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Article an Alternative Method for the Simultaneous Determination of Copper and Lead for Quality Control of Sugar Cane Spirit Using a Nanotube-Based Sensor

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Article an Alternative Method for the Simultaneous Determination of Copper and Lead for Quality Control of Sugar Cane Spirit Using a Nanotube-Based Sensor J. Braz. Chem. Soc., Vol. 23, No. 9, 1614-1622, 2012. Printed in Brazil - ©2012 Sociedade Brasileira de Química A 0103 - 5053 $6.00+0.00 Article An Alternative Method for the Simultaneous Determination of Copper and Lead for Quality Control of Sugar Cane Spirit using a Nanotube-Based Sensor Elisângela de F. L. Tavares,a Leonardo L. Okumura,*,b Maria das Graças Cardoso,a Marcelo F. de Oliveirac, Zuy Maria Magriotisa and Adelir A. Saczk*,a aUniversidade Federal de Lavras, 37200-000 Lavras-MG, Brazil bUniversidade Federal de Viçosa, 36570-000 Viçosa-MG, Brazil cDepartamento de Química, Faculdade de Filosofia Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, 14040-901 Ribeirão Preto, Brazil Um método eletroanalítico foi desenvolvido para determinação simultânea de íons cobre e chumbo em cachaça empregando eletrodo de pasta de carbono modificado com ácido ascórbico e nanotubos de carbono (CPE-AaCNT). A técnica de voltametria de onda quadrada (SWV) com redissolução anódica foi empregada e otimizada com relação aos seguintes parâmetros: frequência (50 Hz), amplitude (100 mV) e incremento de varredura (9 mV). As curvas analíticas apresentaram intervalo linear de 0,09 a 7,00 mg L–1 para o chumbo e para o cobre. Os limites de detecção foram de 48,5 e 23,9 µg L–1 para o chumbo e o cobre, respectivamente. O método desenvolvido foi aplicado na determinação simultânea de chumbo e cobre em cinco amostras comerciais de cachaça. Os resultados apresentaram-se concordantes com os obtidos por F AAS/GF AAS (espectrometria de absorção atômica com chama/espectrometria de absorção atômica com forno de grafite) e revelaram que CPE-AaCNT podem ser empregados com êxito na quantificação de cobre e chumbo em amostras comerciais de cachaça. The development of an electroanalytical method for simultaneous determination of copper and lead ions in sugar cane spirit (cachaça) using carbon paste electrode modified with ascorbic acid and carbon nanotubes (CPE-AaCNT) is described. Squarewave voltammetry (SWV) with anodic stripping was employed, and this technique was optimized with respect to the following parameters: frequency (50 Hz), amplitude (100 mV) and scan increment (9 mV). The analytical curves were linear in the range from 0.0900 to 7.00 mg L–1 for lead and copper. The limits of detection were 48.5 and 23.9 µg L–1 for lead and copper, respectively. The developed method was applied to the simultaneous determination of copper and lead in five commercial samples of sugar cane spirit. The results were in good agreement with those obtained by F AAS/GF AAS (flame atomic absorption spectrometry/graphite furnace atomic absorption spectrometry) and showed that CPE-AaCNT can be successfully employed in the simultaneous determination of these metals in real sugar cane spirit samples. Keywords: copper, lead, carbon paste electrode, multi-walled carbon nanotubes, ascorbic acid, squarewave voltammetry, sugar cane spirit Introduction (Brazil) is the largest producer of sugar cane spirit, representing 44% of the national production. Because The sugar cane spirit (usually known as cachaça in of the high consumption of this beverage, and also for Brazil) is the third most consumed distilled beverage marketing and safety purposes, toxicological aspects and in the world, and in Brazil, occupies the first place. Its evaluation of the levels of accidental or incidental additives consumption was estimated at 1.2 billion L per year in in sugar cane spirit should be taken into account. Overall, 2008.1 According to Sebrae,2 the Minas Gerais State the demand for chemical analysis of beverages worldwide has been increasing in recent years mainly due to the *e-mail: [email protected], [email protected] need for improving the quality standards and industrial Vol. 23, No. 9, 2012 Tavares et al. 1615 control. Additionally, when it comes to the quality control was employed for analysis of five real commercial sugar of sugar cane spirit, it is important to quantify metals for cane spirit samples collected in Lavras City, Minas Gerais various purposes. The most important aspect is to check State, Brazil. the presence of heavy metal species (Cu, Pb, Zn, Cd, As, Al, etc.) in toxic levels and to attend to the specifications Experimental required by both Brazilian and foreign laws. The Brazilian Ministry of Agriculture, Livestock and Reagents Food Supply (MAPA) established many quality criteria for this product (Normative Instruction No. 13, Official Union All the reagents and chemicals used in this work were Diary, Jun/29/2005). Among these important criteria, the of analytical grade. Aqueous solutions were prepared with ones related to maximum copper and lead concentrations are water purified by a Milli-Q system. A solution of 1 mol –1L of major concern since concentrations higher than 5 mg L–1 HCl was prepared as supporting electrolyte for copper and for copper and 200 mg L–1 for lead are not allowed.3 lead determination. Cu+2 and Pb+2 stock solutions were Copper is an essential element in human beings. initially prepared at a concentration of 1000 mg L–1 using However, the consumption of large quantities of this copper nitrate (> 95% purity, Synth, Brazil) and lead nitrate metal can be toxic. Excess copper in the human body can (> 95% purity, Vetec, Brazil), which was followed by lead to diseases such as Wilson, Menkes, MS, Alzheimer, dissolution in ultra-pure water solution or 50% ethanol PA epilepsy, melanoma and rheumatoid arthritis.4-8 Excess (Qhemis, Brazil). The stock solutions containing Cu+2 and lead intake also produces harmful effects on the human Pb+2 standards were used for further dilutions, as necessary. health since several organ systems are affected and nervous, The commercial sugar cane spirit samples were acquired gastrointestinal, reproductive, skeletal and biochemical from different local stores in Lavras (Minas Gerais State, activities are impaired.9,10 Brazil), in a partnership with the laboratory of analysis of Analysis of copper and lead in the sugar cane spirits LAFQA at Universidade Federal de Lavras (Minas spirit is routinely carried out using flame atomic Gerais State, Brazil). Spectroscopic carbon powder from absorption spectrometry (F AAS),11-16 inductively coupled Merck (> 99% purity, Germany) was used in the preparation plasma mass spectrometry (ICP-MS)17 and UV-Vis of the modified electrodes. Mineral oil from Aldrich (> 99% spectrophotometry.18,19 The electroanalytical techniques can purity, USA) was utilized as the binding agent, and ascorbic be used as an alternative method in analytical chemistry and acid also from Aldrich (> 95% purity, USA) was employed offer the additional benefit of being compatible with the as an additional modifier. MWCNT was purchased in principles of green chemistry. research grade from Nanolab, USA, with > 95% purity, Both copper and lead exhibit good redox activities 10-20 nm diameter, 5-20 µm length, as prepared by the at some electrode surfaces. Over the last decade, more chemical vapor deposition (CVD) technique. attention has been given to their inherent electrochemical activity on mercury electrodes.20 However, the use of Apparatus mercury-based electrodes is now considered incompatible with environmental objectives. The modification of electrode Squarewave voltammetry (SWV) with anodic stripping substrates with carbon nanotubes (CNTs) and several types analysis was performed on an mAutolab type III of nanoparticles has been recently documented to result potentiostat/galvanostat (ECO-Chemie, The Netherlands) in low limits of detection, high sensitivity, reduction of coupled to a microcomputer and controlled by a GPES 4.9 overpotentials, resistance to surface fouling and enhanced software. A one-compartment electrochemical cell electrocatalytic properties.21 Despite all these advantages, containing a platinum wire auxiliary electrode and an the CPE-AaCNT electrode has not been evaluated in terms Ag/AgClsat reference electrode was used for all the of electroanalytical use and electrocatalytic effects. To the voltammetric measurements. CPE-AaCNT was employed best of our knowledge, the anodic stripping voltammetric as the working electrode. The compositions of the paste determination of copper and lead using a CPE-AaCNT from chemically modified electrodes were also analyzed electrode has not been reported in the literature yet. So, the by SEM (scanning electron microscopy) to study their aim of the present work was to develop a convenient and morphology and surface area. To obtain the micrographs, sensitive method for the determination of copper and lead the samples were added on aluminum supports (stubs) with based on the unusual, and not fully explored, properties a ribbon on a double-sided film of an aluminum foil and of multi-walled carbon nanotubes (MWCNT) using observed under a scanning electron microscope Leo Evo ascorbic acid (Aa) as a modifier. This modified electrode 40 XVP using a voltage of 25 kV. 1616 An Alternative Method for the Simultaneous Determination of Copper and Lead J. Braz. Chem. Soc. Electrode preparation Recovery experiments Carbon paste electrodes (CPEs) were prepared by The standard addition method was used for investigation substituting the corresponding amounts of the carbon of the developed methodology in terms of accuracy powder with carbon nanotubes and/or ascorbic acid in for sugar cane spirit samples and matrix effect. For order to obtain the desired composition. The carbon this purpose, several samples were analyzed, and the powder/modifier mixture was homogenized in a mortar by squarewave voltammogram signal was recorded. After using a pestle. Subsequently, the mixture was added to the that, four additions of equal amounts of Cu2+ and Pb2+ were mineral oil, whose percentage was always kept at 15-22% accomplished up to a final metal concentration of 10 mg L–1. (m/m), depending on the electrode. The new mixture was Extrapolation of the resulting linear regression plot gave homogenized again, and the resulting carbon pastes were the amount of Cu2+ and Pb2+ that was recovered from the packed in a cylindrical plastic tube (internal diameter samples.
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