In order to provide an initial validation of the SECM methodology, approach curves were obtained in an O2-free solution containing 1.0 mM Ru(NH3)6 3+ and 0.1 M KCl. The potential of the C tip and Pt substrate electrodes were set at -0.4 and 0.4 V vs Ag/AgCl, respectively, to reduce Ru(NH3)6 3+ and oxidize Ru(NH3)6 2+ at their diffusion-controlled rates. In SECM analysis, approach curves showing the dependencies of the tip and substrate currents (it and is) on the distance between the tip and substrate, d, are generally plotted in terms of a normalized distance, L = d/a, where a is the tip radius. Figure shows typical it-L and is-L curves (black) recorded in the absence of S2O8 2-. These curves correspond to the classical positive feedback mechanism between the two electrodes, as depicted in Scheme 2A. At large values of L, it approaches the value of the diffusion-limited current observed in SSV (i.e., ~ -1 nA, Figure ), while is falls to zero.