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Evaluation of the Detection and Quantification Limits in Electroanalysis Using Two Popular Methods: Application in the Case Study of Paraquat Determination

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Evaluation of the Detection and Quantification Limits in Electroanalysis Using Two Popular Methods: Application in the Case Study of Paraquat Determination View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Biblioteca Digital da Produção Intelectual da Universidade de São Paulo (BDPI/USP) Universidade de São Paulo Biblioteca Digital da Produção Intelectual - BDPI Departamento de Físico-Química - IQSC/SQF Artigos e Materiais de Revistas Científicas - IQSC/SQF 2012 Evaluation of the detection and quantification limits in electroanalysis using two popular methods: application in the case study of paraquat determination ANALYTICAL METHODS, CAMBRIDGE, v. 4, n. 8, supl. 2, Part 3, pp. 2348-2354, DEC, 2012 http://www.producao.usp.br/handle/BDPI/41955 Downloaded from: Biblioteca Digital da Produção Intelectual - BDPI, Universidade de São Paulo View Article Online / Journal Homepage / Table of Contents for this issue Analytical Dynamic Article LinksC< Methods Cite this: Anal. Methods, 2012, 4, 2348 www.rsc.org/methods PAPER Evaluation of the detection and quantification limits in electroanalysis using two popular methods: application in the case study of paraquat determination Osmair Benedito da Silva and Sergio A. S. Machado* Received 31st January 2012, Accepted 5th May 2012 DOI: 10.1039/c2ay25111f In this work, the reduction reaction of paraquat herbicide was used to obtain analytical signals using electrochemical techniques of differential pulse voltammetry, square wave voltammetry and multiple square wave voltammetry. Analytes were prepared with laboratory purified water and natural water samples (from Mogi-Guac¸u River, SP). The electrochemical techniques were applied to 1.0 mol LÀ1 Na2SO4 solutions, at pH 5.5, and containing different concentrations of paraquat, in the range of 1 to 10 mmol LÀ1, using a gold ultramicroelectrode. 5 replicate experiments were conducted and in each the mean value for peak currents obtained À0.70 V vs. Ag/AgCl yielded excellent linear relationships with pesticide concentrations. The slope values for the calibration plots (method sensitivity) were 4.06 Â 10À3, 1.07 Â 10À2 and 2.95 Â 10À2 A molÀ1 L for purified water by differential pulse voltammetry, square wave voltammetry and multiple square wave voltammetry, respectively. For river water samples, the slope values were 2.60 Â 10À3, 1.06 Â 10À2 and 3.35 Â 10À2 A molÀ1 L, respectively, showing a small interference from the natural matrix components in paraquat determinations. The detection limits for paraquat determinations were calculated by two distinct methodologies, i.e.,as proposed by IUPAC and a statistical method. The values obtained with multiple square waves voltammetry were 0.002 and 0.12 mmol LÀ1, respectively, for pure water electrolytes. The detection limit from IUPAC recommendations, when inserted in the calibration curve equation, an analytical signal (oxidation current) is smaller than the one experimentally observed for the blank solution under the same experimental conditions. This is inconsistent with the definition of detection limit, thus the IUPAC methodology requires further discussion. The same conclusion can be drawn by the analyses of Downloaded by UNIVERSIDAD SAO PAULO on 10/04/2013 19:25:37. Published on 09 May 2012 http://pubs.rsc.org | doi:10.1039/C2AY25111F detection limits obtained with the other techniques studied. A. Introduction different ways in the literature, but is defined as an analyte concentration that yields a signal in the measurement instrument The growing rate of discarded pesticides and drugs is associated (y) which is significantly different from that obtained for the with environmental contamination and subsequent risks to blank.1 Every discussion about methodologies employed to human health. Considering that most of these substances are calculate the detection limit of a given analytical technique synthetic, tiny amounts can result in effects not yet fully under- emerges from the definition of ‘‘significantly different’’. In this stood to health and reproduction in humans and livestock. Thus, way, the International Union of Pure and Applied Chemistry detection and quantification of trace and ultratrace complex (IUPAC) proposes that a good evaluation is given through organic molecules is a constant challenge in analytical chemistry. considering a linear calibration curve, normal data distribution, Pico to nano moles per litre amounts of e.g. pesticides, and constant variance in every measurement2, i.e.: hormones, drugs, should be determined with precision and reli- ability. Modern analytical techniques provide the possibility to LOD ¼ 3SB/S (1) run such determinations, frequently in situ, and in many cases in real time allowing remediation actions to be taken. where SB is the standard deviation of 10 measurements taken In order to present reliable results on the demanded concen- from the signal obtained from the blank (a solution identical to tration range, analytical techniques should feature, as one of the that analysed but without the analyte) and S the slope of the main characteristics, a low detection limit, together with repro- calibration curve (sensitivity of the analytical method). The ducibility and robustness. This concept has been defined in number 3 comes from the required 90% level of confidence in the difference between the observed signal and the blank response. Such methodology is popular when applied to analytical tech- Instituto de Quımica de Sa˜o Carlos, Universidade, de Sa˜o Paulo, C.P. 780, 13560-970 Sa˜o Carlos, SP, Brazil niques, e.g. electrochemical and spectrophotometry, mainly due 2348 | Anal. Methods, 2012, 4, 2348–2354 This journal is ª The Royal Society of Chemistry 2012 View Article Online to the simplicity of the statistical procedure. Consequently, it is found that a very large number of papers in the literature employ such a methodology.3–7 In Brazil, the Government regulatory agencies adopted a similar concept of detection limit. In this way, the Instituto Nacional de Metrologia, Normalizac¸a˜o e Qualidade Industrial published in March 2003 a guidance only document, where this concept is defined, in a similar fashion to the IUPAC definition.8 On the other hand, the Agencia^ Nacional de Vigi- lancia^ Sanitaria (ANVISA) postulated that the detection limit should be calculated from: LOD ¼ 3A/S (2) where A is the standard deviation of the intercept of the linear Fig. 2 Stationary state cyclic voltammogram on UME-Au in a 4.2 Â À3 À1 À1 À1 calibration curve of, at least, three independent curves. Of 10 mol L paraquat + 0.1 mol L Na2SO4 at 0.1 V s . course, eqn (2) is similar to eqn (1), only with a different meth- odology to calculate S . However, a conceptual criticism of such B This work aims to determine the detection limits of the an approach, although it is widely applied being established by herbicide paraquat in laboratory matrices and in water samples usage methodology, concerns the use of standard deviation of the from the Mogi-Guac¸u River, Brazil, using three different elec- blank signal as the determining parameter in the calculation of troanalytical techniques: differential pulse voltammetry, square detection limit. As observed in eqn (1), the smaller the value of S B wave voltammetry and multiple square wave voltammetry. The the smaller the detection limit of the methodology. But S is B results are compared to those obtained by IUPAC methodology defined as:1 using a far less popular statistical methodology. sPffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðx À xÞ2 S ¼ i i (3) B ðn À 1Þ B. Experimental where xi is the analytical signal value and n is the number of Reagents and equipment repetitions done. As can be observed if n increases, SB diminishes and, thus the detection limit also gets lower. Consequently the n All electrochemical experiments were performed at room value should be carefully standardized, which does not seems to temperature, using a 25 mL volume electrochemical cell made of be the case. Several papers that use eqn (1) report values of n borosilicate glass with a polytetrafluorethylene cover. In this between 3 and 20. In this way, the reliability of detection limit cover there were special holes to accommodate the entrance and determination is bonded to the measurement of the smallest exit of N2 gas (White Martins, SS, bubbled in the solution 15 analytical signal (the blank one), with an unavoidable loss of minutes prior to the experiment with the flow kept over the Downloaded by UNIVERSIDAD SAO PAULO on 10/04/2013 19:25:37. surface during the measurements) and the reference (Ag/AgCl, Published on 09 May 2012 http://pubs.rsc.org | doi:10.1039/C2AY25111F precision. À1 2 Another far from logical consequence in the utilization of such 3.0 mol L KCl), auxiliar (Pt foil with 2 cm geometric area) and methodology is concerned with the fact that, as the standard working electrodes (a home-made Au ultramicroelectrode – deviation of the blank solution is normally very low, the UME-Au – obtained by embedding a Goodfellow 99.95% Au m numerical values of detection limit obtained are so small that wire with 25 m diameter in epoxy resin). The electrochemical they are not detectable in the calibration curve.9–11 In this way, experiments of square wave voltammetry (SWV), multiple the physical meaning of such quantitites is not straightforward. square wave voltammetry (MSWV) and differential pulse vol- Paraquat is used worldwide as a non-selective herbicide and is tammetry (DPV) were performed in a model PGZ 402 Voltalab sold under commercial names of Gramoxone, Weedol or Pan- potentiostat/galvanostat controlled by Voltamaster 4.05 soft- thclear.12 Its molecular structure is presented in Fig. 1. Paraquat ware from Radiometer Analytical. During the voltammetric has been determined by several electrochemical techniques13–19 experiments the electrochemical cell was kept in a Faraday cage due to its reversible electrochemical response, its good solubility to minimize background noise. À3 À1 in water and the importance of trace determination of this A 1.0 Â 10 mol L stock solution of paraquat (Aldrich, pesticide in foodstuff, since it is extremely hazardous to human 98%) was prepared with ultrapurified Milli-Q (Millipore Inc.) health.
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