The substrate generation/tip collection (SG/TC) mode ofscanning electrochemical microscopy (SECM) coupledwith linear voltammetry is proposed as a way to quantifyreaction intermediates generated in the solution at smallsubstrates (100
m diameter). The collection efficiency(CE) for SG/TC mode depends on the collector tip radius(
a), the tip/substrate distance (
d), and the size of theinsulating glass sheath surrounding the collector tip (RG).In this work, we present experimental and simulatedcalibration CE values for different SG/TC geometries.Results of digital simulations in axial 2-D symmetry withthe tip approaching a planar substrate are shown and fitexperimental results obtained using ferrocenemethanolas a redox mediator very well. This model assumes thatthe mediator reacts under stationary-state conditions andundergoes diffusion-controlled electron transfer withoutany heterogeneous or homogeneous kinetic complications. Empirical equations for all SG/TC geometriesreported here are provided as a convenient way to predictthe maximum CE value for any given distance within thecalibration range. Hydrogen peroxide quantification during the oxygen reduction reaction (ORR) at a Hg on Auelectrode in acid pH was carried out using the SG/TCmode of SECM to demonstrate the utility of this techniquein determining the number of electrons transferred (
n)in the ORR. The results (
n = 2.12-2.19) clearly pointout the predominance of the two-electron pathway overthe four-electron pathway when ORR takes place at thiselectrode material. Therefore, this work presents a powerful alternative to the rotating ring-disk electrode (RRDE)as means of obtaining mechanistic information by calculating the number of electrons transferred during anelectrochemical reaction.