Electrochemical Detection - Waterborne Pathogens: Detection Methods and Applications - page 150

in the electrochemical system. Only techniques involving impedance mea-
surement are able to identify the various phenomena.
First measurements of full impedance spectra were performed in the
early 1970s when reliable equipment became available. Among the vari-
ous methods of impedance measurements, the frequency response analyzer
(FRA) is now the standard. FRA is a single-sine method in which a small
potential of 5-15 mV of a given frequency is overlaid on a given bias poten-
tial and applied to a working electrode. The resulting electrical current is
then measured between this working electrode and a counter electrode. This
process is repeated by scanning the frequency and computing the imped-
ances from the voltage and current data. Typically five to ten measurements
are performed for a decade change in frequency ( Fig. 6.5 ).
This two-electrode configuration is most often used. However, it suffers
from the parasitic influence of the electrode-electrolyte interface imped-
ance, which hinders signal detection in the low frequency range. As a con-
sequence, tetrapolar arrangements have also been proposed. 17,18 The nature
Figure 6.5 Typical two-electrode arrangement for an impedimetric sensor. Source:
Adapted from Ref. 16 . (For color version of this figure, the reader is referred to the online
version of this topic.)