14.1.3 Requirements and limitations
14.1.3.1 Ions in running solution
For the three-electrode configuration to work, the solution inside the flow cell should have a low
electrical resistance. This is obtained by running solution through the flow cell that contains at
least 10 mM ions.
Warning:
The solution running through the flow cell should have an ionic strength of
at least 10 mM when the cell is ON.
Either the mobile phase contains the ions (usually the pH buffer) or the ions are added in a post-
column setup. An absence of ions in the solution running through the flow cell results in
destabilization of the feedback loop and potential damage to the electrodes. When the flow is not
yet filled (air has high electrical resistance) or solutions without ions are being flushed through,
the cell should be OFF.
14.1.3.2 Organics in running solution
The effective volume of the cell is defined by the working electrode pressing on the inlet block,
with a spacer in between. Running solutions that contain more than 50% organic solvent through
the flow cell increases the occurrence of leakage: the organic solvent makes the solution creep
under the spacer. For such applications, a different cell design is better suited.
14.1.4 Working electrodes
14.1.4.1 Various WE materials
The surface of the working electrode is where the electrochemical reaction takes place. This puts
specific demands on the working electrode material. The WE should be made of an
electrochemically inert material, it should have a very well defined and flat surface, and it should
have favorable I/E characteristics for the analyte of interest. Ideally, a high signal is obtained at a
low working potential because noise levels increase with potential.
Different working electrode materials are available for the FlexCell: glassy carbon, gold, silver,
platinum, boron-doped diamond (BDD), and copper. For most regular applications, glassy carbon
is the working electrode material of choice. Some components are best detected on specific
materials. For example, the analysis of iodide is best done with a silver working electrode. This
reaction already occurs at a very low working potential (1 mV), which results in extremely high
selectivity. This allows the identification of iodide in urine samples with virtually no sample
pretreatment.
14.1.4.2 Applications
The following table gives the typical applications for various working electrode materials. Different
materials also have different working potential limits. At high positive working potentials, the
water in the mobile phase electrolyzes and results in a strong increase in the background current
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