Electrooxidation of pimozide and its differential pulse voltammetric and HPLC-EC determination

The oxidative behaviour of pimozide was studied in hydroalcoholic media (10+90 methanol-H₂O, pH range 2-7.5) at carbon based electrodes. Pimozide was irreversibly oxidized at high positive potentials, resulting in the formation of a couple with a reduction and re-oxidation peak at much lower potentials. The response was evaluated with respect to pH, scan rate, addition of surfactant and other variables. Using differential pulse voltammetry (DPV), the drug yielded a well-defined voltammetric response in Britton-Robinson buffer, pH 2.1 at +1.1 V (versus Ag/AgCl) on glassy carbon electrode. The process could be used to determine pimozide concentrations in the range 8×10⁻⁷-1×10⁻⁴ M. Applicability to tablets and human serum analysis was illustrated. Furthermore, a high-performance liquid chromatographic method with electrochemical detection (HPLC-EC) was developed, which allowed pimozide to be detected down to a level of 2.7×10⁻¹⁰ M (0.25 ppb).     

Photochemical behavior of inorganic and organic selenium compounds in various aqueous solutions

Selenium possesses interesting chemical, biochemical and geochemical behaviors. However, studies of its photochemical properties in aqueous systems are scarce. A better understanding of these phenomena is of great importance for further application of such properties to selenium speciation. In this work, the photochemical behavior of selenium and some of its organic compounds have been systematically studied in various aqueous matrices under UV irradiation at 300 nm. It was observed that the photochemical oxidation rate of Se(IV) to Se(VI) was greatly enhanced in the presence of HN0₃ at ≥1 × 10⁻³ M, but not by NaNO₃. However this photo-oxidation could be inhibited by the presence of Cl⁻. Under UV irradiation, organoselenium compounds went through two successive photochemical reactions in pure water: a direct photolysis (photo-cleavage) followed by a photo-oxidation to form Se(VI). These two steps could also be greatly accelerated in presence of NO₃⁻ although the second step required an acidic condition. The photo cleavage rates varied from one organic compound to another and 10-fold differences were observed. Similarly to Se(IV), the further oxidation to Se(VI) could be prevented by Cl⁻ for all studied organoselenium compounds. Detailed reaction mechanisms involving OH radicals are proposed to explain Se photochemical behaviors in different matrices.     

Identification and quantification of inorganic and organic selenium compounds with highperformance liquid chromatography

Selenium appears in the natural selenium cycle in the form of several organic and inorganic compounds. The biologically beneficial and deterimental effects of ‘selenium’ must be ascribed to particular selenium compounds. The identification and quantification of selenium compounds in biological and environmental samples is required for an understanding of the role of selenium. The high-performance liquid-chromatographic (HPLC) methods for the separation, identification and quantification of selenite, selenate, hydrogen selenide, methaneselenol, bis(organothio) selenides, trimethylselenonium salts, selenonamino-acids, selenium derivatives of carbohydrates, selenoproteins, selenonucleosides and other miscellaneous selenium compounds are summarized (193 references) and pertinent detection modes discussed. Advantages and disadvantages of the methods are pointed out. The literature is covered since 1974, the year of the first publication in this field.     

Voltammetric behaviour of hyponitrite ion and its analytical applications

Hyponitrite is electrochemically oxidized at a platinized platinum microelectrode. The overall process is characterized by an appreciable overvoltage; in alkaline solution nitrite and nitrous oxide are the ultimate products. The wave is well shaped and diffusion-controlled, and can be used for the voltammetric determination of microamounts of hyponitrite. Alternatively, the oxidation of hyponitrite by hypochlorite in basic solution can be used for the direct determination of hyponitrite with amperometric end-point detection. The limits of determination of the voltammetric and amperometric methods are about 10^-4 M and 5 · 10^-4 M, respectively.     

Subtractive differential pulse voltammetry following adsorptive accumulation of organic compounds

Subtractive differential pulse voltammetry following adsorptive preconcentration of organic compounds at solid electrodes is described. Different preconcentration periods are used, and the difference between the oxidation (stripping) currents is recorded. Background currents which are independent of the preconcentration period cancel out. Combining the enhanced peak current, due to the preconcentration step, with the background current correction of the subtractive mode, gives improved sensitivity and/or allows the use of shorter preconcentration periods. Chlorpromazine and dopamine have been used as test systems. A detection limit of around 1 x 10(-9)M has been obtained for chlorpromazine with a 10-min preconcentration period. Applicability to clinical samples is illustrated by the determination of chlorpromazine in whole blood and urine.     

Surface ionization of organic compounds and its applications

Surface ionization of organic compounds in a weak external electric field involving the formation of many-atomic positive ions is considered (including specific features of the phenomenon, its major characteristics, experimental techniques used and possible applications).

Information is presented on the adsorption of compounds of complex composition which was obtained in studying the phenomenon. A detailed analysis is made of the reactions of particles in the adsorbed layer, as well as of the methods of determining the binding energies of the particles undergoing ionization (primary molecules and many-atomic radicals and complexes produced on the surface) to the surface, and of their lifetimes on the surface against thermal desorption in the charged and neutral states.

Vast possibilities are shown to open up by using the thermally equilibrium processes of many-atomic particle ionization occurring on the surface for the determination of the properties of molecules, radicals and complexes (e.g. thermal stability, the weakest bond energies in ions, ionization potentials of the particles analyzed).

The physicochemical aspects of the surface ionization-based detection of organic compounds and of its applications to analytical mass spectrometry, gas chromatography and analytical chemistry of the atmosphere are considered. Attention is focused upon the fact that the high ionization coefficients observed for a number of compounds ensure a record-high sensitivity when using surface ionization-based detection.     

Retention study of inorganic and organic selenium compounds on a silica-based reversed phase column with mixed ion-pairing reagents

Summary Separation of seven inorganic and organic selenium compounds, namely selenic acid [Se(VI)], selenous acid [Se(IV)], trimethylselenonium iodide (TMSe⁺), selenocystine (SeCys), selenomethionine (SeMet), selenoethionine (Seet), and selenocystamine (SeCM), has been performed on a LiChrosorb C 18 column by using mixed ion-pair reagents; 1-butanesulfonic acid and tetramethylammonium hydroxide. Flame atomic absorption spectrometry (FAAS) was used as an element-specific detector. The retention behaviors of selenium compounds in terms of several chromatographic parameters, such as pH of the mobile phase, the concentrations of ion-pair reagents, and the content of organic modifier (methanol) were investigated. It was found that the separation of both inorganic and organic selenium compounds can be achieved within 12 min with a mobile phase of 10 mM 1-butanesulfonic acid −4 mM tetramethylammonium hydroxide −4 mM malonic acid −0.05% methanol adjusted to pH 4.5 at a flow rate of 1.0 mL min⁻¹. The results obtained in this study showed that the use of mixed ion-pair reagents is very useful to improve the separation of selenium compounds. The applicability of this technique for the speciation of selenium compounds in real samples was demonstrated by the determination of selenium compounds in a selenium nutritional supplement. The results were found to be in good agreement with those obtained by ion-exchange HPLC-ICP-MS.     

Differential pulse cathodic stripping voltammetric speciation of trace level inorganic arsenic compounds in natural water samples

A simple, fast and sensitive speciation method is described for inorganic arsenic in water at the μg/l level, applicable in the laboratory and in the field, based on differential pulse cathodic stripping voltammetry (DPCSV). Only As(III) is deposited on a Hg electrode in the presence of Cu and Se in HCl medium. Determination of total As is performed by reducing As(V) to As(III) using sodium meta-bisulfite/sodium thiosulfate reagent stabilized with ascorbic acid. As(V) is quantified by difference. The detection limit (S/N>3) was 0.5 μg/l with a linear range from 4.5 to 180 μg/l. The relative standard deviation (n=6) was 2.4, 2.5, 4.2% for As(III) and 8.0, 6.8, 9.0% for As(V) at levels of 45, 10, and 5 μg/l, respectively. Analysis of the NIST 1640 natural water standard yielded total arsenic concentration 26.5±3.4 μg/l (n=3) compared to the certified value of 26.7 μg/l. Results obtained on several natural water samples analyzed both in the laboratory and on-site compared well with those obtained by HR ICP-MS, GFAAS and IC-AFS. Ions (phosphate, iron, manganese) commonly found in groundwater containing arsenic were found to have negligible interference.     

The polarographic and voltammetric behaviour of the copper(II) mitoxantrone complex and its analytical application

The polarographic and voltammetric behaviour of the copper(II)-mitoxantrone complex have been studied. A well-defined linear sweep voltammetric peak was obtained at -0.275 V (vs. Ag AgCl ) or -0.325 V (vs. SCE) in ammonia-ammonium chloride (20 mmoll(-1), pH 9.0). The characteristics of the peak have been examined in detail. The experimental results show that the reduction of the copper(II) mitoxantrone complex is irreversible and the peak displays adsorption characteristics at the dropping mercury electrode. A mechanism is proposed for the reduction of the complex, comprising one-electron reduction of the copper(II) of the complex, is reduced directly in the complex form. A single-sweep oscillopolarographic method was develped for the determination of copper(II). The peak current is proportional to the concentration over the range 5 x 10(-8)-2 x 10(-5) mol l(-1). The method reported here has the advantage that the interference of many common metal ions is small.     

Polarographic behaviour of alizarincomplexan,anodic wave and its analytical applications

Summary Polarographic Behaviour of Alizarincomplexan, Anodic Wave and Its Analytical Applications The polarographic DPP and DC techniques have been utilized to carry out the study of the anodic wave of alizarincomplexan (AC) in aqueous medium. At pH>5.7 the anodic current is produced by the complex formed between the AC and Hg(II) proceeding from the dropping electrode. At pH=7.3 theE _1/2 of this anodic wave is 0.000 V vs. Ag/AgCl. The main characteristics of the wave have been studied, and the corresponding electrode mechanism process is proposed. Since Hg(II) forms a complex with AC, a method for the amperometric determination of Hg(II) is proposed. We have also studied the analytical possibilities for doubly charged ions which form complexes with the reagent, and an amperometric method for Co(II) determination is also proposed.     

Voltammetric behaviour of copper(iii) and its analytical applications

Copper(II) and copper(III) complexes with periodate or tellurate ligands are electroactive at a smooth platinum electrode, giving an anodic, cathodic or cathanodic wave in the presence of alkaline hydroxide solutions containing copper(II), copper(III), or copper(II)-copper(III) species, respectively. The corresponding limiting currents are diffusion-controlled. The following analytical applications are proposed: (a) amperometric titration of copper(III) solutions; (b) voltammetric determination of copper. Results of amperometric titrations of copper(III) were similar to those by an established procedure. Voltammetry of copper(II) allows the metal to be determined down to concentrations of 1·10^-5M, even in the presence of different ions; the procedure can be applied to such heat-transfer media for nuclear reactors as sodium and potassium metals and their hydroxides.     

A study of the polarographic behaviour of meso-tetra(4-trimethylammoniumphenyl)porphine and its copper and magnesium complexes and its analytical applications

The investigation of the electrochemical reduction and the adsorption of meso-tetra(4-trimethylammoniumphenyl)porphine (T(4-TMAP)P) at a mercury electrode in alkaline solution shows that the overall reduction involves three two-electron steps, of which the first step is reversible and the latter two are irreversible. In addition, T(4-TMAP)P and its metal complexes of Cu(II) and Mg(II) can be strongly adsorbed on the surface of a mercury electrode. The adsorption phenomena have been utilized as a preconcentration step for the determination of trace amounts of the two ions by single sweep polarography. For copper, the detection limit is 1 × 10^–8 mol dm^–3, for magnesium, 1 × 10^–7 mol dm^–3, the latter being limited by the reagent blank. The proposed method was applied to the determination of Cu and Mg in various types of samples (chemicals, hair and liver tissues) with satisfactory results.     

A polarographic study of the hydrolysis of 1,4-benzodiazepines and its analytical applications

The mechanism of hydrolysis of flurazepam (Dalmane) and six of its metabolites was investigated in mildly acidic solution (pH 0–2) by differential pulse polarography. Simultaneous determinations of the “parent” compound(s) and hydrolytic degradation product(s) are possible because of the different reduction potentials. The kinetic results can be explained if the hydrolytic reaction is considered reversible; this is important for evaluation of the hydrolysis and absorption of 5-(o-fluorophenyl)-1,4-benzodiazepines in the stomach. The rate constants and the pK_a values corresponding to protonation of the azomethine groups are shown to be correlated. Appropriate kinetic data for other 1,4-benzodiazepines make it possible to evaluate the effects of certain substituents on the rate of hydrolysis and on the peak potentials of the “parent” compounds and their hydrolytic degradation products. The results of the kinetic investigations can be used for the identification of an isolated 5-(o-fluorophenyl)-1,4-benzodiazepine or identification of any 1,4-benzodiazepine studied here which exhibits some degree of acid hydrolysis within 24 h.     

Behaviour of solid electrodes in normal and differential pulse voltammetric methods

In order to investigate the behaviour of solid electrodes in normal and differential pulse voltammetry, step functions have been applied to the electrochemical cell containing the electrodes to be tested, in the absence of electroactive species. The large residual current observed could be attributed to electrochemical reactions of the electrode material.     

Determination of selenium and tellurium in organic compounds and organic materials — A review

Summary A review is given of methods available for determination of selenium and tellurium in organic compounds and organic materials. The various techniques for decomposition (wet digestion, combustion and bomb fusion) are covered, and the investigational work on decomposition methods is mentioned. The various types of reactions which have been used as methods of finish are also discussed. Methods which involve only physical measurements are not included.

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Zusammenfassung Eine Übersicht über verfügbare Methoden zur Bestimmung von Selen und Tellur in organischen Verbindungen und Materialien wurde gegeben. Die verschiedenen Verfahren zur Zerstörung (nasse Veraschung, Verbrennung und Bombenaufschluß) wurden berücksichtigt und die über Zerstörungsmethoden vorliegenden Publikationen angeführt. Die für die Endbestimmung verwendeten Reaktionstypen wurden erörtert. Physikalische Meßmethoden wurden nicht eingeschlossen.

     

Thin-layer differential pulse voltammetric determination of chlorpromazine in biological fluids

The combination of a thin-layer electrochemical cell with differential pulse voltammetry can be used to determine chlorpromazine in plasma and urine. The thin-layer cell (23 μl capacity) has a wax-impregnated graphite electrode. Direct determination of chlorpromazine in urine gave a linear calibration curve for the range 4.8 × 10^-3–2.4 × 10^-4 M with 97% recovery. No interference from glutethimide, dextropropoxyphene, meprobamate, diazepam, and methaqualone-HCl was detected. Direct measurement of chlorpromazine in plasma gave a linear calibration curve for the range 2.4 × 10^-5–4.8 × 10^-4 M with 89% recovery. The procedure for plasma and urine requires only 2 min per determination. Detection levels are below that required for monitoring therapeutic levels of chlorpromazine in urine.     

A study of the chemiluminescence behavior of myoglobin with luminol and its analytical applications

A chemiluminescence signal at 425 nm was observed when ferric state myoglobin was mixed with luminol in alkaline medium. Because the signal was remarkably enhanced in the presence of Fe(CN)₆ ^4–, analytical applications were investigated in a flow-injection system. The increase in chemiluminescence was linearly dependent on myoglobin concentration in the range 0.1 to 100 nmol L^–1, and the limit of detection was 0.04 nmol L^–1 with relative standard deviation 3.2% (3). It was also found that binding of Mb with the ligands CN^–, SCN^–, and F^– significantly inhibited the chemiluminescence reaction. The linear dynamic ranges for the ligands were 1.0–300.0, 0.1–3.0, and 0.5–100.0 nmol L^–1, and the limits of detection (S/N=3) 0.4, 0.04, and 0.2 nmol L^–1, for F^–, CN^–, and SCN^–, respectively. The relative standard deviations were 5.32%, 6.13%, and 3.38% for 0.1 nmol L^–1 CN^–, 0.5 nmol L^–1 SCN^–, and 1.0 nmol L^–1 F^–, respectively. At a flow rate of 2.0 mL min^–1 the assay could be accomplished in 1 min, including sampling and washing. The method has been successfully applied to the determination of myoglobin in human urine and F^– in water samples. A possible mechanism of chemiluminescence production by myoglobin and luminol is presented.     

77Se NMR study of some organic selenium compounds formed in the selenium dioxide oxidation of ketones

⁷⁷Se NMR chemical shifts and ¹J(SeC) coupling constants were measured for nine organic selenium compounds: 4,5,6,7-tetrahydro-,4′,4′,6,6-tetramethylspiro[1,3-benzoxaselenole-2,1′-cyclohexane]-2′,4,6′-trione and closely related derivatives, bis(2-hydroxy-4,4,6,6-tetramethyl-3-oxo-1-cyclohexenyl) selenide and derivatives, and 1,5,5-trimethyl-7-selenabicyclo[2.2.1]heptane-2,3-dione. The chemicalshifts ranged from ?107 to 595 ppm from the external dimethyl selenide standard. The bridged selenabicyclic compound showed a small coupling constant (42 Hz).     

Thin-layer chromatography of some organic selenium compounds

The thin-layer chromatographic behavior on silica gel and consequent rapid separation of some selenopyran derivatives together with some intermediates of their synthesis are described. The compounds examined were selenochrom-4-one, selenochroman-4-ol, 2-nitrobenzyl phenyl selenide, 2-aminobenzyl phenyl selenide, and 6H-dibenzo[b,d]selenopyran. The chromatographic results show that it is possible to obtain rapid separations with small amounts (μg) of substance by a simple monodimensional technique using a two-component eluting solvent.     

Anodic oxidation of etodolac and its square wave and differential pulse voltammetric determination in pharmaceuticals and human serum

A voltammetric study of the oxidation of etodolac has been carried out at the glassy carbon electrode. The electrochemical oxidation of etodolac was investigated by cyclic, linear sweep, differential pulse and square wave voltammetry using glassy carbon electrode. Different parameters were tested to optimize the conditions for the determination of etodolac. The dependence of intensities of currents and potentials on pH, concentration, scan rate, nature of the buffer was investigated. For analytical purposes, a very well resolved diffusion controlled voltammetric peak was obtained in Britton-Robinson buffer at pH 2.15 for differential pulse and square wave voltammetric techniques. The linear response was obtained in the ranges of 2.10(-6)-8.10(-5) M with a detection limit of 6.8x10(-7) and 6x10(-6)-8x10(-5) M with a detection limit of 1.1x10(-6) M for differential pulse and square wave voltammetric techniques, respectively. Based on this study, simple, rapid, selective and sensitive two voltammetric methods were developed for the determination of the etodolac in tablet dosage form and human serum.