Arnaldo César Pereira

Solid-phase extraction system for Pb (II) ions enrichment based on multiwall carbon nanotubes coupled on-line to flame atomic absorption spectrometry

The present paper proposes the application of multiwall carbon nanotubes (MWCNTs) as a solid sorbent for lead preconcentration using a flow system coupled to flame atomic absorption spectrometry. The method comprises the preconcentration of Pb (II) ions at a buffered solution (pH 4.7) onto 30mg of MWCNTs previously oxidized with concentrated HNO(3). The elution step is carried out with 1.0molL(-1) HNO(3). The effect of the experimental parameters, including sample pH, sampling flow rate, buffer and eluent concentrations were investigated by means of a 2(4) full factorial design, while for the final optimization a Doehlert design was employed. Under the best experimental conditions the preconcentration system provided detection and quantification limits of 2.6 and 8.6mugL(-1), respectively. A wide linear range varying from 8.6 up to 775mugL(-1) (r>0.999) and the respective precision (relative standard deviation) of 7.7 and 1.4% for the 15 and 200mugL(-1) levels were obtained. The characteristics obtained for the performance of the flow preconcentration system were a preconcentration factor of 44.2, preconcentration efficiency of 11min(-1), consumptive index of 0.45mL and sampling frequency estimated as 14h(-1). Preconcentration studies of Pb (II) ions in the presence of the majority foreign ions tested did not show interference, attesting the good performance of MWCNTs. The accuracy of the method was assessed from analysis of water samples (tap, mineral, physiological serum and synthetic seawater) and common medicinal herbs submitted to the acid decomposition (garlic and Ginkgo Biloba). The satisfactory recovery values obtained without using analyte addition method confirms the feasibility of this method for Pb (II) ions determination in different type of samples.

Simultaneous determination of zinc, cadmium and lead in environmental water samples by potentiometric stripping analysis (PSA) using multiwalled carbon nanotube electrode

The present paper has focused on the potential application of multiwalled carbon nanotube for the development of a new, simple and highly selective electrochemical method for simultaneous Zn (II), Cd (II) and Pb (II) monitoring in water samples (lake and effluent waters), by using potentiometric stripping analysis (PSA). The electrochemical method is based on simultaneous preconcentration/reduction of metal ions onto a multiwall carbon nanotube electrode at -1.3V (versus Ag/AgCl(sat)) in 0.3 mol L(-1) acetate solution containing 15 mg L(-1) Hg (II) ions during 180s, followed by subsequent chemical stripping. The analytical curve for all analytes covered the linear range varying from 58.4 up to 646.2 microg L(-1) with correlation coefficients higher than 0.981. The limits of detection for Zn (II), Cd (II) and Pb (II) were found to be 28.0, 8.4 and 6.6 microg L(-1), while the relative standard deviation (RSD) at 352 microg L(-1) was 5.6, 7.1 and 5.6% (n=5), respectively. The behavior of the simultaneous determination in the presence of following ions Co (II), Cr (III) and Cu (II) was affected by using the analyte:interferent ratio 1:10. Therefore, by using standard addition method, Zn (II), Cd (II) and Pb (II) ions in lake and effluent water samples were determined after the spiking procedure and the results were successfully compared with those obtained by atomic absorption spectrometry (AAS). The obtained results suggest that the proposed method can be applied as a simple and efficient alternative for the simultaneous monitoring of heavy metals in water samples, according to those established requirements from environmental organizations. In addition, this method demonstrates the powerful application of carbon nanotubes in the field of potentiometric stripping analysis.

Tendências em modificação de eletrodos amperométricos para aplicações eletroanalíticas

The most relevant advances on the analytical applications of chemically modified electrodes (CME) are presented. CME have received great attention due to the possibility of electrode surface modification including chemisorption, composite generation and polymer coating. In recent years, the interest in CME has increased overall to improve the sensitivity and selectivity of the electroanalytical probes, considering the electron mediator incorporation and the new conducting polymers development. The general procedures employed for the electrode modification and the operational characteristics of some electrochemical sensors are discussed.

Molecularly imprinted pipette-tip solid phase extraction for selective determination of fluoroquinolones in human urine using HPLC-DAD.

A simple method using HPLC-DAD was developed for the determination of fluoroquinolones in human urine including ciprofloxacin (CIPRO), enrofloxacino (ENRO), marbofloxacino (MARBO) and norfloxacin (NOR). In addition, it was studied the extraction of fluoroquinolones in human urine samples using pipette tip-based molecularly imprinted polymers solid phase extraction (PT-MIPs-SPE). With the goal of finding the best procedure for extraction of four fluoroquinolones in human urine, several parameters that are likely to affect the efficiency of extraction during sample preparation, including the washing solvent, type and volume of eluent, amount of material, the volume of the sample, pH and the ionic strength were systematically optimized. Chromatographic separations of fluoroquinolones were hit within 10min using a Synergi(®) C18 (250×4.6mm, 4μm) column and mobile phase consisting of water (10mM of phosphoric acid, the pH adjusted at 3.29 with triethylamine) : acetonitrile (85.7: 14.3, v/v) at a flow rate of 1.5mLmin(-1). Detection was performed at 290nm. The average extraction recoveries/standard deviation relative to ENRO, CIPRO, NOR and MARBO were 96.40±5.51%, 42.47±4.81%, 41.82±7.99% and 87.49±4.70, respectively. The method was liner from 39 to 1260ngmL(-1) for each fluoroquinolone with correlation coefficient of 0.9904, 0.9910, 0.9914 and 0.9919, to ENRO, CIPRO, NOR and MARBO, respectively. The assays of within-day and between-day precision and accuracy for all analytes were studied at three concentration levels and were lower than 15%. The method was successfully employed in a preliminary cumulative urinary excretion study after administration of CIPRO to a healthy volunteer.

Self-assembly pipette tip-based cigarette filters for micro-solid phase extraction of ketoconazole cis-enantiomers in urine samples followed by high-performance liquid chromatography/diode array detection

A simple stereoselective HPLC/DAD method was developed for the determination of ketoconazole (KTZ) cis-enantiomers in human urine. In addition, the potential use of pipette tip-based cigarette filters for micro-solid phase extraction (PT-CFs-μ-SPE) of KTZ cis-enantiomers in human urine samples prior to HPLC/DAD was studied in this paper. In order to find a suitable procedure for extraction of KTZ cis-enantiomers in human urine, various parameters that probably affect the extraction efficiency including the washing solvent, eluent kind and volume, pH, quantity of the material, sample volume and ionic strength were systematically optimized. Optimum chromatographic separations among the KTZ cis-enantiomers have been achieved within 10 minutes using a Chiralpak® IA column (100 × 4.6 mm, 3 μm) as the stationary phase with a mobile phase consisting of 100% ethanol at a flow rate of 1 mL min−1. Detection was performed at 250 nm. The performance criteria for linearity, sensitivity, precision, accuracy, recovery, robustness and stability have been assessed and were within the recommended guidelines. The mean extraction recoveries/relative standard deviation for (+)-(2R,4S)-KTZ and (−)-(2S,4R)-KTZ were 100.74 ± 9.52% and 106.90 ± 9.26%, respectively. The method showed to be linear over the concentration range of 12.5 to 400 ng mL−1 for each enantiomer with the correlation coefficients of 0.9946 and 0.9910, for (+)-(2R,4S)-KTZ and (−)-(2S,4R)-KTZ, respectively. Within-day and between-day precision and accuracy assays for these analytes were studied at three concentration levels and were lower than 15%. The method was successfully employed in a preliminary cumulative urinary excretion study after administration of racemic cis-KTZ to a healthy volunteer.

Otimização da preparação de eletrodo de pasta de carbono contendo riboflavina imobilizada em suporte inorgânico

The aim of this work was to optimize the preparation of electrodes with riboflavin (RF) immobilized on a silica surface modified with niobium oxide and carbon paste. Electrode preparation was optimized employing a factorial design consisting of two levels and three factors. The electrochemical properties of immobilized RF were investigated by cyclic voltammetry. The factorial analysis was carried out analysing the current intensity (Ipa). It was possible to optimize the electrode to get the best reversibility in the redox process, i. e. the lowest separation between anodic and cathodic peak potentials and a current ratio close to unity. The concentration of supporting electrolyte has a small effect. The proportion has the highest effect and the interaction factor between proportion and mixture has also a significant effect on the current intensity.

Development of pipette tip-based on molecularly imprinted polymer micro-solid phase extraction for selective enantioselective determination of (−)-(2S,4R) and (+)-(2R,4S) ketoconazole in human urine samples prior to HPLC-DAD

In this work a simple and selective sample preparation technique employing pipette tip-based on molecularly imprinted polymer micro-solid phase extraction (PT-MIP-μ-SPE) coupled to HPLC/DAD was developed for the determination of the cis-enantiomers of ketoconazole (KTZ) in human urine samples. Several parameters affecting the proposed PT-MIP-μ-SPE technique were investigated, namely the washing solvent, type and volume of the eluent, amount of material (molecularly imprinted polymer, MIP), sample volume, pH and salt addition. The recoveries were around 100% using 5 μg mL−1 for the (−)-(2S,4R) and (+)-(2R,4S) KTZ enantiomers. Optimum chromatographic conditions for determination of the (−)-(2S,4R) and (+)-(2R,4S)-KTZ enantiomers were found using the chiral stationary phase polysaccharide derived of amylose tris(3,5-dimethylphenylcarbamate). The mobile phase consisted of ethanol:water (95:5, v/v) at a flow rate of 1 mL min−1 and the detection was performed at 250 nm. The performance criteria for linearity, sensitivity, precision, accuracy, recovery, robustness and stability were assessed and they were within the recommended guidelines. The method showed to be linear over the concentration range from 6.25 to 650.00 ng mL−1 for each enantiomer, with the correlation coefficients of 0.9962 and 0.9952 for (+)-(2R,4S)-KTZ and (−)-(2S,4R)-KTZ, respectively. Within-day and between-day precision and accuracy assays for these analytes were studied at three concentration levels and were lower than 15%. The results for the application of the method in preliminary urinary excretion showed that there are not significant differences in the analyzed concentrations of KTZ cis-enantiomers in urine samples.

o-Phenylenediamine adsorbed onto silica gel modified with niobium oxide for electrocatalytic NADH oxidation

Abstract A study of a modified carbon paste electrode employing o -phenylenediamine (PDA) adsorbed onto silica gel modified with niobium oxide (SN) for electrocatalytic oxidation of nicotinamide adenine dinucleotide (NADH) is described. The species adsorbed on SN was used to prepare a modified carbon paste electrode to investigate its electrochemical properties. The formal potential ( E 0′ ) of the adsorbed PDA was −140 mV vs. SCE (saturated calomel electrode). The electrochemical behavior of the adsorbed PDA, compared to that of PDA dissolved in aqueous solution, was completely different. In solution, pH between 3.0 and 8.0, E 0′ remained almost constant and the response was very stable. A linear response range for NADH between 4.0×10 −5 and 8.0×10 −4 mol l −1 , at pH 7.0, was observed for the electrode, with an applied potential of −50 mV vs. SCE. The formation of an intermediate charge transfer complex is proposed for the charge transfer reaction between NADH and adsorbed PDA. The heterogeneous electron transfer rate, k obs , was 5480 mol −1 l s −1 and the apparent Michaelis–Menten constant, 1.04×10 −4 mol l −1 at pH 7.0, evaluated with rotating disk electrode (RDE) experiments with an electrode with a coverage of PDA of 5.7×10 −9 mol cm −2 . The slight increase in the reaction rate with the solution pH was assigned to the thermodynamic driving force.

A highly improved method for sensitive determination of amitriptyline in pharmaceutical formulations using an unmodified carbon nanotube electrode in the presence of sulfuric acid

The present paper describes a novel, simple and reliable differential pulse voltammetric method for determining amitriptyline (AMT) in pharmaceutical formulations. It has been described for many authors that this antidepressant is electrochemically inactive at carbon electrodes. However, the procedure proposed herein consisted in electrochemically oxidizing AMT at an unmodified carbon nanotube paste electrode in the presence of 0.1 mol L(-1) sulfuric acid used as electrolyte. At such concentration, the acid facilitated the AMT electroxidation through one-electron transfer at 1.33 V vs. Ag/AgCl, as observed by the augmentation of peak current. Concerning optimized conditions (modulation time 5 ms, scan rate 90 mV s(-1), and pulse amplitude 120 mV) a linear calibration curve was constructed in the range of 0.0-30.0 μmol L(-1), with a correlation coefficient of 0.9991 and a limit of detection of 1.61 μmol L(-1). The procedure was successfully validated for intra- and inter-day precision and accuracy. Moreover, its feasibility was assessed through analysis of commercial pharmaceutical formulations and it has been compared to the UV-vis spectrophotometric method used as standard analytical technique recommended by the Brazilian Pharmacopoeia.

A nanocomposite based on multi-walled carbon nanotubes grafted by molecularly imprinted poly(methacrylic acid–hemin) as a peroxidase-like catalyst for biomimetic sensing of acetaminophen

In the present study the synthesis of a nanocomposite based on multi-walled carbon nanotubes grafted by poly(methacrylic acid–hemin) is described. The presence of functional groups in the nanocomposite was evaluated by FT-IR, while morphological, textural and thermal stability data were evaluated by means of SEM, TEM, nitrogen adsorption–desorption assays and TGA, respectively. The material was evaluated as a biomimetic and catalytic sensor toward the electrochemical determination of acetaminophen (AC) by depositing it onto a glassy carbon electrode surface. Acetaminophen has been widely used as an analgesic and antipyretics drug and for patients with a high sensitivity to aspirin; therefore the development of analytical methods for AC determination in pharmaceutical formulations is of paramount importance for quality control. The biomimetic electrochemical sensor operation is similar to that of the biosensor based on horseradish peroxidase, where the hemin in the presence of H2O2 catalyses the oxidation of acetaminophen into N-acetyl-p-benzoquinoneimine, which in turn is reduced back to AC on the electrode at a potential of −0.27 V. The variables that exert influence on the performance of the biomimetic response, including H2O2 concentration (300 μmol L−1), pH (8.0) and concentration (0.1 mol L−1) and type of buffer solution (Trizma) were investigated. Under optimized conditions the electrochemical determination of AC using multi-walled carbon nanotubes grafted by poly(methacrylic acid–hemin) immobilized on the surface of the glassy carbon electrode was carried out by square wave voltammetry (SWV), showing a limit of detection of 1.1 μmol L−1. The selectivity of material, ascribed to imprinted effect, was assured by comparison to a non-imprinted material (NIP). The proposed method was applied to AC determination in a pharmaceutical formulation, whose obtained values were very similar to those declared and statistically equal to the HPLC method, thus illustrating the feasibility of method for analysis of real samples.