Microdialysis Coupled with Electrochemical Detection - Drugs of Abuse: Neurological Reviews and Protocols - page 407

2. Materials
2.1. ED Equipment
1. Detectors and electrodes: There are basically two types of electrochemical
detectors ( see Notes 2 and 3 ): amperometric detectors (BAS that has been a
leading company for years and Antec-Leyden market excellent detectors and
electrodes) and the coulometric detectors (ESA is the leading company in this
fi eld). They both can oxidize or reduce the substance to be detected when a
specifi c potential, that depends on the substance, is applied . The major difference
between these two detectors is the type of electrode used.
An amperometric electrode is capable of oxidizing or reducing the analyzed
substance that fl ows in a thin layer onto its surface and can be made of carbon
paste or glassy carbon. The fi rst type can be made by fi lling the electrode site with
carbon paste. This electrode offers a high sensitivity (much better than that of
glassy carbon available 10 yr ago) but only a high skilled person can make them,
and they have to be replaced often. Sensitivity for DA can be about 5-10 fmol/
sample. Today excellent glassy carbon electrodes are available. The major
advantage of the glassy carbon electrodes is the long life (more than 10 yr)
while sensitivity is quite high and constant, with repolishing necessary not more
than once a year.
A Coulometric electrode, instead, is based on a porous graphite in which
the solution containing the substance to analyze fl ows through. Because all the
substances that pass trough the electrode become oxidized, a second electrode
in series after the fi rst can be used to reduce the substance previously oxidized.
This increases selectivity, as a specifi c oxidizing and reducing potential can be
selected. This feature permits cleaning of the chromatogram from disturbing
peaks of substances that can become oxidized irreversibly. Coulometric electrodes
have a great sensitivity when new, but they lose it even after a few months, and
their average life (in my experience) is about 1 yr. When sensitivity decreases they
can be still used for measuring samples with a high concentration of transmitter
(50-100 fmol/sample). Their cost is rather high if compared with their average
life (high unexplained price differences exist among coulometric electrodes from
different countries).
Reference electrodes have the function of producing a reference signal for
the working electrode. They are generally Ag/AgCl electrodes in saturated KCl
or NaCl solution. They are never problematic, but air bubbles that eventually
stop in their vicinity have to be washed out ( see Note 10 ). Their average life is
generally long (many years).
2. HPLC equipment: The basic HPLC equipment includes an HPLC pump, a pulse
damper, an injector, a chromatographic column, a guard column, and connections
to the detectors.
The HPLC pump delivers the solvent at high pressure through the system
(column and electrode). Because detection of neurotransmitters in areas where