Fast scan cyclic voltammetry concentrations with

Typical steady-state tip and substrate current approach curves for the oxidation of different concentrations of ArCT are shown in Figure 23. A general observation is that as the concentration of ArCT increases, the tip and substrate currents—at a particular distance—decrease, due to the second-order nature of the follow-up chemical reaction. The experimental approach curves are shown alongside theoretically derived curves for a spread of normalized rate constants, K2, from which it can be seen that there is reasonable agreement between the observed and predicted trends. From measurements of both feedback currents, for all three ArCT concentrations investigated, and collection efficiencies, for the lowest two concentrations, a radical dimerization rate constant of 1.2 ( 0.3) X 10s M 1 s 1 was determined (5), which was in reasonable agreement with that determined earlier using fast scan cyclic voltammetry (36). 

The slice is electrically stimulated with an enamel-coated bipolar wire electrode causing action potentials that evoke DA release. In the slice preparation, the stimulation is applied directly at the neuron terminals. This is in contrast to an in vivo experiment, where the stimulation is performed at the cell body of the neuron and the DA release is monitored remotely at the presynaptic terminals. DA concentration in the extracellular fluid rises and quickly returns to baseline at the cessation of the stimulation [3]. Fast scan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes is used to detect the resulting concentration changes in the extracellular fluid. This analytical technique provides a method for the determination of uptake kinetics in intact brain tissue. Thus, the secretion and subsequent clearance of DA in the tissue is observed in real time. 

The earliest experiments of this type employed voltammetry as a detection scheme. Fast-scan cyclic voltammetry can be used to discriminate between released substances with different oxidation/reduction properties. Voltammetric measurements of absolute concentrations released from single vesicles undergoing exocytosis have proven to be difficult however, because the amount of transmitter released is only at zeptomole levels and because the events occur on the millisecond time scale [13]. Furthermore, although elicited by chemical stimulation, these events do not occur at precise times. Although several neurotransmitters have been identified on the basis of characteristic voltammograms, it is difficult to distinguish catecholamines with this technique. However, fast-scan cyclic voltammetry has been used to identify the seaeted catecholamines norepinephrine and epinephrine [14]. 

Various polarographic techniques have been successfully used to measure elemental sulfur, S , and sulfide (H2S and HS ) concentrations in laboratory solutions [116], salt water [117], saline lake [118], freshwater [119], lake porewaters [120], salt marsh sediments [109,120], and marine porewaters [121], Using stripping square wave voltammetry, researchers have been able to measure sulfide species at nanomolar concentrations [118-120, 122,123], Recently, cyclic and linear sweep voltammetries have been used to quantify elemental sulfur S , hydrogenosulfide (HS ) and polysulfides (S ) with fast scan rates (IV s ) [124] for example, in estuarine sediments (from Rehoboth Bay, an inland bay located in Delaware), the sulfur speciation was found to change throughout a core profile, with dominant in the top layers (0-6 cm), dominant... 

Figure 7 Simple model of the concentration of vesicular events as determined by fast-scan rate cyclic voltammetry. (A) Pictorial demonstration that the area of the electrode used for oxidation/reduction of the DA species is very different for the cellular case compared with that in standard solution. (B) A head-on view depicting the difference in electrode area used in the above two cases. Bevehng a carbon fiber on a 45° angle creates an elliptical surface with major and minor radii of about 3.5 and 2.5 i,m, respectively. It is apparent that a large difference exists between the vesicular area and that of the total electrode. (Reproduced from Ana/. Chem. with permission [13].)...

Figure 9 Apparent distributions of vesicular concentrations detected by fast-scan rate cyclic voltammetry following K -stimulated exocytosis. (A) Distribution generated from control PC12 cells (n = 29 cells, 77 total events). (B) Distribution generated from L-DOPA-exposed (100 pM for 1 hr) PC12 cells (n = 11 cells, 145 total events). For each data set, the peak current of the anodic wave of all events is matched to its respective calibration plot to determine the detected concentration. All data are collected into bins having increments of 0.25 pM and plotted as the percent of the total number of release events detected. (Reproduced from Anal. Chem. with permission [13].)...

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