Characterization of Nonlinear Background Components in Voltammetry by Use of Large Amplitude Periodic Perturbations and Fourier Transform Analysis Alan M. Bond,* ,† Noel W. Duffy, † Darrell M. Elton, ‡ and Barry D. Fleming † School of Chemistry, Monash University, Clayton, Victoria 3800, Australia, and Department of Electronic Engineering, Latrobe University, Bundoora, Victoria 3083, Australia Under most experimental conditions, a distinctly nonlin- ear background current is encountered in all forms of voltammetry which arises from the potential dependence of the capacitance. The nonlinear background current has been successfully modeled under large amplitude sinu- soidal ac voltammetric conditions with a fourth order polynomial. The model was applied to a dummy cell containing a nonideal ceramic capacitor and commonly used electrodes. The nonlinearity in behavior of the background capacitance is particularly significant when considering the discrimination between the Faradaic and background contributions in the higher order harmonics resolved in ac voltammetry by Fourier transform-inverse Fourier transform approaches and in the simulation of the background current and hence double-layer capacitance as a function of potential. Typically, measurable back- ground current under large amplitude conditions is detectable in the dc and fundamental to fourth harmonic components in large amplitude ac voltammetry. For analytical purposes, this background current can be corrected on a per harmonic basis without the need for any model. Background correction has been successfully applied to the first four harmonics for the oxidation of ferrocenemonocarboxylic acid over the concentration range of 5-500 μM in aqueous 0.5 M NaCl solution. In instrumentally based analytical techniques, an understanding of the background processes onto which the signal of interest is superimposed is of paramount importance. In dynamic electro- chemical methods such as voltammetry, this background signal can originate from non-Faradaic (e.g., double-layer capacitance, C dl ) and unwanted Faradaic (e.g., oxygen reduction, solvent/ electrolyte reduction or oxidation) processes which may arise at working electrode-solution (electrolyte) interfaces. The importance of understanding the response of both the analyte and background signals can be emphasized by noticing that the limit of analyte detection is intimately related to the ratio of the signal that needs to be measured (depends on the analyte concentration) to that of the background signal (usually independent of analyte concentration). As a consequence, instrumentally based advances in analytical performance are commonly achieved by implementation of strategies that enhance the “signal-to-noise” ratio, which can be achieved by amplifying the signal of interest and/or suppressing the background signal. 1,2 In the case of voltammetric techniques that are of interest in this paper, commonly the analyte (A) that needs to be detected is oxidized to species B or reduced to C at the working electrode as in eq 1 (oxidation example given). A h B + e - (1) * To whom correspondence should be addressed. E-mail: Alan.Bond@ sci.monash.edu.au. † Monash University. ‡ Latrobe University. (1) Bond, A. M. Modern Polargraphic Methods in Analytical Chemistry; Marcel Dekker: New York, 1980. (2) Smith, D. E. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1966; Vol. 1, pp 1-155. Figure 1. Simulated dc cyclic voltammograms showing the Faradaic current, I F (a, blue) resulting from eq 1 when both A and B are soluble in the solvent (electrolyte) of interest and the rate of electron transfer is fast (reversible); the background current, I B (b, red) resulting from the electrolyte solution; and the sum of I F and I B (c, black). Simulation conditions: [A] ) 0.1 mM, [B] ) 0, temperature T ) 25 °C, ν ) 100 mV s -1 , diffusion coefficient D A ) D B ) 3 × 10 -6 cm 2 s -1 , reversible formal potential E 0 ) 0.3 V, A ) 0.007854 cm 2 , R u ) 10 Ω and C dl ) 0.5 μF. Anal. Chem. 2009, 81, 8801–8808 10.1021/ac901318r CCC: $40.75  2009 American Chemical Society 8801 Analytical Chemistry, Vol. 81, No. 21, November 1, 2009 Published on Web 10/06/2009