Diamond paste-based electrodes for the determination of sildenafil citrate (Viagra)

Three types of monocrystalline diamond: natural diamond 1 μm, synthetic diamond 50 μm (synthetic-1), and synthetic diamond 1 μm (synthetic-2) were used for the design of diamond paste electrodes for the determination of sildenafil citrate (Viagra) using square wave voltammetry. The linear concentration ranges recorded for sildenafil citrate when natural diamond, synthetic-1, and synthetic-2 based electrodes were used were between 10⁻¹² and 10⁻⁸, 10⁻¹² and 10⁻⁹, and 10⁻¹¹ and 10⁻⁹ mol/L, respectively. Low detection limits which lie between 0.1 and 1 pmol/L proves the sensitivity of the electrodes. It was found that sildenafil citrate yielded a peak at about +0.175 ± 0.025 V (versus Ag/AgCl) for all the electrodes. Sildenafil citrate was determined with high reliability from its pharmaceutical formulation.     

Diamond paste based electrodes for the determination of Pb(II) at trace concentration levels

Three types of monocrystalline diamond: natural diamond 1 μm, synthetic diamond 50 μm (synthetic-1), and synthetic diamond 1 μm (synthetic-2) were used for electrodes’ construction. The linear concentration ranges recorded for Pb(II), when natural diamond, synthetic-1 and synthetic-2 based electrodes were used were between 10⁻⁹ and 10⁻⁶; 10⁻¹⁰ and 10⁻⁷; and between 10⁻¹⁰ and 10⁻⁸ mol l⁻¹, respectively. Low detection limits which lie between 10 and 100 pmol l⁻¹ proves the sensitivity of the electrodes. It was found that Pb(II) yielded a peak at about +0.3±0.02 V (versus Ag/AgCl) for all the electrodes. Lead was determine with high reliability from water and tea samples at trace concentration levels using the proposed diamond paste based electrodes.     

Rapid and sensitive spectrophotometric determination of trace amounts of iron(III) using leuco Xylene cyanol FF

A new, simple, sensitive, and reliable method is presented for the rapid spectrophotometric determination of trace amounts of iron(III) using leuco Xylene cyanol FF. The method is based on the oxidation of leuco Xylene cyanol FF (LXCFF) to its blue form of xylene cyanol FF by iron(III) in sulfuric acid medium (pH 2.0–3.0), the absorbance of the formed dye is measured in an acetate buffer medium (pH 2.8–4.4) at 615 nm. The method obeys Beer's law over a concentration range of 0.15–0.9 g mL^-1 iron, having a molar absorptivity of 5.6×10⁴ L mol^-1 cm^-1 and a Sandell's sensitivity of 0.0001 g cm^-2. The optimum reaction conditions and other analytical parameters have been evaluated. The developed method has been successfully applied to the determination of iron in water, soil, industrial effluent, plant material, pharmaceutical preparations, synthetic mixtures, and aluminum alloys.     

Three useful bromimetric methods for the determination of salbutamol sulfate

Three different methods developed for the determination of salbutamol sulfate (SBS), in pure drug form and in dosage forms, are discussed. The methods are based on the oxidation–bromination reaction of the drug by bromine generated in-situ by the interaction of bromate with bromide in acid medium. In titrimetry the drug is titrated directly with bromate in the presence of a large excess of bromide and in sulfuric acid medium using methyl red as indicator. Spectrophotometry is based on addition of a measured excess of bromate–bromide mixture to the sample solution in sulfuric acid medium followed by the estimation of surplus bromine by reacting it with a definite amount of methyl orange dye and measuring the absorbance at 510 nm. The amount of bromate reacting corresponds to the sample content. The kinetic method depends on the linear relationship between the concentration of the drug and time for oxidation and bromination as indicated by the bleaching of the methyl orange acid colour by the bromine generated in situ. Titrimetry is applicable in the 2–20 mg range. In spectrophotometry, Beer's law is obeyed in the 0.5–5.0 g mL^–1 range whereas concentrations in the 5.0–25.0 g mL^–1 range can be determined by the kinetic method. The effect of common excipients and additives in tablets is discussed. The procedures have been successfully applied to dosage forms; the results agree well with those obtained by use of a reference method. The methods can be used to determine SBS at mg or g levels.     

Thermodynamic Study of the Aqueous Rubidium and Manganese Bromide System

Crystallization of 2RbBr · MnBr₂ · 2H₂O, the only double salt obtained under standard conditions from saturated aqueous rubidium–manganese bromide solutions, was theoretically predicted using the hard and soft Lewis acids and bases concept and Pauling's rules. The RbBr—MnBr₂—H₂O system was thermodynamically simulated by the Pitzer model assuming a solubility diagram of three branches only: RbBr, 2RbBr · MnBr₂ · 2H₂O and MnBr₂ · 4H₂O. The theoretical result was experimentally proved at 25°C by the physicochemical analysis method and formation of the new double salt 2RbBr · MnBr₂ · 2H₂O was established. It was found to crystallize in a triclinic crystal system, space group –P1, a = 5.890(1) Å, b = 6.885(1) Å, c = 7.367(2) Å, = 66.01(1)°, = 87.78(2)°, = 84.93(2)°, V = 271.8(1) ų, Z = 1, D _x = 3.552 g-cm^–3. The binary and ternary ion interaction parameters were calculated and the solubility isotherm was plotted. The standard molar Gibbs energy of the synthesis reaction, _rG _m ^o , of the double salt 2RbBr · MnBr₂ · 2H₂O from the corresponding simple salts RbBr and MnBr₂ · 4H₂O, as well as the standard molar Gibbs energy of formation, _fG _m ^o , and standard molar enthalpy of formation _fH _m ^o of the simple and double salts were calculated.     

Chromatographic behavior of cadmium in an ion-pair reversed-phase micro HPLC system and its application to the determination of bio-available cadmium in soil samples

A novel reversed phase ion-pair micro HPLC system with on-line fluorescence detection has been developed systematically and studied for the determination of cadmium in its bio-available fractions of soil samples. In this system, a micro ODS column of 1.0 mm i.d.×150 mm length and a mobile phase containing 6 mmol L^-1 8-hydroxyquinoline 5-sulphonic acid (HQS), 3 mmol L^-1 cetyltrimethylammonium bromide (CTMABr), 10 mmol L^-1 acetic acid-acetate buffer (pH 5.1) as well as 50% acetonitrile at 50 L min^-1 flow rate were employed to determine cadmium with a 2 L flow cell through its fluorescence at 518 nm under 338 nm excitation. Furthermore, the composition of Cd-HQS chelate formed on the column was confirmed to be [Cd(HQS)₂]^2- through a log–log plot method, and then combined with the ion-pair reagent by the electrostatic force under the chromatographic condition proposed. With such a method, the detection limit of cadmium was 8.48 ng mL^-1 (3) with 1 L sample injection, and the linear range for the determination of cadmium was 30–800 ng mL^-1 (R²=0.992). This method has been successfully applied to determination of cadmium in its bio-available fractions of BCR-483 and soil samples without interference from other coexistent metal ions. The RSD (n=6) was less than 7.3%. The results were in agreement with the indicative value for BCR-483 and those for the soil samples obtained by ICP-MS with a pretreatment of bis(1,1,3,3-tetramethylbutyl) phosphinic acid extraction.     

Determination of hydroxyl radical by capillary electrophoresis and studies on hydroxyl radical scavenging activities of Chinese herbs

High-performance capillary electrophoresis (CE) with electrochemical detection (ED) was employed to determine hydroxyl radicals in the Fenton reaction. Hydroxyl radicals can react with salicylic acid to produce 2,3-dihydroxy benzoic acid and 2,5-dihydroxy benzoic acid, which can be analyzed by CE-ED. Based on this principle, hydroxyl radicals were determined indirectly. In a 20 mmol/L phosphate running buffer (pH 7.4), 2,3-dihydroxy benzoic acid and 2,5-dihydroxy benzoic acid would elute simultaneously from the capillary within 6 min. As the working electrode, a 300 m diameter carbon-disk electrode exhibits good responses at +0.60 V (vs. SCE) for the two analytes. Peak currents of the two analytes are additive. Excellent linearity was obtained in the concentration range from 1.0×10^-3 mol/L to 5.0×10^-6 mol/L for 2,3-dihydroxy benzoic acid. The detection limit (S/N=3) was 2.0×10^-6 mol/L. This method was successfully applied for studying hydroxyl radical scavenging activities of Chinese herbs. It is testified that Apocynum Venetum L., Jinkgo bibola L., Morus alba L. and Rhododendron dauricum L. have strong hydroxyl radical scavenging activities.     

Automated coupling of capillary-HPLC to matrix-assisted laser desorption/ionization mass spectrometry for the analysis of small molecules utilizing a reactive matrix

This study describes the application of a novel, reactive matrix for the mass spectral analysis of steroids by capillary-high performance liquid chromatography (capillary-HPLC) coupled to matrix-assisted laser desorption/ionization (MALDI). The mass spectral analysis of steroids was accomplished after fully automated peak deposition of chromatographic peaks onto MALDI targets. The seven corticosteroids used as test compounds were: triamcinolone, prednisone, cortisone, fludrocortisone, dexamethasone, deoxycorticosterone, and budesonide. They were separated using a PepMap C₁₈ (3 m particle size, 100 Å pore width) column at five different concentration levels of 25, 15, 7.5, 2.5 and 1 ng/L, and the peaks were detected at a wavelength of 237 nm. The column effluent was mixed with 2,4-dinitrophenylhydrazine (DNPH) directly following the UV detector. The chromatographic peaks were then deposited onto the MALDI target with a robotic micro-fraction collector triggered by the UV detector signals. A special hydrophobic surface coating allowed the deposition of up to 4 L (up to 90 % of the chromatographic peak volume) onto one sample spot. The compounds were then identified by MALDI mass spectrometry. Depending on the nature of the analyte, radical cations ([M]^+.) and sodium adduct ions ([M+Na]⁺) of the steroids as well as protonated steroid-dinitrophenylhydrazone derivatives ([M_D+H]⁺) were detected in positive ion mode. The detection limits were between 0.5 and 15 ng injected with capillary-HPLC-MALDI-TOF-MS and between 0.3 and 3 ng on target with MALDI-TOF when deposited manually.     

Speciation of phytate ion in aqueous solution. Alkali metal complex formation in different ionic media

The acid–base properties of phytic acid [myo-inositol 1,2,3,4,5,6-hexakis(dihydrogen phosphate)] (H₁₂Phy; Phy^12–=phytate anion) were studied in aqueous solution by potentiometric measurements ([H⁺]-glass electrode) in lithium and potassium chloride aqueous media at different ionic strengths (0<I mol L^–13) and at t=25 °C. The protonation of phytate proved strongly dependent on both ionic medium and ionic strength. The protonation constants obtained in alkali metal chlorides are considerably lower than the corresponding ones obtained in a previous paper in tetraethylammonium iodide (Et₄NI; e.g., at I=0.5 mol L^–1, logK₃^H=11.7, 8.0, 9.1, and 9.1 in Et₄NI, LiCl, NaCl and KCl, respectively; the protonation constants in Et₄NI and NaCl were already reported), owing to the strong interactions occurring between the phytate and alkaline cations present in the background salt. We explained this in terms of complex formation between phytate and alkali metal ions. Experimental evidence allows us to consider the formation of 13 mixed proton–metal–ligand complexes, M_jH_iPhy^{(12–i–j)–}, (M⁺=Li⁺, Na⁺, K⁺), with j7 and i6, in the range 2.5pH10 (some measurements, at low ionic strength, were extended to pH=11). In particular, all the species formed are negatively charged: i+j–12=–5, –6. Very high formation percentages of M⁺–phytate species are observed in all the pH ranges investigated. The stability of alkali metal complexes follows the trend Li⁺Na⁺K⁺. Some measurements were also performed at constant ionic strength (I=0.5 mol L^–1), using different mixtures of Et₄NI and alkali metal chlorides, in order to confirm the formation of hypothesized and calculated metal–proton–ligand complex species and to obtain conditional protonation constants in these multi-component ionic media.Presented at SIMEC–02, Santiago de Compostela, 2–6 June 2002     

A toxicity view of the pesticide picloram when immobilized onto a silica gel surface

The pesticide picloram (4-amino-3,5,6-trichloropicolinic acid) was anchored onto silica gel to yield a new surface. Isothermal microcalorimetry was applied to study the toxic effects caused to microbial activity of a typical Brazilian agricultural soil by application of free and immobilized picloram. The activity of the microorganisms in 1.50 g of soil sample was stimulated by addition of 6.0 mg of glucose plus 6.0 mg of ammonium sulfate under 34.8% controlled humidity at 298.15±0.02 K. The activity was recorded through power–time curves for increasing amounts of the active principle, varying from zero to 10.00 g g^–1. The increasing amounts of picloram, either free or immobilized, caused a decrease of the original thermal effect. The calorimetric data showed that the anchored pesticide presented a much lower toxic effect than the free picloram on the microbial activity.     

High-performance liquid chromatographic determination of diltiazem in human plasma after solid-phase and liquid–liquid extraction

A selective, sensitive, and accurate high-performance liquid chromatographic method for determination of diltiazem in plasma samples has been developed and validated. The effects of mobile phase composition, buffer concentration, mobile phase pH, and concentration of organic modifiers on retention of diltiazem and internal standard were investigated. Solid-phase and liquid–liquid extraction were examined and proposed for isolation of the drug and elimination of endogenous plasma interferences. A 5 m Lichrocart Lichrospher 60 RP-select B chromatographic column was used; the mobile phase was acetonitrile–0.025 mol L^–1 KH₂PO₄ (pH 5.5), 35:65 ( v / v) at a flow-rate of 1.5 mL min^–1. The detection wavelength was 215 nm. The calibration plots were linear in the concentration range 20.0–500.0 ng mL^–1. The method has been implemented to monitor diltiazem levels in patient samples.     

Stopped-flow kinetic determination of a binary mixture: Simultaneous determination of europium and cerium

A kinetic method is proposed for the simultaneous determination of europium(II) and cerium(III), based on their inductive action on the Cr(VI)-iodide redox reaction in weak acidic medium. The reaction rate was followed by the stopped-flow spectrophotometric technique using starch-iodine complex as indicator. The method can be used for the determination of europium and cerium after reduction with zinc metal powder in the concentration ranges 0–2.1 and 0–1.8 g/ml, respectively. The detection limits are 0.015 g/ml Eu and 0.010 g/ml Ce. 100 to 1000 times higher concentrations of other rare earth metal ions and Th⁴⁺, Al³⁺, Mn²⁺, Hg²⁺, Co²⁺, Ni²⁺, Cu²⁺, Pb²⁺ do not interfere. The method is applied to the determination of europium and cerium in a synthetic sample and in barium yttrium fluoride fluorescent material, with relative standard deviation of about 4%.     

Simultaneous determination of phenylenediamine isomers and dihydroxybenzene isomers in hair dyes by capillary zone electrophoresis coupled with amperometric detection

The simultaneous determination of three isomers of phenylenediamines (o, m, and p-phenylenediamine) and two isomers of dihydroxybenzenes (catechol and resorcinol) in hair dyes was performed by capillary zone electrophoresis coupled with amperometric detection (CZE–AD). The effects of working electrode potential, pH and concentration of running buffer, separation voltage, and injection time on CZE–AD were investigated. Under the optimum conditions the five analytes could be perfectly separated in 0.30 mol L⁻¹ borate–0.40 mol L⁻¹ phosphate buffer (pH 5.8) within 15 min. A 300 μm diameter platinum electrode had good responses at +0.85 V (versus SCE) for the five analytes. Their linear ranges were from 1.0 × 10⁻⁶ to 1.0 × 10⁻⁴ mol L⁻¹ and the detection limits were as low as 10⁻⁷ mol L⁻¹ (S/N = 3). This working electrode was successfully used to analyze eight kinds of hair dye sample with recoveries in the range 91.0–108.0% and RSDs less than 5.0%. These results demonstrated that capillary zone electrophoresis coupled with electrochemical detection using a platinum working electrode as detector was convenient, highly sensitive, highly repeatable and could be used in the rapid determination of practical samples. Figure Electropherograms obtained from 10 mg mL⁻¹ hair dye sample solutions at a platinum working electrode under optimum CZE–AD conditions: (a) natural black (I), (b) golden: (1) p-phenylenediamine, (2) m-phenylenediamine, (3) o-phenylenediamine, (4) resorcinol, and (5) catechol     

β-Naphthoquinone sulfonate as electrochemical labelling reagent for high-sensitivity LC analysis of drugs in biological fluids

Summary A new sensitive, high-performance liquid chromatography (HPLC) method using electrochemical detection (ECD) is reported for the determination of 21 amphetamine-like compounds such as amphetamine, methamphetamine, methoxyphenamine, phentermine, ephedrine, mescaline, STP and methylenedioxyamphetamine, in biological fluids. Primary and secondary amines in biological specimens were easily labelled with -naphtho-quinone-4-sulfonate (BNQS) at 60°C for 30 min. Hydrodynamic voltamograms of BNQ-derivatives showed a maximum response at an applied potential of around zero V (against Ag/AgCl) in reducing model. BNQ derivatives of amines were analyzed by HPLC using a reducing ECD at zero V (Ag/AgCl). Separation was on a Bondasphere C18 with mobile phase: acetonitrile-methanol-0.01 M H₂SO₄. Good separations of twenty one amphetamine-analogs were obtained. Only 50 l of biological fluid containing those drugs at a concentration of over 1 g ml^–1 were needed direct for determination with good accuracy. When extraction was carried out prior to the labelling reaction, the detection limits were much improved to less than 10 ng ml^–1.This method was successfully applied to determination of methamphetamine in hair.     

Deposition and characterisation of a porous Sn(IV) semiconductor nanofilm on boron-doped diamond

Nanofilm deposits of a porous Sn(IV) oxide are formed by anodic electrodeposition on a polished boron-doped diamond electrode immersed in an aqueous Sn²⁺ solution. Mechanically and electrochemically stable deposits of 10–15 nm thickness are formed irrespective of the Sn²⁺ concentration and mass-transport enhancement by power ultrasound. Atomic force microscopy images indicate the presence of a smooth and noncrystalline film, which is stable under ambient conditions. n-type semiconducting characteristics are observed for the aqueous solution redox couples Fe(CN)₆ ^3–/4– and Ru(NH₃)₆ ^3+/2+. However, preliminary results from voltammetric experiments indicate that the small and neutral organic molecule N,N,N′,N′-tetramethylphenylenediamine is able to diffuse through the porous film to undergo oxidation directly at the surface of the boron-doped diamond electrode. Electronic Publication     

Determination of cadmium and copper with ET-AAS after electrochemical deposition on a graphite electrode

The electrodeposition of cadmium and copper on a special graphite disk electrode has been performed at controlled potential. The electrode with the deposit has been inserted into the graphite atomizer HGA-400 by an adapted automatic sampler for the final determination by ET-ASS. The sensitivity of determination has been 0.371 (gl^-1)^-1 for cadmium and 0.025 (gl^-1)^-1 for copper for 2 min electrodeposition and increased linearly with the time of deposition. The limit of detection (3s_bl) has been 7.9 ngl^-1 Cd²⁺ and 0.11 gl^–1 Cu²⁺ for 2 min deposition and it has been improved with increased time of electrodeposition. The technique has been applied to the determination of both metals in seawater and to speciation in the presence of EDTA complexing agent.     

Chromotropic acid,a fluorogenic chelating agent for aluminium(III)

Complexation of aluminium(III) with the fluorogenic ligand chromotropic acid (4,5-dihydroxynaphthalene-2,7-disulfonic acid) has been revisited with the aim of using enhancement of the fluorescence intensity as an analytical tool. Complexation at the optimum pH4 was shown to lead to a 1:1 complex with a stability constant log ₁₁₀=18.4±0.7. The fluorogenic effect was thoroughly investigated. Nearly selective excitation of the chelate rather than the ligand could be achieved at wavelengths longer than 360 nm. For analytical purposes the main interfering ion was Ga³⁺. The strongest competing ligand was shown to be citric acid. Competitive complexation by acetate or formate ions can also make their use in a buffer at the usual concentration, 0.2 mol L^–1, questionable, whereas a 10^–2 mol L^–1 formic acid buffer was shown to be a good alternative. The calibration plot showed that the dependence of response on Al(III) concentration was linear up to 500 g L^–1; the detection limit was 0.65 g L^–1 (3SD _blank, n=10, SD=±1.4% at 10 g L^–1 and ±0.8% at 100 g L^–1). The analytical procedure was successfully applied to several samples of tap water and the results were in good agreement with those from AAS determination.     

Separation of Monovalent and Divalent Cations on a Pure Silica Gel Column with Dilute Oxalic Acid Containing 18-Crown-6 as Mobile Phase

Summary Pure silica gel (Pia Seed 5S-60-SIL) has been investigated as a cation-exchange stationary phase for ion chromatography of common monovalent and divalent cations (Li⁺, Na⁺, NH₄⁺, K⁺, Mg²⁺, and Ca²⁺) with conductimetric detection; dilute oxalic acid (0.05 mm oxalic acid, pH 4.1, to 1 mm oxalic acid, pH 3.0) was used as mobile phase. The Pia Seed 5S-60-SIL silica gel acted as a cation-exchange stationary phase for these cations when 0.2 mm oxalic acid at pH 3.6 was used as the mobile phase. Excellent simultaneous separation and highly sensitive indirect conductimetric detection of these cations were achieved in 20 min on a 150 mm × 4.6 mm i.d. Pia Seed 5S-60-SIL silica gel column with 0.2 mm oxalic acid containing 4 mm 18-crown-6 (1,4,7,10,13,16-hexaoxacycloctadecane), pH 3.7, as mobile phase (detection limits (signal-to-noise ratio, 3, injection volume, 20 L), were 0.15 m for Li⁺, 0.16 m for Na⁺, 0.21 m for NH₄⁺, 1.0 m for K⁺, 0.17 m for Mg²⁺, and 0.25 m for Ca²⁺). The proposed IC–CD method was successfully applied to the separation and detection of major cations (Na⁺, NH₄⁺, K⁺, Mg²⁺, and Ca²⁺) in rain and river water samples.     

Electrochemical Behavior of a Copper Electrode in Solid RbCu4Cl3I2Electrolyte

The electrochemical behavior of a copper electrode in solid RbCu₄Cl₃I₂electrolyte is studied by galvanostatic and potentiostatic methods. It is found that a Cu₂O layer 1 m thick exists at the interface between the Cu electrode and RbCu₄Cl₃I₂. The layer blocks the electrochemical reaction Cu⁰– e Cu⁺, which involves with metallic copper. At low overpotentials, the Cu electrode acts as an inert redox electrode. At the Cu₂O/RbCu₄Cl₃I₂interface, the electrochemical reaction Cu⁺– e Cu²⁺occurs, which involves Cu²⁺ions. The reaction rate is limited by slow diffusion of Cu²⁺ions in RbCu₄Cl₃I₂. The initial concentration of Cu²⁺ions in the electrolyte near this interface is about 1.4 × 10¹⁷cm^–3. The exchange current density is (4 ± 2) × 10^–6A/cm². At potentials exceeding 8–10 mV, an electric breakdown of the Cu₂O layer occurs, and the reaction with metallic copper becomes unblocked. At 10 mV < < 100 mV, the rate of this reaction is limited by the nucleation of copper crystals and the nuclei growth. At > 120 mV, the reaction rate is limited by charge transfer.