Amperometric detectors flow cell designs

Fluorescence detection, because of the limited number of molecules that fluoresce under specific excitation and emission wavelengths, is a reasonable alternative if the analyte fluoresces. Likewise, amperometric detection can provide greater selectivity and very good sensitivity if the analyte is readily electrochemically oxidized or reduced. Brunt (37) recently reviewed a wide variety of electrochemical detectors for HPLC. Bulk-property detectors (i.e., conductometric and capacitance detectors) and solute-property detectors (i.e., amperometric, coulo-metric, polarographic, and potentiometric detectors) were discussed. Many flow-cell designs were diagrammed, and commercial systems were discussed. 

The ClinLab flow cell Sputnik [123] was originally developed for H PLC applications but is a separate device which in principle can be combined with every amperometric detector which is designed for three-electrode operation (Figure 4.74). Two PEEK capillaries and three electronic plugs are the only external interfaces. 

Figure 3.8 Amperometric detectors (a) measure the current that flows between the working electrode, usually a glassy carbon electrode, and a reference electrode, at a fixed voltage, usually close to the discharge potential for the compound. Coulometric detectors (b) are less common and are designed with a porous carbon flow cell so that all the analyte reacts in the cell, the amount of current consumed during the process being proportional to the amount of the substance.

Cells are classified according to how the working electrode is positioned relative to the flow stream. There are three major configurations tubular, thin layer, and wall jet. The tubular cell (open or packed) with its greater working electrode surface area is used for coulometric detection. The thin layer and wall jet designs are used for amperometric detector cells. In thin layer cells, the eluent flow is in the same plane as... 

As noted earlier, one of the shortcomings of flow techniques is the lack of flow cells which would allow the instruments currently available to be coupled to flow systems. However, especially common among available cells are amperometric and coulometric cells for solid electrodes. Some have been designed for HPLC detectors, not being all useful for flow systems owing to the high pressure required to pass liquids through them. 

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