA in isoelectric buffers, worsening of resolution was observed in electropherograms as a function of time passed from the preparation of the separation solution, which consisted of 0.7% hydroxypropylcellulose, HPC, Mr 10(6), diluted in 150 mM histidine buffer. The DNA standards used were: kilobase pair-ladder, Marker V and Marker VI. In order to understand what happens in the histidine-HPC solution with ageing, the absorbance spectrum (200-500 nm), the conductivity and the pH of the solutions as a function of time were monitored. Fresh His gave a distinct peak at 206 nm. For all the solutions a significant diminution in the maximum absorbance value at 206 nm was observed as a function of ageing, with the concomitant appearance of a peak at 278 nm as the solutions became older. Also the conductivity increases dramatically with the ageing of the solutions and seemed to reach a plateau after ca. 40 days. In concomitance with the conductivity increments with time, the pH of the His solution (isoelectric point, pI=7.6) grew slowly up to pH 7.9; these combined data indicated that a new species contributing to the conductivity and altering the pH was formed from the His molecule, suggesting that His degraded in time. When the dipeptide His-Gly was used instead, a similar ageing phenomenon was observed, but with much reduced kinetics. Mass spectrometry, coupled to RP-HPLC, detected, in aged His solutions, in addition to intact His, two main degradation products: a 110.1 u species and a 93.2 u compound. The mass of the former coincides with the protonated species derived from the formation of a Schiff base on the alpha-amino group of His and subsequent decarboxylation without transformation of the final Schiff base into a chetonic group (a histamine-like molecule terminating with an imino, rather than with an amino group).     

Carbon-fiber microsensor for in vivo monitoring of trace zinc(II) based on electrochemical stripping analysis

The optimization and characterization of a microsensor for trace zinc based on stripping voltammetry at a single carbon-fiber microelectrode are reported. Despite the absence of a mercury coverage, the carbon-fiber stripping sensor displays a significant hydrogen overvoltage in the physiological pH and a well-defined zinc stripping peak (at ca. −1.17 V). 5 s deposition periods allow convenient quantitation of zinc over the 100–1000 ng/ml (ppb) range. Detection limits of 0.8 ng/ml (1.2×10⁻⁸ M) and 20 ng/ml (3.0×10⁻⁷ M) zinc were estimated in connection to 60 and 1 s deposition, respectively. The response is reversible with the stripping/measurement step completely removing the accumulated zinc. Measurement frequencies as high as 30 runs per min can thus be realized (in connection to 1 s deposition and measurement steps). The influence of relevant variables of the deposition and stripping steps is examined. The attractive behavior of the stripping-voltammetric microsensor offers great promise for in vivo monitoring of trace levels of zinc.     

The determination of nanomolar levels of nitrite in fresh and sea water by cathodic stripping voltammetry

Adsorptive cathodic stripping voltammetry (CSV) is used for the determination of nitrite in natural waters, including sea water. The nitrite is first derivatised by diazotization with sulphanilamide, followed by coupling to 1-naphthylamine to produce an azo dye. This azo dye adsorbs on the mercury drop electrode and its reduction is fully reversible. The concentration of the dye is linearly related to the height of the reduction peak for nitrite concentrations between 0.3 and 200 nM. The optimal concentrations of sulphanilamide and naphthylamine are 2 mM and 0.1 mM, respectively, at a pH of 2.5. After diazotization and coupling, the differential-pulse CSV of the azo dye is done at pH 8.4 with an adsorption potential of ?0.3 V. The standard deviation of a determination of 4 nM nitrite is ±2% and the limit of detection is 0.3 nM for an adsorption time of 60 s. The linear range can be extended by using shorter adsorption times. These sensitivity in sea water is the same as that in fresh water. The concentration of the dye can also be measured by using anodic stripping voltammetry preceded by adsorptive collection at ?0.7 V, but this procedure is less sensitive than CSV.     

Anodic Stripping Voltammetry Using a Vibrating Electrode

This work proposes a vibrating microwire electrode as working electrode in stripping voltammetry. The vibration was found to maintain a constant and thin (1–2 μm) diffusion layer during the deposition step. The electrode vibration eliminated the need for external stirring of the solution, thus facilitating in situ detection in the environment. The vibration was effected by fixing a low‐voltage (3 V), asymmetric, electrical rotor to the working electrode (a gold microwire of either 5 or 25 μm). The sensitivity of the vibrated electrode was ca. 22×greater than stationary. Measurements of copper (4 nM) by anodic stripping voltammetry using the vibrating electrode had a low standard deviation (1% for n=6) indicating that the diffusion layer had only minor variability. The agitation mechanism was unaffected by water moving at >2 m s^?1 and by water pressure equivalent to a depth of >40 m, indicating its suitability for in situ measurements. The vibrating probe was used for in situ detection of copper by anodic stripping voltammetry to a depth of 6 m. Using a 5 min deposition time, the limit of detection for labile copper was 38 pM.     

A coverage-dependent study of pt spontaneously deposited onto Au and Ru surfaces: direct experimental evidence of the ensemble effect for methanol electro-oxidation on Pt

Direct experimental evidence that can be unambiguously attributed to the need of an ensemble of a minimum number of neighboring Pt atoms for methanol electro-oxidation has been observed for the first time. This was realized by a Pt coverage-dependent investigation of methanol and CO electro-oxidation on Pt sites generated via spontaneous deposition onto both Au and Ru surfaces. CO stripping voltammograms also show clear evidence of a substantially strengthened CO-Pt bonding for submonolayer Pt deposited on the Au substrate over a range of ca. 0.22 to 0.77, which is in qualitative agreement with the theoretical prediction based on the Hammer-N?rskov d-band center model. However, the degree of the bond strengthening depends on the Pt coverage, being stronger for lower coverage. Additionally, evidence of an Ostwald ripening process for Pt islands formation has also been observed.     

Potential binding of borate ions to mono- and oligonucleotides: a capillary electrophoresis investigation

The potential binding of borate to oligonucleotides and DNA fragments is here investigated. In case of free nucleotides, such as AMP, there appears to be a weak binding, although no free versus complexed species could ever be separated under any experimental condition. The binding was suggested by the strong peak asymmetry and by the fact that, at progressively lower borate molarities in the background electrolyte, the peak shape suddenly switched from fronting to tailing. This indicated, as also confirmed by theoretical simulations, that the AMP-borate complex was the slow, not the fast moving species. On the contrary, in the case of free adenosine, strong binding ensued, since in Tris-acetate buffer this compound was only eluted with the electroendoosmotic flux, being neutral, whereas in Tris-borate it had a much higher mobility, comparable to, although lower than, that of AMP. When running oligonucleotides, at standard borate molarities (ca. 45 mM), and under strict iso-ionic strength conditions, no binding to borate could be demonstrated, since the free mobility of a 24-mer DNA was identical in TA and TB buffers. However, at very high borate molarities (200 mM) and high pH values (pH 8.92), some binding to oligonucleotides could occur, since in these latter conditions the mobility of a 24-mer was seen to be ca. 20% lower than at pH 7.69, a pH value that should discourage any complex formation.     

Novel use of doublet peaks in flow injection analysis : Simultaneous Spectrophotometric Determination of Nickel(II) and Iron(II)

A simple, non-separation method for the simultaneous, single-injection determination of nickel(II) and iron(II) is described. The method is based on doublet peaks in a single-line system, with multiple vertical (absorbance) measurements of the doublet peak profile. Doublet peaks occur when the center of the sample zone remains unmixed. Nickel(II), 0.17–0.24 M, in the presence of 2.7–5.4 mM iron(II) is determined by direct spectrophotometry of the nickel(II) ion at the center of the sample zone, Iron(II) is first oxidized on-line by peroxodisulfate to iron(III), which complexes with thiocyanate to form the intensely red complex; this is measured at the peak maximum corresponding to the trailing edge of the sample zone. Correction are made for absorbance of nickel and its reduction in the iron thiocyanate complex formation. The absorbance of nickel(II) ion and the iron thiocyanate complex are both measured at 395 nm from a single injected sample. The general utility of the doublet peak method is discussed.     

A kinetic-spectrophotometric method for the determination of glucose in solutions

A kinetic-spectrophotometric method is proposed to determine glucose in solutions. Measurements were performed at 400 nm; the negative peak was obtained by subtracting the absorption spectra of myoglobin (Mb) before and after oxidation. In this method, glucose is added to a mixture of Mb and glucose oxidase. Glucose is oxidized by glucose oxidase and oxygen to gluconate and hydrogen peroxide is generated. The liberated hydrogen peroxide oxidizes the Mb heme (Fe²⁺) into Fe³⁺. The higher the glucose concentration added, the more the H₂O₂ generation, and the more the Mb oxidation (Fe²⁺ to Fe³⁺) and, as a result, the higher the absorbance at 400 nm (negative peak, lower absorbance value). The increments of added glucose are monitored by measuring the absorbance decay versus time (0–250 s) at 400 nm. Each glucose concentration has an accompanying unique absorbance value at 250 s. The higher the glucose concentrations, the lower the absorbance at 250 s (measured at 400 nm). The calibration curve for glucose was linear from 0.1 to 3.0 mM; the detection limit was found to be 0.025 mM. There was no interference from major substances present; the only interference was from species that react with H₂O₂ (ascorbic acid, uric acid, and urea) or that react with glucose (Cu²⁺ and Fe³⁺). Standard deviation in the determination was ±0.01 mM for a 1.3 mM glucose solution (n = 10). The text was submitted by the author in English.     

Cyclic Square Wave Voltammetry: Theory and Experimental

Abstract

The voltage wave form of cyclic square wave voltammetry (CSWV) and its theory are presented, and experimental verification of theoretical work for CSWV is given. Indications are that the peak potential difference, E_p, of the differential current between the forward sweep and the reverse sweep is zero. Their peak heights have the same value for a reversible electrode reaction, zero, a different value for a reversible electrode reaction with stripping process, not zero, and the same value for a quasi-reversible electrode reaction. According to this property, the reversibility of an electrode reaction can be easily determined.     

Signal processing and detection limits for graphite furnace atomic absorption with Zeeman background correction

The signal handling requirements for graphite furnace atomic absorption are much more demanding than those for flame atomic absorption. Graphite furnace signals change rapidly, background levels are higher, and signal interpretation needs are more extensive.We have identified a number of signal generation and processing factors that are important for success in graphite furnace analyses. These include: use of the transverse, a.c. Zeeman technique with the magnet on the analyte for background correction; production of a series of signal integrals at line frequency to accurately represent the shape of the furnace peak; use of interpolation techniques to better correct for rapidly changing background levels; use of integrated peak absorbance (A.s) signals rather than peak height absorbance for quantitative measurements; use of baseline correction to improve the accuracy of integrated peak absorbance signals; and use of graphical techniques to facilitate data interpretation and methods development.Examples are presented that illustrate the contribution of these factors to precision and detection limit performance. It is possible to improve detection limits over those previously reported by choosing appropriate signal handling parameters.     

Quantitation of P-Aminohippuric Acid and N-Acetyl-P-Aminohippuric Acid from Blood by HPLC

Abstract

P-aminohippuric acid, N-acetyl-p-aminohippuric acid, and p-aminobenzoic acid can be separated by a reverse-phase, isocratic high-pressure liquid chromatographic procedure in less than 4 minutes. The eluent is 10 mM sodium phosphate buffer, pH 3.5, containing 30% methanol. Detection is by absorbance at 270 nm; quantitation is accomplished by peak height. Linear response was obtained from 3 to 1600 nmoles of each compound in aqueous standards and in blood deproteinized with perchloric acid.     

Cathodic adsorption stripping analysis of vitamin B12

Amalgam formation preconcentration is usually performed in electrochemical stripping analysis, but adsorption also can be employed for preconcentration. Vitamin B₁₂ is adsorbed on a mercury electrode in the potential region between ca. ?0.3 and ?1.5 V vs. SCE. Vitamin B₁₂ also yields a polarographic catalytic wave at ca. ?1.6 V. It is considered that the adsorbed protonated reduced form of vitamin B₁₂ plays an important role in producing the catalytic current. Stripping analysis for vitamin B₁₂ employing adsorption preconcentration was found to be possible. Ammonium acetate was used as the supporting electrolyte. It was found that the peak for vitamin B₁₂ was maximal in 0.3 M ammonium acetate. The suitable preconcentration potential was ?1.45 V. The preconcentration time was 5 min while stirring the solution. The scan rate used was 50 mV/s, considering the pen speed of the X-Y recorder. The optimum temperature was 35°C. Under the optimum conditions described above, the calibration curve was linear in the concentration range up to 0.1 μ M. The detection limit was 2 n M.     

Experimental and computational study of species formed during electrochemical stripping oxidation of copper in chloride media determination of copper(II) in the ng l(-1) range by stripping potentiometry

A novel glassy carbon electrode design, permitting medium exchange in batch mode without loss of electrode potential control, has been used for the study of copper(I) and copper(II) species formed during constant current stripping oxidation of copper in chloride media. It was found that copper(II) species dominated at chloride concentrations below about 1 mM and that soluble copper(I) species dominated at chloride concentrations above about 100 mM. In the concentration range 1-100 mM, soluble copper(I) and copper(II) species are formed as well as solid copper(I) chloride, the latter giving rise to a split peak as it is further oxidised to copper(II). The experimental results agreed satisfactorily with computer calculated equilibria data using the haltafall program. The medium exchange procedure has, furthermore, been used for the determination of copper(II) in seawater reference samples, 7.5 M ammonium acetate/2.5 M acetic acid being used as stripping medium. The detection limit, after 15 min of electrolysis, was found to be 6 ng l(-1) (0.10 mM) and the relative precision 6-10%.     

A comprehensive study for selective removal of Cr(VI) by asymmetric imidazolium bromide salts as environmentally-friendly extractant

The study was to determine selective removal and recovery of Cr(VI) from acidic media by solvent extraction (SX) using asymmetric imidazolium-based room temperature imidazolium bromide salts (ARTILs) as the extractants. The relevant parameters on the extraction and the stripping of Cr(VI) were investigated to identify optimum conditions. The optimum conditions were determined as equilibration time 5 minutes, acid concentration and type 0.5?mol/L H₂SO₄, stirring speed 1200?rpm, extractant concentration and type 0.5?mol/L ARTIL5, phase ratio 2.0 and stripping reagent type, and concentration 2.0?mol/L NH₃. In optimum conditions, decyl substituted ARTIL was best in extraction process about 99.7% of extraction rate, whereas moderately hydrophobic pentyl substituted ARTIL was best in stripping process about 70.0% of stripping rate. Also, the optimized process was found as so selective toward Cr(VI) in the presence of the other metal ions with an environmental-friendly liquid–liquid–based SX method.     

Quantitative study on selective stacking of zwitterions in large-volume sample matrix by moving reaction boundary in capillary electrophoresis

The paper advanced the theoretical procedures for quantitative design on selective stacking of zwitterions in full capillary sample matrix by a cathodic-direction moving reaction boundary (MRB) in capillary electrophoresis (CE) under control of electroosmotic flow (EOF). With the procedures, we conducted the theoretical computations on the selective stacking of two test analytes of L-histidine (His) and L-tryptophan (Trp) by the MRB created with 30 mM pH 3.0 formic acid-NaOH buffer and 2-80 mM sodium formate. The results revealed the following three predictions. At first, the MRB cannot stack His and Trp plugs if less than 12.5 mM sodium formate is used to form the MRB and prepare the sample matrix. Second, the MRB can stack His and/or Trp sample plugs completely if higher than 50 mM sodium formate is chosen to form the MRB. Third, the MRB can only focus His plug completely, but stack Trp plug partially if 20-50 mM sodium formate is used; this implied the complete MRB-induced selective stacking to His rather than Trp. All the three predictions were quantitatively proved by the experiments. With great dilution of sample matrix and control of EOF, controllable, simultaneous and MRB-induced selective stacking and separation of zwitterions were achieved. The theoretical results hold evident significances to the quantitative design of selective stacking conditions and the increase of detection sensitivity of zwitterions in CE. In addition, the control of EOF by cetyltrimethylammonium bromide (CTAB) can evidently improve the stacking efficiency to both His and Trp.     

Determination of trace thorium using catalytic-adsorptive stripping voltammetry of the thorium-cupferron complex

A sensitive stripping voltammetric procedure for trace measurement of thorium, based on the catalytic-adsorptive peak of the thorium-cupferron complex, is reported. Optimal experimental conditions include the use of 1mM BES buffer solution (pH 5.5), containing 20muM cupferron, an accumulation potential of -0.80 V (vs. Ag/AgCl), and a differential pulse potential scan. The resulting stripping procedure offers improved sensitivity over a previous stripping scheme for thorium. The limit of detection after 5 min preconcentration is 50 ng/l. (2 x 10(-10)M), the response is linear up to 8 x 10(-8)M, and the relative standard deviation at the 2.1 x 10(-8)M level is 4.4%. Possible interferences are evaluated.