Determination of Arsenate Ions by Stripping Voltammetry at a Silver Electrode

Trace amounts of arsenate ions can be determined by stripping voltammetry at a silver electrode. Cathodic polarization curves exhibited one current peak corresponding to the electrostripping of a compound formed by arsenate ions with silver. The areas of stripping peaks expressed in Coulomb can serve as analytical signals in the determination of arsenate ions by stripping voltammetry.     

Peak Area: the Instrumental Datum to Improve the Determination at Ultratrace Level Concentration of Mercury(II) at Gold Electrode

Peak area as instrumental datum for determining the concentration of metals in solution instead of peak height is proposed for analytical voltammetric determinations. In the case of species present at ultra‐trace concentration level or having low reversibility degree electrodic processes, the employment of peak area permits to achieve limits of detection lower even more of one order of magnitude. The present work shows the possibility of determining, at ultratrace level concentration, mercury(II) by differential pulse anodic stripping voltammetry (DPASV).     

A new mathematical function for describing electrophoretic peaks

A new model is proposed for characterizing skewed electrophoretic peaks, which is a combination of leading and trailing edge functions, empirically modified to get a rapid recovery of the baseline. The peak model is a sum of square roots and is called thereby "combined square roots (CSR) model". The flexibility of the model was checked on theoretical and experimental peaks with asymmetries in the range of 0-10 (expressed as the ratio of the distance between the center and the trailing edge, and the center and the leading edge of the chromatographic peak, measured at 10% of peak height). Excellent fits were found in all cases. The new model was compared with other three models that have shown good performance in modelling chromatographic peaks: the empirically transformed Gaussian, the parabolic Lorentzian-modified Gaussian, and the Haarhoff-van der Linde function. The latter model was proposed recently to describe electrophoretic peaks. The CSR model offered the highest flexibility to describe electrophoretic peak profiles, even those extremely asymmetrical with long tails. The new function has the advantage of using measurable parameters that allow the direct estimation of peak areas, which is useful for quantitative purposes.     

Determination of sulfur species by cathodic square wave stripping voltammetry; Compounds relevant to natural sulfur mineralization

The feasibility of rapid analysis of a number of environmentally important sulfur compounds by cathodic square wave stripping voltammetry at a mercury electrode has been investigated. For cysteine/cystine a relatively anodic peak was identified, which is ascribed to the stripping of a mercurous species. The dependence of the peak currents on pH was found to be different for cysteine and cystine. Methionine and thioproline gave similar stripping peaks to those for cysteine. It is proposed that they arise from species deposited by oxidative hydrolysis. A complex ion is proposed to cause the stripping peak of thiosulfate and tetrathionate, while polysulfides give both a HgS stripping peak and a non-adsorptive reduction peak. Limits of detection are in the 10(-8)-10(-9)M range.     

Temporally convoluted Gaussian equations for chromatographic peaks

Both spatial and temporal peaks that are produced by the discrete parcel model can be mathematically approximated by Gaussian functions, but the transformation from a spatial pattern to a temporal image requires a convolution treatment. A first-order convolution is given for temporal peaks under a linear isotherm, whereas a second-order convolution is proposed for those under non-linear isotherms. Numerical tests show that the peak shapes generated by the proposed temporally convoluted Gaussian equations (TCG) match perfectly with those obtained by the discrete parcel model. Although the full TCG equation may be quite complicated, it can be made easier by a recursion calculation technique, and a group of peak curves can be plotted simultaneously on computer worksheet. The results also suggest that the temporal distortion effect should be predominately considered, in addition to those known-to-exist spatial effects, for explaining the peak asymmetry.     

H-point standard additions method for resolution of overlapped chromatographic peaks with a conventional fluorescence detector. Determination of phenol and cresols in waters

Summary The H-Point Standard Additions Method (HPSAM) is proposed in order to resolve overlapping peaks in liquid chromatography by using a conventional fluorescence detector. The method uses as analytical signals the heights or the areas obtained at two previously selected emission wavelengths, and good results are obtained for highly overlapping peaks with highly overlapping fluorescence spectra. The principal benefits of the method are the ease of finding the required wavelengths, its insensitivity to changes in the retention time of the peak from one injection to another, and the possibility of using it in highly or only partially overlapping peaks. We have applied the method to the determination of phenol and cresols in water, resolving by the proposed method the overlapping peaks ofm- andp-cresol.     

Simulation of Asymmetric Peak-Shaped Analytical Signals by the Frame Representation of Their Shape Using Stripping Voltammetry as an Example

To simulate peak-shaped analytical signals, the shape characteristics and geometrical properties of four asymmetric basic functions (the derivative of the logistic function, the Gaussian function, the Cauchy function modified by Fraser and Suzuki, and the Poisson function with the Stromberg correction) are systematically studied. The frame representation, which was proposed earlier by the two authors of this paper, is used to characterize the peak shape. Some regularities are found in peak properties expressed in terms of the frame parameters and considered as functions of the coefficient of skewness b. Peak shapes are compared using two approaches to obtaining dimensionless frame parameters: one using relative parameters and another using parameters of the unit frame. It is shown that the frame representation of the peak shape can be used in the phenomenological simulation of metal peaks in stripping voltammetry. The practical use of information on the shape of the analytical signal in the solution of various problems of analytical chemistry is discussed.     

A preliminary study of non-EMG peaks

Summary It is shown that part of non-EMG peaks are gamma peaks. The describing function of a gamma peak is derived. The peak area equation and the formation mechanism of a gamma peak are discussed.     

Fitting Lorentzian peaks with evolutionary genetic algorithm based on stochastic search procedure

A global search technique for curve fitting based on evolutionary random search was modified and applied for quantifying a combination of Gaussian and Lorentzian peaks. This stochastic search procedures based on randomized operators is a modified Monte Carlo method. The proposed method tested on self obtained several overlapped Lorentzian peaks with random noise, Lennard particles in three dimensions and discrete mathematical functions previously used for optimization in literature. It was found to be the proposed method is suitable for complex and large scale optimization. The results of the new method have been compared with those obtained by two peak fitting programs. Developed method was found to be very fast and thus it is time saving.     

Correction method for quantitative area determination of overlapping chromatographic peaks based on the exponentially modified Gaussian (EMG) model

A simple method is presented for peak area correction of overlapping peaks. This correction is necessary for the normal approach of dealing with overlapping peaks by a vertical line at the valley point. The relative area errors caused by this vertical line are calculated as the correction factors in three dimensions of peak separation, peak ratio, and peak tailing skew. The calculation is based on the exponentially modified Gaussian asymmetric peak model.     

Studies on substituted phenols by differential pulse cathodic stripping voltammetry: 1. Pentachlorophenol

A study of chlorophenols using differential pulse cathodic stripping voltammetry (dpcsv) is reported. Of the wide range of chlorophenols investigated, only pentachlorophenol and 2,3,4,6-tetrachlorophenol yield stripping peaks.For pentachlorophenol in water, two peaks are obtained at stripping potentials of –1.2 V and –1.58 V versus SCE. In methanol, pentachlorophenol shows one peak with a stripping potential at –1.6 V versus SCE.In the case of 2,3,4,6-tetrachlorophenol in water, two peaks are observed at –1.06 V and –1.6 V versus SCE, while in methanol only one peak arises at a stripping potential of –1.6 V. A procedure was developed for the determination of pentachlorophenol in natural waters.     

Effect of injected sample amount on the shape of chromatographic peaks under condition of linear partition isotherm

In a previous paper a model function was tested in order to approximate the peak shape obtained on non-polar column by injecting different compounds. The simulation of the symmetrical or non-symmetrical shape of gas chromatographic peaks was satisfactory. In this paper, the influence of the amount of injected substance was investigated at different values of inlet pressure and carrier gas velocity, in order to evaluate the relative contribution to the total peak area and shape of the symmetrical distribution due to partition phenomena and of the non-symmetrical and tailing distribution due to adsorption-desorption kinetics. The effect of the molecular mass and of the chain length of compounds belonging to the homologous series of 1-alcohols and n-alkanes on the adsorption phenomena was evaluated.     

Direct determination of molybdenum in seawater by adsorption cathodic stripping square-wave voltammetry

A reliable and very sensitive procedure for the determination of trace levels of molybdenum in seawater is proposed. The complex of molybdenum with 8-hydroxyquinoline (Oxine) is analyzed by cathodic stripping square-wave voltammetry based on the adsorption collection onto a hanging mercury drop electrode (HMDE). This procedure of molybdenum determination was found to be more favorable than differential pulse cathodic stripping voltammetry because of inherently faster scan rate and much better linearity obtained through the one-peak (instead of one-of-two peaks) calibration. The variation of polarographic peak and peak current with a pH, adsorption time, adsorption potential, and some instrumental parameters such as scan rate and pulse height were optimized. The alteration of polarographic wave and its likely mechanism are also discussed. The relationship between peak current and molybdenum concentration is linear up to 150 mug l(-1). Under the optimal analytical conditions, the determination limit of 0.5 mug l(-1) Mo was reached after 60 s of the stirred collection. The estimated detection limit is better than 0.1 mug l(-1) of Mo. The applicability of this method to analysis of seawater was assessed by the determination of molybdenum in two certified reference seawater samples (CASS-2 and NASS-2) and the comparison of the analytical results for real seawater samples (study on a vertical distribution of Mo in the seawater column) with the results obtained by Zeeman-corrected electrothermal atomization atomic absorption spectrometry (Zeeman ETAAS). A good agreement between two used methods of molybdenum determination was obtained.     

The use of derivatives for establishing integration limits of chromatographic peaks

Summary The paper deals with composite peaks in which the resolution is not sufficient to allow simple area determinations with conventional integrator procedures. It is proposed to use the second derivatives of composite peaks, since the derivatives accentuate envelope perturbations due to overlapped peaks. In particular, when there are two solutes in the composite, and when the peak separation is between 2 and 4σ, the second derivative of the composite has two minima and three maxima. The second maximum is indicative of the cross point of the two solutes. This point can be used to initiate and/or terminate the integration of the components in the composite. Similarly, the second minimum occurs at a point close to the true maximum of the second peak in the composite. This point can also be used for the quantitative determination of the second component in the composite. The second derivative traces can also be integrated, but their utility in quantitative analysis of the peaks is questionable. An inversion procedure is given in which the second derivative trace is inverted to yield a trace similar to the conventional chromatograms but with better apparent resolution. In special circumstances, the inverted derivatives can be used for integration purposes.     

The peak width of nearly Gaussian peaks

Summary In chromatography the peak shape is often described by the method of statistical moments and therefore the second central moment is considered as the correct measure of peak width. Using the exponentially modified Gaussian peak model as an example and a criterion more related to chromatography, the extent of separation, it is shown that for nearly Gaussian peaks the width measured at 1/8 of the peak height is a more meaningful width parameter to evaluate the efficiency of chromatography and the resolution. The second central moment gives too much weight to the remote parts of the tail of the usual, somewhat skewed, peak.     

Use of differentiation and smoothing in linear-sweep and staircase stripping voltammetry of some metals

Two methods of signal smoothing were considered: square-wave (moving average) and triangular (weighted average) filters. It was found that smoothing by these two filters did not distort noticeably the signal shape and the linearity of calibration plots. Calibration plots for several series of analytical signals of heavy metals were studied. Analytical signals were obtained by stripping voltammetry using a mercury thin-film electrode and processed using the peak area, the peak height, and the range of the extrema of the first and second peak derivatives. It was shown that, for staircase stripping voltammetry, the linearity of calibration plots decreases substantially in this series of signal processing methods. For linear sweep, calibration plots were linear over the entire concentration range for all signal-processing methods. Presented at the V All-Russian Conference with the Participation of CIS Countries on Electrochemical Methods of Analysis (EM A-99), Moscow, December 6–8, 1999.     

Determination of palladium by stripping voltammetry in raw gold ores

The electroconcentration of gold(III) and palladium(II) on the electrode surface gives a binary alloy whose components are electrochemically oxidized at almost the same potential. Two methods were proposed for the separation of overlapping anodic peaks: the chemical reduction of gold(III) under UV irradiation and the use of a special computer software built into the analyzer. The results of determining palladium(II) in raw gold ore using stripping voltammetry and two proposed methods of peak separation are compared.