Determination of fenoterol and salbutamol in pharmaceutical formulations by electrogenerated chemiluminescence

Fenoterol and salbutamol were determined by electrogenerated chemiluminescence (ECL) coupled with flow injection analysis (FIA), using Ru(bpy)₃²⁺ as the luminescent substance. Fenoterol and salbutamol oxidize together with the ruthenium 2,2-bipyridyl at a platinum electrode, which leads to an increase in the luminescent intensity, and this increase is proportional to the analyte concentration. For fenoterol a linear calibration curve within the range from 1.0 × 10⁻⁵ to 1.0 × 10⁻⁴ mol l⁻¹ was obtained with a correlation coefficient of 0.998 (n = 5) and for salbutamol the linear analytical curve was also obtained in this range with a correlation coefficient of 0.995 (n = 5). The relative standard deviation was estimated as ≤2.5% for 3 × 10⁻⁵ mol l⁻¹ for fenoterol solution and as ≤1.3% for 5.0 × 10⁻⁵ mol l⁻¹ salbutamol solution for 15 successive injections. The limit of detection for fenoterol was 2.4 × 10⁻⁷ mol l⁻¹ and for salbutamol was 4.0 × 10⁻⁷ mol l⁻¹. Fenoterol and salbutamol were successfully determined in drug tablets and the soluble components of the matrix did not interfere in the luminescent emission. The results obtained using the luminescent methodology were not statistically different from those obtained by UV-spectrophotometry at 95% confidence level.     

Voltammetric determination of glucose based on reduction of copper(II)–glucose complex at lanthanum hydroxide nanowire modified carbon paste electrodes

A novel voltammetric method for the determination of β-d-glucose (GO) is proposed based on the reduction of Cu(II) ion in Cu(II)(NH₃)₄²⁺–GO complex at lanthanum(III) hydroxide nanowires (LNWs) modified carbon paste electrode (LNWs/CPE). In 0.1 mol L⁻¹ NH₃·H₂O–NH₄Cl (pH 9.8) buffer containing 5.0 × 10⁻⁵ mol L⁻¹ Cu(II) ion, the sensitive reduction peak of Cu(II)(NH₃)₄²⁺–GO complex was observed at −0.17 V (versus, SCE), which was mainly ascribed to both the increase of efficient electrode surface and the selective coordination of La(III) in LNW to GO. The increment of peak current obtained by deducting the reduction peak current of the Cu(II) ion from that of the Cu(II)(NH₃)₄²⁺–GO complex was rectilinear with GO concentration in the range of 8.0 × 10⁻⁷ to 2.0 × 10⁻⁵ mol L⁻¹, with a detection limit of 3.5 × 10⁻⁷ mol L⁻¹. A 500-fold of sucrose and amylam, 100-fold of ascorbic acid, 120-fold of uric acid as well as gluconic acid did not interfere with 1.0 × 10⁻⁵ mol L⁻¹ GO determination.     

Sequential injection spectrophotometric determination of iron as Fe(II) in multi-vitamin preparations using 1,10-phenanthroline as complexing agent

A sequential injection analysis (SIA) system is proposed for the determination of iron (II). Fe(II) was determined by SIA based on the reaction between 1,10-phenanthroline and iron (II), yielding an orange–red colour complex with absorption maximum at 512 nm. The method involved aspiration of 187 μl sample/standard zone followed by a zone of a reagent solution containing 140 μl of 7.8 × 10⁻⁴ mol l⁻¹ 1,10-phenanthroline into a carrier stream to be stacked inside a holding coil and flow reversed through a reaction coil to a detector. The optimum condition was evaluated and the calibration curve is linear over a range of 0.25 to 5.0 mg l⁻¹ of Fe(II) with detection limit of 18 μg l⁻¹. A sample throughput of 40 h⁻¹ was established. This technique is found to be simple, accurate, reproducible and sensitive. The proposed method was successfully applied for the determination of total iron as Fe(II) in pharmaceutical products (multi-vitamin tablets) and is especially useful for the determination of iron (II) in tablets with lower iron (II) contents. The results were found to be in good agreement with the results obtained by manual UV/Vis spectrophotometry and flame atomic absorption spectrometry (FAAS) and with claimed values by the manufacturers.     

Determination of naringin in grapefruit juice by cathodic stripping differential pulse voltammetry at the hanging mercury drop electrode

A highly sensitive cathodic stripping voltammetric method for the determination of naringin is presented. It is based on the formation and accumulation of two naringin–mercury complexes at the electrode surface, followed by reduction of the surface species during a differential pulse voltammetric scan. The cathodic stripping responses at −0.25 V and −0.42 V, are evaluated with respect to various experimental conditions, such as composition and pH of the supporting electrolyte, naringin concentration, accumulation potential and preconcentration time. The new method is suitable for the determination of naringin concentrations between 0.1 mg l⁻¹ (1.72×10⁻⁷ mol l⁻¹) and 40 mg l⁻¹ (6.88×10⁻⁵ mol l⁻¹). A 3σ limit of detection of 32 μg l⁻¹ (55 nmol l⁻¹) can be reached. The relative standard deviation (r.s.d.) is <1.5%. Recovery experiments yielded a mean recovery of 97% (r.s.d.=4.1%). The application of the procedure to the selective determination of naringin in grapefruit juice is demonstrated.     

Electrochemical detection of DNA hybridization using methylene blue and electro-deposited zirconia thin films on gold electrodes

A novel electrochemical DNA biosensor based on methylene blue (MB) and zirconia (ZrO₂) thin films modified gold electrode for DNA hybridization detection is presented. Zirconia thin films were electrodynamically deposited onto the bare gold electrode in an aqueous electrolyte of ZrOCl₂ and KCl by cycling the potential between −1.1 and +0.7 V (versus Ag/AgCl) at a scan rate of 20 mV s⁻¹. Oligonucleotide probes with phosphate group at the 5′ end were attached onto the zirconia thin films because zirconia is affinity for phosphoric group. The surface density of the immobilized DNA molecules at the zirconia interface was investigated by fluorescence spectroscopy method. Hybridization was induced by exposure of the ssDNA-containing Au electrode to complementary ssDNA in solution. The decreases in the peak currents of MB, an electroactive label, were observed upon hybridization of probe with the target. The cathodic peak current (i_p) of MB after hybridization with the target DNA was linearly related to the logarithmic value of the target DNA concentration ranging from 2.25×10⁻¹⁰ to 2.25×10⁻⁸ mol l⁻¹. A detection limit of 1.0×10⁻¹⁰ mol l⁻¹ of oligonucleotides can be estimated.     

Preparation of a novel fluorescence probe based on covalent immobilization by emulsion polymerization and its application to the determination of metronidazole

A novel method of preparing fluorescence particle probe by emulsion polymerization with covalent immobilization of indicator dye was described. A terminal double bond was attached to 3-amino-9-ethylcarbazole (AEC) via methacryloyl chloride, and the resultant compound was copolymerized with butyl methacrylate. The obtained polymer particles were used as a fluorescence probe, which is almost free of dye leaching, and has higher photostability and lower toxicity in comparison with free AEC. This probe holds great potential for the applications in intracellular measurements. In present study the prepared probe was used for the determination of metronidazole. The results revealed that the probe showed good selectivity and had a linear response to the analyte in the range from 2.0 × 10⁻⁵ to 1.0 × 10⁻³ mol l⁻¹ with a detection limit of 9.0 × 10⁻⁶ mol l⁻¹.     

Square wave adsorptive stripping voltammetric determination of famotidine in urine

Electrochemical studies of famotidine were carried out using voltammetric techniques: cyclic voltammetry, linear sweep and square wave adsorptive stripping voltammetry. The dependence of the current on pH, buffer concentration, nature of the buffer, and scan rate was investigated. The best results for the determination of famotidine were obtained in MOPS buffer solution at pH 6.7. This electroanalytical procedure enabled to determine famotidine in the concentration range 1 × 10⁻⁹–4 × 10⁻⁸ mol L⁻¹ by linear sweep adsorptive stripping voltammetry (LS AdSV) and 5 × 10⁻¹⁰–6 × 10⁻⁸ mol L⁻¹ by square wave adsorptive stripping voltammetry (SW AdSV). Repeatability, precision and accuracy of the developed methods were checked. The detection and quantification limits were found to be 1.8 × 10⁻¹⁰ and 6.2 × 10⁻¹⁰ mol L⁻¹ for LS AdSV and 4.9 × 10⁻¹¹ and 1.6 × 10⁻¹⁰ mol L⁻¹ for SW AdSV, respectively. The method was applied for the determination of famotidine in urine.     

Threonine-FX dosimeter for food irradiation

A detailed study for the spectrophotometric readout method for L-threonine powder, [CH₃CH(OH)CH(NH₂)COOH], was done. In this method, 400 mg unirradiated/irradiated L-threonine powder was dissolved in 10 ml of a solution which contains 3×10⁻⁴ mol dm⁻³ ferrous ammonium sulphate and 1.7×10⁻⁴ mol dm⁻³ xylenol orange (XO) in aerated aqueous 0.17 mol dm⁻³ sulphuric acid (FX). The peroxy radicals produced from irradiated threonine oxidize ferrous ions and XO forms a complex with ferric ions as well as controls the chain length of ferrous ion oxidation. The plot of absorbance at 556 nm against dose is linear in the dose range 20–400 Gy and doses down to about 1 Gy can be measured using 10-cm path cells. Response of the dosimeter is independent of irradiation temperature above 20. A dose of 50 Gy–10 kGy can be measured dissolving 50 mg threonine powder in 10 ml of a solution which contains 3×10⁻⁴ mol dm⁻³ ferrous ammonium sulphate and 1.3×10⁻⁴ mol dm⁻³ XO in aerated aqueous 0.06 mol dm⁻³ sulphuric acid (FX). The plot of absorbance at 552 nm against dose is non-linear. However dosimeter shows linear dose response up to 1000 Gy. Irradiated threonine powder is stable for about 3 months. The reproducibility of the method is better than ±2%. This dosimeter is very useful as transfer dosimeter for food irradiation programme.     

A new flow-injection determination of glucose based on the redox reaction of hydroquinone with iron(III) in the presence of 1,10-phenanthroline

A new spectrophotometric flow-injection (FI) method is proposed for the determination of glucose based on the redox reaction of hydroquinone with iron(III). When a glucose solution containing quinone is passed through the immobilized glucose oxidase column introduced in FI system, quinone is reduced to hydroquinone by glucose. In the presence of 1,10-phenanthroline (phen), iron(III) is then quantitatively reduced by hydroquinone to iron(II) followed by the formation of iron(II)-phen complex (λ_max=510 nm). An FI peak observed at 510 nm corresponds to the concentration of glucose. The wide dynamic range for glucose was obtained in the range of 1×10⁻⁶–1×10⁻³ mol l⁻¹ at a sampling rate of 24 h⁻¹ and the detection limit (S/N=3) was 5×10⁻⁷ mol l⁻¹. Relative standard deviations were 0.78, 0.44 and 0.23% (n=5) for 5×10⁻⁶, 5×10⁻⁵ and 5×10⁻⁴ mol l⁻¹ of glucose, respectively. The proposed method was successfully applied to the determination of glucose in control blood sera, human blood plasma and wine.     

Flow-injection potentiometric determination of triiodide by plasticized poly(vinyl chloride) membrane electrodes and its application to the determination of chlorine-containing disinfectants

Plasticized poly(vinyl chloride) (PVC) membranes of different compositions were tested for use in the construction of potentiometric flow detectors for triiodide. A membrane with a 2:1 (w/w) 2-nitrophenyl octyl ether to PVC ratio was selected. The influence of thiosulphate in the carrier solution composition and of the flow-injection variables on the determination of triiodide was studied. In the selected conditions, a linear relationship between peak height and log[I₃⁻] was obtained between 5 × 10⁻⁶ and 1 × 10⁻⁴ mol l⁻¹ triiodide. Peak height relative standard deviations for 2 × 10⁻⁵ and 1 × 10⁻⁴ mol l⁻¹ triiodide were ±0.4 and ±1.8%, respectively, and sampling frequency was 80 samples per hour. The method proposed was applied satisfactorily to the iodometric determination of different chlorine-containing disinfectants, among them trichloroisocyanuric acid and dichloroisocyanurate in several types of commercial sample.     

Application of lead film electrode for simultaneous adsorptive stripping voltammetric determination of Ni(II) and Co(II) as their nioxime complexes

An adsorptive stripping voltammetric (AdSV) procedure for simultaneous determination of Ni(II) and Co(II) in the presence of nioxime as a complexing agent at an in situ plated lead film electrode was described. The Co(II) signal was enhanced by exploitation of the catalytic process in the presence of nitrite. Ni(II) and Co(II) signals are better separated than in the case of bismuth film electrodes. Calibration graphs for an accumulation time of 120 s are linear from 1 × 10⁻⁹ to 1 × 10⁻⁷ mol L⁻¹ and from 1 × 10⁻¹⁰ to 5 × 10⁻⁹ mol L⁻¹ for Ni(II) and Co(II), respectively. The proposed procedure was applied for Ni(II) and Co(II) determination in water certified reference materials.     

Flow injection visible diffuse reflectance quantitative analysis of nickel

Flow injection (FI) methodology, using diffuse reflectance in the visible region of the spectrum, for the analysis of nickel, precipitated in the form of dimethylglyoximate, is presented. A reflectance cell, constructed in polytetrafluoroethylene, using a LED (light emitting diode) as light source and a LDR (light dependent resistor) as detector, is described. The analytical signal (S) correlates with nickel concentration (C) between 1.6 × 10⁻⁴ and 6.6 × 10⁻⁴ mol L⁻¹. This correlation is described by the equation S = −1.108 + 3.314 × 10⁴C − 2.081 × 10⁷C² (r = 0.9996). The experimentally observed limit of detection is about 1.3 × 10⁻⁴ mol L⁻¹, as in lower concentrations the formation of precipitate is not observed. The experimental quantitation limit is about 1.6 × 10⁻⁴ mol L⁻¹. The mean R.S.D. (relative standard deviation) is about 2.7%. Samples containing nickel were analyzed and the results obtained in this method were compared with those of other methods using the statistical Student's t-test.     

Electrochemiluminescent behavior of allopurinol in the presence of Ru(bpy)(3)(2+)

The electrochemiluminescent (ECL) response of allopurinol was studied in aqueous media over a wide pH range (pH 2–13) using flow injection (FI) analysis. It was revealed that allopurinol itself had no ECL activity, but could greatly enhance the ECL of Ru(bpy)₃²⁺ in alkaline media giving rise to a sensitive FI-ECL response. The effects of experimental conditions including the mode of applied voltage signal, the potential of working electrode, pH value, the flow rate of carrier solution, and the concentration of Ru(bpy)₃²⁺ and allopurinol on the ECL intensity were investigated in detail. The most sensitive FI-ECL response of allopurinol was found at pH 12.0, where the FIA-ECL intensity showed a linear relationship with concentration of allopurinol in the range 1 × 10⁻⁸ mol L⁻¹ to 5 × 10⁻⁷ mol L⁻¹, and the detection limit was 5 × 10⁻⁹ mol L⁻¹.     

A peroxidase-tetrathiafulvalene biosensor based on self-assembled monolayer modified Au electrodes for the flow-injection determination of hydrogen peroxide

The construction and performance under flow-injection conditions of an integrated amperometric biosensor for hydrogen peroxide is reported. The design of the bioelectrode is based on a mercaptopropionic acid (MPA) self-assembled monolayer (SAM) modified gold disk electrode on which horseradish peroxidase (HRP, 24.3 U) was immobilized by cross-linking with glutaraldehyde together with the mediator tetrathiafulvalene (TTF, 1 μmol), which was entrapped in the three-dimensional aggregate formed.

The amperometric biosensor allows the obtention of reproducible flow injection amperometric responses at an applied potential of 0.00 V in 0.05 mol L⁻¹ phosphate buffer, pH 7.0 (flow rate: 1.40 mL min⁻¹, injection volume: 150 μL), with a range of linearity for hydrogen peroxide within the 2.0 × 10⁻⁷–1.0 × 10⁻⁴ mol L⁻¹ concentration range (slope: (2.33 ± 0.02) × 10⁻² A mol⁻¹ L, r = 0.999). A detection limit of 6.9 × 10⁻⁸ mol L⁻¹ was obtained together with a R.S.D. (n = 50) of 2.7% for a hydrogen peroxide concentration level of 5.0 × 10⁻⁵ mol L⁻¹. The immobilization method showed a good reproducibility with a R.S.D. of 5.3% for five different electrodes. Moreover, the useful lifetime of one single biosensor was estimated in 13 days.

The SAM-based biosensor was applied for the determination of hydrogen peroxide in rainwater and in a hair dye. The results obtained were validated by comparison with those obtained with a spectrophotometric reference method. In addition, the recovery of hydrogen peroxide in sterilised milk was tested.     

Excited species in the FBX dosimeter system

In the FBX dosimeter solution, the excitation of xylenol orange (XO) produces maximum emission at 550–575 nm both at room and liquid nitrogen temperatures (about 85%) having a lifetime of 0.20–0.36 ns. In addition, at room temperature there is an emission at 350 nm for the excitation at 260 nm (about 15%) having a longer lifetime of 3.71–4.01 ns. Benzoic acid (BA) has excitation at 284–295 nm and emission at 320–365 nm having a lifetime of 1.38 ns. In an aqueous solution containing 5×10⁻³ mol dm⁻³ BA, 2×10⁻⁴ mol dm⁻³ XO and 0.04 mol dm⁻³ H₂SO₄ there is no XO emission at 550 nm due to UV absorption at 260 nm by BA. In this solution, 2 emissions are observed near 350–360 nm, having lifetimes of 1.25 ns (89%) and 2.86 ns (11%). The wavelengths for the emission of XO and absorption of ferric-XO complex are nearly the same. Excited XO produces oxidation of ferrous ions and BA increases the chain length.     

Determination of potassium ions in biodiesel using a nickel(II) hexacyanoferrate-modified electrode

In this work, nickel hexacyanoferrate-modified electrode was developed to determine potassium ions in biodiesel by potentiometry. The modified electrodes exhibit a linear response to potassium ions in the concentration range of 4.0 × 10⁻⁵ to 1.0 × 10⁻² mol L⁻¹, with a detection limit of 1.9 × 10⁻⁵ mol L⁻¹, and a near-Nernstian slope (53–55 mV per decade) at 25 °C. The method developed in this work was compared with flame photometry and the potassium concentration found in biodiesel showed that the modified electrode method gives results similar to those obtained by flame photometry.     

Determination of folic acid by chemiluminescence based on peroxomonosulfate-cobalt(II) system

Based on the chemiluminescence (CL) phenomena of folic acid in peroxomonosulfate-cobalt(II) system, a rapid and sensitive CL method was developed for determination of folic acid in pharmaceutical preparations and its urinary metabolism processes. Under the optimum conditions, the relative CL intensity was linear over the concentration ranging from 10⁻⁹ to 8 × 10⁻⁷ mol L⁻¹ (R² = 0.9991) with a detection limit as low as 6 × 10⁻¹⁰ mol L⁻¹ (S/N = 3) and relative standard deviation was 2.63% for 2 × 10⁻⁸ mol L⁻¹ folic acid (n = 11). This method has been successfully applied to the determination of folic acid in tablets and human urine. The blank CL emission was yielded owing to the formation of singlet oxygen molecular pair from the quenching experiment of 1,4-diazabicyclo[2.2.2]octane, and pterine-6-carboxylic acid might be the degradation intermediate in this system and it also acts an energy acceptor and sensitizes the chemiluminescence based on the studies of the CL and fluorescence spectra.     

The nano-Au self-assembled glassy carbon electrode for selective determination of epinephrine in the presence of ascorbic acid

Gold nanoparticles were self-assembled to the modified glassy carbon electrode (GC) with cysteamine (CA) to prepare the nano-Au/CA/GC modified electrode. The electrochemical behavior of epinephrine (EP) on the modified electrode was explored with cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Epinephrine gave a pair of redox peaks at E_pa = 0.190 mV and E_pc = −0.224 mV (versus SCE), respectively. The nano-Au/CA/GC modified electrode shows an excellent electrocatalytic activity for the oxidation of EP. The modified electrode could be used to determine EP in the presence of ascorbic acid (AA). The response of catalytic current with EP concentration shows a linear relation in the range of 1.0 × 10⁻⁷ to 5.0 × 10⁻⁴ mol L⁻¹ with the correlation coefficient of 0.998. The detection limit is 4.0 × 10⁻⁸ mol L⁻¹. The modified electrode exhibited a good reproducibility, sensitivity and stability for the determination of EP injection.     

Enhancement effect of sodium dodecyl benzene sulfonate (SDBS) and its application into voltammetric determination of telmisartan

A sensitive and rapid electrochemical method was developed for the determination of telmisartan based on the enhancement effect of sodium dodecyl benzene sulfonate (SDBS). In 0.1 mol L⁻¹ HClO₄ and in the presence of 7.5 × 10⁻⁵ mol L⁻¹ SDBS, a well-defined and sensitive oxidation peak was observed for telmisartan at the acetylene black (AB) paste electrode. However, the oxidation peak is poor-shaped and the peak current is very low in the absence of SDBS, suggesting that SDBS shows obvious enhancement effect for the determination of telmisartan. After all the experimental parameters were optimized, a sensitive and simple electrochemical method was developed for determining telmisartan. The oxidation peak current is proportional to the concentration of telmisartan over the range from 2.5 × 10⁻⁷ to 2.0 × 10⁻⁵ mol L⁻¹. The detection limit is 7.5 × 10⁻⁸ mol L⁻¹ after 2 min of accumulation. This new voltammetric method was successfully used to detect telmisartan in drugs.     

Kinetic method for the determination of traces of thyroxine by its catalytic effect on the Mn(III) metaphosphate-As(III) reaction

A new, highly sensitive and simple kinetic method for the determination of thyroxine was proposed. The method was based on the catalytic effect of thyroxine on the oxidation of As(III) by Mn(III) metaphosphate. The kinetics of the reaction was studied in the presence of orthophosphoric acid. The reaction rate was followed spectrophotometrically at 516 nm. It was established that orthophosphoric acid increased the reaction rate and that the extent of the non-catalytic reaction was extremely small. A kinetic equation was postulated and the apparent rate constant was calculated. The dependence of the reaction rate on temperature was investigated and the energy of activation and other kinetic parameters were determined.

Thyroxine was determined under the optimal experimental conditions in the range 7.0 × 10⁻⁹ to 3.0 × 10⁻⁸ mol L⁻¹ with a relative standard deviation up to 6.7% and a detection limit of 2.7 × 10⁻⁹ mol L⁻¹. In the presence of 0.08 mol L⁻¹ chloride, the detection limit decreased to 6.6 × 10⁻¹⁰ mol L⁻¹. The proposed method was applied for the determination of thyroxine in tablets. The accuracy of the method was evaluated by comparison with the HPLC method.