Electrochemical Detection - Waterborne Pathogens: Detection Methods and Applications - page 146

In a second step, the deposited metal is stripped from the electrode by
selective oxidation of each metal phase species during a potential sweep
toward more positive values. The detection limit of stripping voltammetry
is much smaller than other electrochemical techniques (i.e. 3 orders of
magnitude) thanks to the preconcentration step. Cathodic and adsorptive
stripping voltammetry differs from anodic stripping voltammetry by their
deposition mechanisms. In a cyclic voltammetry experiment, the working
electrode potential is ramped linearly versus time. When it reaches a set
potential, the working electrode's potential ramp is inverted. This inversion
can happen multiple times during a single experiment. Cyclic voltamme-
try is carried out in an unstirred solution. Scanning the potential in both
directions gives the opportunity to explore the electrochemical activity
of species generated at the electrode. This is a major advantage of cyclic
voltammetry over other voltammetric techniques. Finally, amperom-
etry is a voltammetric technique where a constant potential is applied
to the working electrode, and one can measure a current as a function
of time. One advantage of amperometry from other forms of voltam-
metry is that the current readings are averaged over time, thus bringing
greater precision. In most voltammetric techniques, current readings must
be considered independently at individual time intervals. One of the first
amperometric sensors measured the dissolved O 2 in blood and was devel-
oped by Clark in 1956. An extension of single-potential amperometry is
pulsed amperometry which prevent fouling of the working electrode. In
pulsed amperometry, the potential is applied for a short time (usually a few
100 ms), followed by higher or lower potentials that are used for cleaning
the electrode.
Limit of detection in the micromolar range is routine. For some ana-
lytes and for some voltammetric techniques, nanomolar and picomo-
lar levels can be reached, in particular with stripping voltammetry. The
accuracy is limited by residual currents (i.e. charging of the electrode).
At the micromolar range, an accuracy of ±1%-3% is routine. Precision is
generally limited by the uncertainty in measuring the limiting current or
the peak current. Under most conditions, a precision of ±1%-3% is nor-
mal. Sensitivity can be improved by a better control of the experimental
conditions.
Voltammetry is used for the analysis of trace metals in environmental
samples, including groundwater, rivers, and seawater. Levels of dissolved
oxygen and anionic surfactants in waters and wastewaters, as well as concen-
tration of CO 2 , H 2 SO 4 , and NH 3 in atmospheric gases, have been measured