scans 687–690 from the signal of scan 693. (D) Background-subtracted voltammogram of a standard 25 µM DA solution collected using the flow system described in the text. The same buffer medium is used for both cellular experiments and for standards. (Reproduced from Anal. Chem. with permission [13].)

The model allows determination of relative concentrations from individual release events and has been used to examine events at control cells and cells incubated with the dopamine precursor, L-3,4-dihydroxyphenylalanine (L-DOPA) [13]. Exposure to LDOPA (100 µM for 1 hr) results in 145 detectable exocytosis events for 11 cells compared to 77 events for 29 control cells, again suggesting that the released substance is indeed dopamine and that vesicles can be “loaded” with the catecholamine. Additionally, each event has a larger half-width (t1/2) in the release of vesicular contents when compared to cells that are not incubated with L-DOPA. This data is significant kinetically, as it suggests a longer release time for the increased quantity of dopamine. The data are summarized in Figure 8. As described previously, the non-faradaic background current in Figure 8A and Figure 8B is stable, whereas the individual current transients rise very sharply and decay rapidly.