Flow cells voltammetry

Electrochemical Detectors Another common group of HPLC detectors are those based on electrochemical measurements such as amperometry, voltammetry, coulometry, and conductivity. Figure 12.29b, for example, shows an amperometric flow cell. Effluent from the column passes over the working electrode, which is held at a potential favorable for oxidizing or reducing the analytes. The potential is held constant relative to a downstream reference electrode, and the current flowing between the working and auxiliary electrodes is measured. Detection limits for amperometric electrochemical detection are 10 pg-1 ng of injected analyte. 

In the application of tubular electrodes and electrodes in flow cells, there may often be hydrodynamic complications, especially in voltammetry. 

Stripping analysis with inherent pre-concentration seems attractive to CFA, but until recently such a procedure appeared rather exceptional. It has been used in the automatic determination of heavy metals in water by anodic stripping voltammetry (ASV12S) in a continuous flow cell with a mercury-covered graphite electrode, having the advantage that one can distinguish... 

Wang and Dewald have investigated the possibility of using rapid scan differential pulse voltammetry for the detection of chlorpromazine in flow-injection analysis [160]. The measurement was made with the use of a flow-cell equipped with a carbon paste or a vitreous-carbon disc electrode, a carbon rod auxiliary electrode, and a silver-silver chloride reference electrode. Potential scanning was effected at 2 V/min. 

The mechanism of the iodide formation at platinum immersed in aqueous electrode was recently studied by laser-activated voltammetry in a channel flow cell system [161]. In this technique, solid deposits of iodine are removed from the electrode continuously by short nanosecond high-power laser pulses. By removing deposits on electrode surfaces within a channel flow cell, the voltammetric measurements becomes time independent and data can be analyzed and modeled quantitatively. Laser activation using a 10-Hz pulsed Nd YAG 532-nm laser was shown to remove bulk iodine from the electrode surface so that under sustained pulsed... 

Voltammetry experiments are not often performed in flow cells for analytical purposes. One reason for this is the special problem of ohmic potential losses (iR drops) at an electrode in a confined stream. Another reason is the problem of precisely pumping solution at a carefully controlled velocity. In general, rotating electrodes are more easily controlled and do not involve serious plumbing problems. On the other hand, flow cells operated at a fixed potential (i.e., at one point along the steady-state voltammetric curve) are eminently useful for electrosynthesis, chromatographic detection, and automated analysis systems. These features will be described in later chapters. 

Hydrodynamic electrodes — are electrodes where a forced convection ensures a -> steady state -> mass transport to the electrode surface, and a -> finite diffusion (subentry of -> diffusion) regime applies. The most frequently used hydrodynamic electrodes are the -> rotating disk electrode, -> rotating ring disk electrode, -> wall-jet electrode, wall-tube electrode, channel electrode, etc. See also - flow-cells, -> hydrodynamic voltammetry, -> detectors. 

The Au layer - which is a prerequisite for surface-plasmon optics - can be used simultaneously as the working electrode in a regular three-electrode electrochemical set-up (also shown in Fig. 3a) with the reference and the counter electrode being immersed in the flow-cell attached to the Au coated substrate. In this way, various electrochemical techniques, e.g., cyclic voltammetry or impedance spec-... 

Yunus K. and Fisher A. C., Voltammetry under microfluidic control, a flow cell approach, Electroanalysis, 15(22), 1782-1786, 2003. 

Voltammetric techniques may be broadly divided into steady-slate techniques, such as channel flow cell [44 6], rotating disc [47, 48], or microelectrode [49] voltammetry at sufficiently low potential scan rate to give a current response independent of time, and transient techniques, such as cyclic voltammetry or chronoamperometry, giving a current response which is dependent on time. 

Some EC detectors include software to perform simple cyclic voltammetry on analytes held at the electrode. This can be performed by either FI A, as above, or by catching a peak from the column in the flow-cell by rapidly stopping the flow. This is best done by quickly disconnecting the column to cell tubing since the de-pressurisation of the system after switching off the pump usually sweeps the peak from the detector. From the cyclic voltammogram the optimum applied potential can be ascertained. However, most of the restraints mentioned for the FIA approach apply. 

Colombo C, van den Berg CMG, Daniel A (1997) A flow cell for on-line monitoring of metals in natinal waters by voltammetry with a mercury drop electrode. Anal Chim Acta 346 101... 

The amperometric detection uses less than 10% of the analyte in the flow cell, unlike the coulometric detector, and can be operated in a pulsed mode (cydic voltammetry, with a gold working electrode) in addition to the constant potential mode. The pulsed mode helps cleaning the working electrode. 

CSV responds to chromiumlV/) as well as to (///) but the response to chromium(///) diminishes rapidly with time approaching zero after about 15 min due to chromium(///) adsorption on the cell wall. Thereafter the response is due to chromium(V/) only. For this reason it is recommended to convert all chromium into the V7 state prior to the analysis. The only way to prevent the chromium(//7) signal from diminishing is to use a flow cell instead of the standard, batch, voltammetric cell. This adsorption is not special to voltammetry the chromium(///) adsorbs readily on almost anything, and its rapid removal from water samples is normally prevented by acidification. Acidification causes all chromium(V7) to... 

The basic electric circuit in flow-through voltammetry and coulometry is the same in principle. The indicating electrode is polarized against a reference electrode to a constant potential. In the close vicinity of the indicating electrode a constant convection is maintained e.g. by a stirrer. The sample flows through the cell and the electric current proportional to the concentration of the species to be measured is recorded. The basic difference between the two methods is in the physical meaning of the current. 

Pungor made other important contributions to analytical chemistry, especially to oscillometry and conductometry [90], hydrodynamic voltammetry (flow-cell, flow analysis) [91] (Fig. 12.28), coulometric analysis (coulometric titration) [92], and flame photometry [93] (Fig. 12.29). 

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