Determination of textile dyes by means of non-aqueous capillary electrophoresis with electrochemical detection

Eight textile dye compounds including five cationic dyes, namely, basic blue 41, basic blue 9, basic green 4, basic violet 16 and basic violet 3, and three anionic dyes, acid green 25, acid red 1 and acid blue 324, were separated and detected by non-aqueous capillary electrophoresis (NACE) with electrochemical detection. Simultaneous separations of acid and basic dyes were performed using an acetonitrile-based buffer. Particular attention was paid to the determination of basic textile dyes. The optimized electrophoresis buffer for the separation of basic dyes was a solvent mixture of acetonitrile/methanol (75:25, v/v) containing 1 M acetic acid and 10 mM sodium acetate. The limits of detection for the basic dyes were in the range of 0.1–0.7 μg mL⁻¹. An appropriate solid-phase extraction procedure was developed for the pre-treatment of aqueous samples with different matrices. This analytical approach was successfully applied to various water samples including river and lake water which were spiked with textile dyes.     

Oligonucleotides-based biosensors with high sensitivity and selectivity for mercury using electrochemical impedance spectroscopy

A novel electrochemical biosensor with high sensitivity and selectivity for mercuric ion detection, based on DNA self-assembly electrode, is designed. Thiol functionalized poly-T oligonucleotides were used as gold electrode modifier through formation of Au–S bond between DNA and gold electrode. In presence of Hg²⁺ ions, the specific coordination between Hg²⁺ and thymine bases can change parallel ss-DNA from linear to hairpin structures, which can cause the release of partial DNA molecules from the surface of the electrode. The density of DNA on the surface of electrode correlated with the concentration of mercury in the solution and can be monitored by electrochemical impedance spectroscopy. The limit of detection of this method is pM level of mercuric ions which is far below the upper limit of Hg²⁺ mandated by United States Environmental Protection Agency (EPA), 2 ppb (10 nM). In addition, this method showed excellent selectivity. A series of divalent metal ions, including Ni²⁺, Co²⁺, Mg²⁺, Zn²⁺, Ba²⁺ and Cd²⁺, have little interference with the detection of Hg²⁺.     

Determination of dye precursors in hair coloring products by liquid chromatography with electrochemical detection

The simultaneous determination of seven aminophenols, resorcinol and p-phenylenediamine in hair coloring products was performed by liquid chromatography (HPLC) with amperometric detection (ED). The aminophenols were separated on a ODS C18 reversed-phase column by isocratic elution with a mobile phase based on 0.1 M acetate buffer pH 4.5-methanol (90:10%, v/v) at a flow rate 0.8 mL min⁻¹. The limit of detection (S/N = 3) for the aminophenols was in the 15-40 pg (injected mass) range at an applied potential of 0.950 V versus Ag/AgCl. Peak heights for the aminophenols and the two others compounds were found to be linearly related to the amount injected, from 0.3 to 300 ng (r > 0.994-0.999).The relative standard deviation (R.S.D., n = 10) for 1 ng injected was comprised in the range from 2.5 to 6.2%, depending on the aminophenol tested. The present method minimizes troublesome and time-consuming pretreatment procedures and it was applied to the determination of aminophenols, resorcinol and phenylenediamine in hair coloring formulations.     

Monitoring selectivity in kinase-promoted phosphorylation of densely packed peptide monolayers using label-free electrochemical detection

This paper describes remarkably high sensitivities in the label-free detection of kinase-promoted phosphorylation for 14 different peptide substrates on electrode-immobilized monolayers (gold or nitride) using serine/threonine kinases PKA, PKC, and CaMK2. Peptide substrates were preselected using (33)P-labeling in a microarray of 1024 substrates. The three most active peptides (A1-A3, C1-C3, and M1-M3) were investigated using electrochemical impedance spectroscopy (EIS) and ion-sensitive field effect transistors (ISFETs). Some of the peptide substrates, for example, the PKC-specific substrate PPRRSSIRNAH (C1), showed a remarkably high sensitivity in the EIS-based sensor measurements. Our studies revealed that this high sensitivity is primarily due to the monolayer's packing density. Nanoscopic studies demonstrated a distinct disordering of the C1-monolayer upon phosphorylation, while phosphatase-promoted dephosphorylation regenerated the highly ordered peptide monolayer. As a matter of fact, the initial surface packing of the peptide monolayer mainly determined the level of sensitivity, whereas electrostatic repulsion of the redox-active species was found to be much less important.     

Catalytic phenomena in the electrochemical reduction of triphenylphosphine and triphenylphosphine oxide in non-aqueous solvents with tetraalkylammonium supporting electrolytes

Polarography, cyclic voltammetry and coulometry reveal that a catalytic reduction of the tetraalkylammonium cation, R₄N⁺, of the supporting electrolyte is involved in the electrochemical reduction of triphenylphosphine (TPP) and its oxide (TPPO) in aprotic solvents such as acetonitrile, dimethylformamide and hexamethylphosphoramide. There is however progressive consumption of TPP and TPPO resulting in the final formation of phenyl substitution products (RPØ₂ and ROPØ₂). Comparison with the reduction of the BuPØ₃⁺ cation allows to propose the following mechanism which involves a chemical type catalytic process:

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Redox type catalytic mechanisms are discussed and shown to be unlikely. Values of the alkylation rate constant are derived from the polarographic or the coulometric data or from cyclic voltammetry according to its magnitude which varies with the solvent. TPP anion radical appears as more readily alkylable than TPPO anion radical.     

Enabling electrochemical studies of chemically-modified carbon nanotubes in non-aqueous electrolytes using superparamagnetic nanoparticle-nanotube composites co-modified by diazirine molecular “tethers”

Multiwalled carbon nanotubes (MWCNTs) were covalently modified with polymer-coated superparamagnetic Fe₃O₄ nanoparticles via amide bond formation to surface oxo-groups located predominantly at the ends of the nanotubes to form “magnetic MWCNTs”. The sidewalls of the magnetic MWCNTs were then selectively covalently modified with ferrocenyl groups via the photolysis of 3-[3-(trifluoromethyl) diazirin-3-yl] phenyl ferrocene monocarboxylate, which uses an aryldiazirine moiety as a molecular “tether”. We demonstrate that the assembly of the chemically-modified magnetic MWCNTs onto the surface of a magnetic carbon electrode enables one to obtain stable voltammetric signals of chemically-modified carbon nanotubes in non-aqueous electrolytes even under vigorous hydrodynamic conditions of stirring at 3000 rpm for up to twenty minutes. In contrast, non-magnetic chemically modified MWCNTs are removed from the electrode surface within the first two minutes of stirring.     

Determination of bisaramil and its metabolite in plasma using high-performance liquid chromatography with electrochemical detection.

A column liquid chromatographic method using electrochemical detection has been developed for determination of an antiarrhythmic agent, bisaramil, and its metabolite in plasma. The plasma was fractionated by extraction with chloroform and chloroform-ethanol, and each fraction was dried and dissolved in ethyl acetate. After back-extraction into an acidic buffer, bisaramil was chromatographed on a reversed-phase column, and the metabolite, which has a higher polarity, was analysed by ion-pairing chromatography. Calibration curves were linear over the concentration range 2-200 ng/ml with coefficients of variation, within-day or day-to-day, not exceeding 5% at any level. The limits of detection of bisaramil and its metabolite were 0.5 and 1 ng/ml, respectively, using 0.5 ml of plasma. The dual-electrode detector was operated in the screening mode of oxidation (electrode 1, +0.55 V; electrode 2, +0.8 V), providing a greater specificity and reducing the background noise. This procedure was applied to a large number of samples in a pharmacokinetic study at the therapeutic dose.     

Determination of catecholamines in urine using hydrophilic interaction chromatography with electrochemical detection

The determination of catecholamines in urine was investigated using hydrophilic interaction chromatography (HILIC) as an alternative to the commonly used reversed-phase (RP) method. A number of different approaches were explored, including per-aqueous liquid chromatography (PALC), and HILIC using bare silica, bonded amide and zwitterionic phases. The bonded phases gave superior results in terms of both peak shape and selectivity. The mechanism of the HILIC separation was investigated particularly with respect to the contribution of ion exchange to retention. The electrochemical detection of catecholamines was studied and optimised in typical HILIC mobile phases that contain high concentrations of acetonitrile. HILIC offered a number of advantages over the conventional RP approach, giving good retention of the solutes without use of ion pair reagents, the absence of which also would facilitate detection by mass spectrometry. HILIC used in conjunction with solid phase extraction based on RP also gives orthogonal separation mechanisms in the cleanup and analysis steps. Furthermore, good recoveries from the cleanup stage were obtained, as high concentrations of acetonitrile can be used as eluting solvent that are fully compatible with HILIC, and lipophilic impurities are eluted close to the void volume of the HILIC column.     

Enhancement of detection sensitivity and cleanup selectivity for tobramycin through pre-column derivatization.

Reversed-phase high-performance liquid chromatography, UV detection and reversed-phase solid-phase extraction (SPE) as analytical methods for pharmaceutical compounds face a challenge when the compound is rather polar and lack UV absorptivity. A good example is tobramycin. To overcome these problems, a method has been developed using pre-column derivatization of tobramycin with o-phthalaldehyde and automated with an autosampler with microrobotic routines. The detection enhancement of the derivatives was achieved by using fluorescence detection which was forty times more sensitive than using UV detection. Recovery studies of standards and spiked serum samples show that pre-SPE derivatization significantly enhances the recoveries (by at least a factor of 3) and the quality of cleanup over post-SPE derivatization.     

Determination of conjugated bile acids in human bile by isotachophoresis in a non-aqueous solvent using a.c. conductivity and UV detection

A method for the determination of conjugated bile acids in human bile using isotachophoresis in 95% methanol is described. The leading ion is 0.01 M chloride, the counter ion is hydroxylamine at its pK value and the terminating ion is N-2-hydroxyethylpiperazine-N'-2-ethanesulphonic acid (HEPES). The sample preparation consists of C18-silica cartridge adsorption. Microlitre amounts of the methanol eluate are injected and analysed within 20 min in a 0.2 mm I.D. PTFE capillary. The sensitivity of the method is better than 50 ng of each of the conjugated bile acids using a.c. conductivity detection.     

Thermodynamics of dissolution of non-electrolytes and electrolytes in non-aqueous solvents

Experimental results on the H _soln of noble gases, polar non-electrolyte molecules, porphyrins, their metallocomplexes and organic electrolytes in individual and mixed solvents are discussed. Xe was shown to indicate changes in solvent mixtures and also to exert a noticeable influence on the character of intermolecular interactions in solutions. The thermodynamic characteristics of the solvation of a series of non-polar and polar molecules are given and their dependence on the solvent nature, composition, isotopy and temperature is shown. For the first time the thermodynamic characteristics of porphyrins and metalloporphyrins in different non-aqueous solvents have been found by direct calorimetry. The possibility of axial coordination of porphyrin metallocomplexes is treated and a method is proposed for estimation of the influence of the functional substituents and the metal atom on the electronic effects in them. Compared to simple inorganic electrolyte solutions, the dependence of H _soln on various factors reveals a specific character.

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Zusammenfassung Experimentelle Daten zur Lösungswärme H _soln von Edelgasen, polaren Nichtelektrolyten, Porphyrinen und ihren Metallkomplexen und organischen Eletrolyten in einfachen und gemischten Lösungsmitteln werden diskutiert.Es wird gezeigt, dass Xe nicht nur auf Veränderungen in der Zusammensetzung von Lösungsmittelmischungen anspricht, sondern selbst einen merklichen Einfluss auf den Charakter der intermolekularen Wechselwirkungen ausübt. Thermodynamische Charakteristika der Solvatation einer Reihe unpolarer und polarer Moleküle werden aufgeführt und die Abhängigkeit von der Natur, Zusammensetzung (auch isotopisch) und Temperatur des Lösungsmittels wird gezeigt. Erstmalig werden die thermodynamischen Charakteristika von Porphyrinen und Metallporphyrinen in verschiedenen Lösungsmitteln direkt kalorimetrisch bestimmt. Die Möglichkeit einer axialen Koordination an Metall-Porphyrin-Komplexen wird gezeigt und eine Methode zum Abschätzen des dabei wirkenden Einflusses der funktioneilen Substituenten und des Metallatoms auf die elektronischen Effekte vorgeschlagen. Verglichen mit einfachen anorganischen Elektrolyt-Lösungen ist' die Abhängigkeit von H _soln von verschiedenen Faktoren sehr substanzspezifisch.

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Determination of tranquilisers and carazolol residues in animal tissue using high-performance liquid chromatography with electrochemical detection.

A multi-residue method for the determination of tranquiliser residues in animal tissue is described. The procedure may be used to determine residues of the tranquilisers acepromazine, azaperone, chlorpromazine, haloperidol, propionylpromazine, xylazine, the metabolite of azaperone, azaperol, and the beta-adrenoreceptor blocking agent carazolol. Existing methods of analysis for tranquilisers are based on ultraviolet and fluorescence detection and have been used for pig kidney analysis. Determination in this method was by high-performance liquid chromatography with electrochemical detection in the screen mode. The enhanced selectivity offered by the electrochemical detector allowed determination in liver extracts, which often give rise to more interferences on chromatographic traces when using conventional methods of detection. The method offers up to a ten-fold improvement in limits of determination over methods using ultraviolet and fluorescence detection. Recoveries and coefficients of variation have been determined in the range 2-25 micrograms/kg in pig kidney and liver. This electrochemical detection method has been used to measure residues in routine surveillance programmes.     

Determination of tetraethyllead and tetramethyllead using high performance liquid chromatography and electrochemical detection

Summary A new method for the separation of tetramethyllead (TML) and tetraethyllead (TEL) was developed using high-performance liquid chromatography. The electrochemical detection was examined with different electrodes. Amperometric and pulse-amperometric techniques were investigated and the optimal working potential for each electrode was determined. Linearity for the glassy carbon electrode was observed between 350 ng and 30 g; the detection limit is 310 ng (TML) resp. 340 ng (TEL). In case of the mercury gold electrode the linearity range was 300 g–3 g and the detection limit 1.5 m (TML) resp. 1.7 g (TEL).