Page 108 - Analytical Electrochemistry 2d Ed - Jospeh Wang

3-6 FLOW ANALYSIS 93

surface. This is usually accomplished with a three-step potential waveform,
combining anodic and cathodic polarizations (e.g., Figure 3-25). The analytical
response results primarily from adsorbed analyte, with detection limits approaching
50 ng (for 50 mL samples). Other automated multistep potential waveforms are
possible. Such waveforms are commonly executed at a frequency of 1±2 Hz, in
connection with gold or platinum working electrodes.
The power of electrochemical detection can be improved by using more than one
working electrode (67). Different strategies, based primarily on dual-electrode
detection, can thus be employed. For example, in the series mode (Figure 3-26a)
the ®rst upstream electrode can be used to generate an electroactive species that is
more easily detected at the downstream electrode. Discrimination against
compounds with irreversible redox chemistry can also be achieved. Signi®cantly
improved qualitative information can be achieved using a parallel (side-by-side)
dual-electrode con®guration (Figure 3-26b). Two simultaneous chromatograms can
be generated by holding these electrodes at different potentials. The current ratios at
these two potential settings provide real-time ``®ngerprints'' of the eluting peaks.
Such ratio values are compared with those of standards to con®rm the peak identity.
Further improvements in the information content can be achieved using multi-
channel amperometric detection (analogous to diode array optical detection) (68).
For example, Figure 3-27 displays a three-dimensional chromatogram for a mixture
of several biologically signi®cant compounds at a 16-electrode detector array. By
rapidly applying a ®ve-potential sequence to the individual electrodes, an 80-channel
chromatographic detection can be obtained. Such an electrochemical pro®le across

Series
(a)

Parallel adjacent
(b)
FIGURE 3-26 Dual-electrode thin-layer detector con®gurations for operation in the series
(a) and parallel (b) amperometric modes.