Orlando Fatibello-Filho

Simultaneous voltammetric determination of paracetamol and caffeine in pharmaceutical formulations using a boron-doped diamond electrode.

A simple and highly selective electrochemical method was developed for the single or simultaneous determination of paracetamol (N-acetyl-p-aminophenol, acetaminophen) and caffeine (3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione) in aqueous media (acetate buffer, pH 4.5) on a boron-doped diamond (BDD) electrode using square wave voltammetry (SWV) or differential pulse voltammetry (DPV). Using DPV with the cathodically pre-treated BDD electrode, a separation of about 550 mV between the peak oxidation potentials of paracetamol and caffeine present in binary mixtures was obtained. The calibration curves for the simultaneous determination of paracetamol and caffeine showed an excellent linear response, ranging from 5.0 x 10(-7)mol L(-1) to 8.3 x 10(-5)mol L(-1) for both compounds. The detection limits for the simultaneous determination of paracetamol and caffeine were 4.9 x 10(-7)mol L(-1) and 3.5 x 10(-8)mol L(-1), respectively. The proposed method was successfully applied in the simultaneous determination of paracetamol and caffeine in several pharmaceutical formulations (tablets), with results similar to those obtained using a high-performance liquid chromatography method (at 95% confidence level).

Biosensor based on paraffin/graphite modified with sweet potato tissue for the determination of hydroquinone in cosmetic cream in organic phase

An organic-phase biosensor based on paraffin/graphite modified with sweet potato (Ipomoea batatas (L.) Lam.) tissue as the source of peroxidase was developed and used for determining hydroquinone in cosmetic creams. This enzyme in the presence of hydrogen peroxide catalyses the oxidation of hydroquinone to p-quinone which electrochemical reduction back to hydroquinone was obtained at a peak potential of -0.22 V. The recovery of hydroquinone from two samples ranged from 99.1 to 104.1% and a rectilinear analytical curve for hydroquinone concentration from 7.5x10(-5) to 1.6x10(-3) M (r=0.9991) were obtained. The detection limit was 8.1x10(-6) M and relative standard deviation was <1.0% for a solution containing 7.3x10(-4) M hydroquinone and 1.0x10(-3) M hydrogen peroxide in 0.10 M tetrabutylammonium bromide methanol-phosphate buffer solution (95:5% v/v) (n=10). The results obtained for hydroquinone in cosmetic creams using the proposed biosensor are in close agreement with those obtained using a Pharmacopoeia procedure at the 95% confidence level.

Square-wave voltammetric determination of propranolol and atenolol in pharmaceuticals using a boron-doped diamond electrode

The independent determination of two beta-blocker agents, namely propranolol (PROP) and atenolol (ATN), in pharmaceutical formulations using square-wave voltammetry and a cathodically pretreated boron-doped diamond electrode is described. These electroanalytical determinations of propranolol or atenolol were carried out in 0.1molL(-1) H(2)SO(4) or 0.5molL(-1) NaNO(3) (pH 1.0, adjusted with concentrated HNO(3)), respectively. Excellent linear calibration curves, ranging from 0.20 to 9.0micromolL(-1) for PROP and from 2.0 to 41micromolL(-1) for ATN, with detection limits of 0.18 and 0.93micromolL(-1), respectively, were obtained. The obtained recoveries range from 93.9% to 105.0%, for PROP, and from 92.5% to 106.0%, for ATN. The proposed method was successfully applied in the determination of both beta-blockers in several pharmaceutical formulations (tablets), with results in close agreement at a 95% confidence level with those obtained using official spectrophotometric methods.

An improved flow system for phenols determination exploiting multicommutation and long pathlength spectrophotometry.

A greener and sensitive procedure for spectrophotometric determination of phenols based on a multicommuted flow system with a 100cm optical path flow cell is presented. The method exploited the oxidative coupling of phenolic compounds with 4-aminoantipyrine in alkaline medium containing potassium hexacyanoferrate(III). Sensitivity was 80-fold higher than that achieved with a 1cm flow cell, making feasible the determination of phenols in the 10-100mugl(-1) range with a detection limit estimated as 1mugl(-1) phenol. The sampling rate and the coefficient of variation were estimated as 90 determinations per hour and 0.6% (n=10), respectively. The multicommutation approach allowed a 200-fold reduction of the reagent consumption in comparison with the reference batch method. Moreover, the chloroform extraction for analyte concentration is unnecessary in view of the increase in sensitivity. Recoveries within 93.3 and 106% were achieved for determination of phenol in natural and wastewater samples. Results agreed with the obtained by a reference method at the 95% confidence level.

Simple Flow Injection Analysis System for Simultaneous Determination of Phenolic Antioxidants with Multiple Pulse Amperometric Detection at a Boron-Doped Diamond Electrode

A method for simultaneous determination of butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) in food was developed that uses multiple pulse amperometry (MPA) with flow injection analysis (FIA). Determination of these phenolic antioxidants was carried out with a cathodically pretreated boron-doped diamond electrode and an aqueous ethanolic (30% ethanol, v/v) 10 mmol L⁻¹ KNO₃ solution (pH(cond) = 1.5) as supporting electrolyte. A dual-potential waveform, at E(det1) = 850 mV/200 ms and E(det2) = 1150 mV/200 ms versus Ag/AgCl (3.0 mol L⁻¹ KCl), was employed. The use of E(det1) or E(det2) caused the oxidation of BHA or of BHA and BHT, respectively; hence, concentration subtraction could be used to determine both species. The respective analytical curves presented good linearity in the investigated concentration range (0.050-3.0 μmol L⁻¹ for BHA and 0.70-70 μmol L⁻¹ for BHT), and the detection limits were 0.030 μmol L⁻¹ for BHA and 0.40 μmol L⁻¹ for BHT. The proposed method, which is simple, quick, and presents good precision and accuracy, was successfully applied in the simultaneous determination of BHA and BHT in commercial mayonnaise samples, with results similar to those obtained by HPLC, at a 95% confidence level.

Comparative study of different cross-linking agents for the immobilization of functionalized carbon nanotubes within a chitosan film supported on a graphite-epoxy composite electrode.

The effectiveness of immobilization of functionalized carbon nanotubes into chitosan using different cross-linking agents has been evaluated. The cross-linkers used were glyoxal (GO), glutaraldehyde (GA), epichlorohydrin (ECH), and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide together with N-hydroxysuccinimide (EDC-NHS), and the nanotubes were retained on graphite epoxy resin composite electrodes. The nanotube modified electrodes have been characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Using CV and EIS in the presence of potassium hexacyanoferrate(III), the electroactive area of all types of electrodes was determined and the redox process analyzed, leading to the conclusion that ECH and EDC-NHS are better for immobilization of functionalized carbon nanotubes inside the chitosan matrix. The modified electrodes were successfully applied to the determination of hydrogen peroxide by fixed potential amperometry at -0.1 V vs SCE, the highest response being exhibited when using ECH.

Spectrophotometric determination of methyldopa and dopamine in pharmaceutical formulations using a crude extract of sweet potato root (Ipomoea batatas (L.) Lam.) as enzymatic source.

A rapid, precise and low cost spectrophotometric method is proposed for the determination of methyldopa and dopamine in pharmaceutical formulations. The crude extract of sweet potato root (Ipomoea batatas (L.) Lam.) was used as an enzymatic source of polyphenol oxidase (PPO; EC.1.14.18.1). This enzyme catalyses the oxidation of catecholamines to the corresponding methyldopaquinone and dopaminequinone. Those compounds are converted by a rapid spontaneous auto-oxidation to methyldopachrome and dopaminechrome which have a strong absorption at 480 or 470 nm, respectively. The calibration graphs are linear from 2.0x10(-4) to 6.0x10(-3) M. The results obtained by the proposed enzymatic method are in close agreement with those obtained using a Pharmacopoeia procedure and also with the label values. The detection limit (three times the signal blank/slope) was 3.4x10(-5) and 3.0x10(-5) M for methyldopa and dopamine, respectively, the recovery of methyldopa and dopamine from three samples ranged from 97.5 to 102.9% of the added amount.

Uso analítico de tecidos e de extratos brutos vegetais como fonte enzimática

This article describes the current status of several analytical methodologies using vegetal tissue and crude extracts as enzymatic source. In this divulgation paper the obtention of vegetal crude extract and/or tissue and selected enzymatic procedures are presented emphasizing its characteristics and peculiarities. Examples of many biosensors and/or flow injection procedures using vegetal tissues or crude extracts for the determination of many analytes, such as amines, ascorbic acid, ethanol, glutamate, hydrogen peroxide, oxalic acid, pectins, phenolic compounds and urea of biologic, environmental, food, pharmaceutical and industrial interests are also given and discussed.

Simultaneous voltammetric determination of synthetic colorants in food using a cathodically pretreated boron-doped diamond electrode

Differential pulse voltammetry (DPV) and a cathodically pretreated boron-doped diamond (BDD) electrode were used to simultaneously determine two pairs of synthetic food colorants commonly found mixed in food products: tartrazine (TT) and sunset yellow (SY) or brilliant blue (BB) and sunset yellow (SY). In the DPV measurements using the BDD electrode, the reduction peak potentials of TT and SY or BB and SY were separated by about 150 mV. The detection limit values obtained for the simultaneous determination of TT and SY or BB and SY were 62.7 nmol L(-1) and 13.1 nmol L(-1) or 143 nmol L(-1) and 25.6 nmol L(-1), respectively. The novel proposed voltammetric method was successfully applied in the simultaneous determination of these synthetic colorants in food products, with results similar to those obtained using a HPLC method at 95% confidence level.

Chronoamperometric determination of paracetamol using an avocado tissue (Persea americana) biosensor

A biosensor based on vaseline/graphite modified with avocado tissue (Persea americana) as the source of polyphenol oxidase was developed and used for the chronoamperometric determination of paracetamol in pharmaceutical formulations. This enzyme catalyses the oxidation of paracetamol to N-acetyl-p-benzoquinoneimine whose electrochemical reduction back to paracetamol was obtained at a potential of -0.12 V. After addition of paracetamol reference solutions in glass cell and stirring for 60 s for the accumulation of N-acetyl-p-benzoquinoneimine at the electrode surface under open-circuit conditions, the current response was monitored by 120 s without stirring. The currents obtained at 70 s were proportional to the paracetamol concentration from 1.2x10(-4) to 5.8x10(-3) mol l(-1) (r=0.9927) with a detection limit of 8.8x10(-5) mol l(-1). The recovery of paracetamol from two samples ranged from 97.9 to 100.7% and a relative standard deviation lower than 0.5% for a solution containing 5.0x10(-3) mol l(-1) paracetamol in 0.10 mol l(-1) phosphate buffer solution (pH 7.0; n=10) was obtained. The results obtained for paracetamol in pharmaceutical formulations using the proposed biosensor and those obtained using a pharmacopoeial procedure are in agreement at the 95% confidence level.