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There are several modes of operation of the SECM. In the tip generation-substrate collection (TG/SC) mode, the tip is used to generate a reactant that is detected at a substrate electrode. For example, the reaction O + ne —> R occurs at the tip, and the reverse reaction occurs at the substrate. This mode of operation is similar to that at the rotating ring-disk electrode (7). Similar behavior is observed for a pair of side-by-side microband electrodes (8,9) and in thin-layer cells (10). In the SECM, TG/SC is usually used in studies of homogeneous chemical reactions, where the reaction of species R as it transits between tip and substrate causes a decrease in the substrate current (see Chapter 7). An alternative mode, where the substrate is the generator and tip the collector (SG/TC mode), can also be employed and is used in studies of reactions at a substrate surface (Chapters 6, 9, 11, and 12). The SG/TC mode was first used to study concentration profiles near an electrode surface without scanning and imaging (11-13).