Talanta 69 (2006) 1200–1207 Multiple square wave voltammetry for analytical determination of paraquat in natural water, food, and beverages using microelectrodes Djenaine De Souza a,∗ , Sergio A.S. Machado a , Roberto C. Pires b a Instituto de Qu´ ımica de S ˜ ao Carlos, Universidade de S ˜ ao Paulo, CP 780, 13560-970 S˜ ao Carlos, SP, Brasil b Escola de Engenharia de S ˜ ao Carlos, Universidade de S ˜ ao Paulo, Cx. P. 359, 13566-590, S˜ ao Carlos, SP, Brasil Received 13 September 2005; received in revised form 14 December 2005; accepted 16 December 2005 Available online 7 February 2006 Abstract This paper reports on the use of multiple square wave voltammetry (MSWV) for analytical determination of paraquat herbicide at gold micro- electrode (Au-ME) in different samples of natural water, food, and beverages. In this work, the MSWV consisted in a sequence of four pairs of potential pulse in the same step and the interval potential evaluated was of the 0.0V at -1.2 V versus Ag/AgCl 3.0 mol L -1 . The paraquat herbicide presented two reduction peaks, in -0.69 V and -0.99 V, with profile of the redox process totally reversible, and the use of multiple pulses allowed a detection of nanomolar levels after the optimization of experimental and voltammetric conditions. Analytical curves were constructed for pulse potential frequency of 250 s -1 , pulse amplitude of 50 mV, scan increment of 2 mV and pulse number of 8 pulses in a same step. The two reduction peaks showed that the peak currents were found to be directly proportional to the pesticide concentration in the range comprised between 5.0 × 10 -7 mol L -1 and 1.04 × 10 -5 mol L -1 . With this, it was possible to determine detection limits (DL), which resulted in 0.044 gL -1 (0.044 ppb) and 0.146 gL -1 (0.146 ppb), respectively, for peak 1 and peak 2. DL results, obtained using MSWV, were 2–3 orders of magnitude lower (10 -2 to 10 -3 ) less than those observed for traditional square wave voltammetry or published in literature, clearly pointing to the advantages arising from the possibility of using a MSWV for analytical purposes in contaminated matrices. In addition, the proposed methodology was applied in different samples of natural water, food and beverages without pre-treatment or pre-concentration step, where a recovery measurement indicated that the methodology could be employed to analyze paraquat in such matrices. © 2006 Elsevier B.V. All rights reserved. Keywords: Multiple square wave voltammetry; Microelectrodes; Pesticides 1. Introduction The development of electronic components has promoted recent innovations in the electroanalytical instrumentation, gen- erating new possibilities for the use of some techniques in order to increase the signal-to-noise ratio and to lower detectable concentration of the organic and inorganic species in different samples, using different electrodic surfaces. Among the electro- analytical techniques, the “classical” pulse voltammetry, such as the differential pulse or square wave voltammetry, has been successfully employed for analytical [1–5] and mechanistic pur- poses [6,7] in the last few years. In order to obtain the lower detection limits at nanomolar levels, the use of an extremely sensitive electroanalytical tech- ∗ Corresponding author. Fax: +55 16 3373 9952. E-mail address: djenaine@iqsc.usp.br (D.D. Souza). nique named multiple square wave voltammetry (MSWV) can be employed. The MSWV is a multipulse technique whose per- turbation mode is similar to the square wave voltammetry (SWV) [6,8–10], but with the difference that onto each step of the stair- case, it can be applied more than one pair of potential pulses of opposing sign [11–13]. The voltammograms present profile similar to those obtained for SWV and the response is produced in a few seconds, in which the sampling or integration of the currents is carried out on each step of the staircase, from the dif- ference between the sum of the response which was measured during the forward pulse, and the sum of the response which was measured during the reverse pulse. This mode of multiple pulse potential application is suit- able to substitute SWV in the electroanalytical analysis of inorganic and metallorganic compounds since its use allows for the improvement of the analytical sensitivity to about 2–3 orders of magnitude and it has been successfully applied for trace anal- ysis even in complex samples [13–17]. 0039-9140/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2005.12.045