Detection with constant working potential

While the working potential required for the desired electrochemical reaction may be determined with voltammetric experiments, amperometry is used as the detection method in ion chromatography. A distinction is made between amperometry with constant working potential and pulsed amperometry. 

This kind of amperometry is the most widely used electrochemical detection method in liquid chromatography. A constant DC potential is continuously applied to the electrodes of the detector cell. The theory of amperometry with constant working potential does not differ from the theory of hydrodynamic voltammetry, even though the applied potential remains constant. 

Fig. 7-9. Separation of monosaccharides in a chocolate milk on a latexed anion exchanger a) with pulsed amperometric detection at a gold working electrode, b) with conventional amperometric detection applying a constant working potential. Analytes (1) glucose, (2) fructose, (3) lactose, and (4) sucrose.

Two distinctly different coulometric techniques are available (1) coulometric analysis with controlled potential of the working electrode, and (2) coulometric analysis with constant current. In the former method the substance being determined reacts with 100 per cent current efficiency at a working electrode, the potential of which is controlled. The completion of the reaction is indicated by the current decreasing to practically zero, and the quantity of the substance reacted is obtained from the reading of a coulometer in series with the cell or by means of a current-time integrating device. In method (2) a solution of the substance to be determined is electrolysed with constant current until the reaction is completed (as detected by a visual indicator in the solution or by amperometric, potentiometric, or spectrophotometric methods) and the circuit is then opened. The total quantity of electricity passed is derived from the product current (amperes) x time (seconds) the present practice is to include an electronic integrator in the circuit. 

The decay of the valence state of the bare TDMAE molecule has been analysed in a cold supersonic beam after excitation at 266 nm, in a former work[6]. The ionisation potential of TDMAE is sufficiently low that excitation of the valence state at 266 nm allows probing the V state at 800 nm with an excess energy of 0.8 eV. The monotonic decay of the V state was observed with a 300 fs time constant. This decay is accompanied by the simultaneaous rise of the Z state detected with two 800 nm photons. This latter state further decays in 100 ps. This results in a bi-exponential signal for the TDMAE+ ions. 

The conductivities of PAn films were also examined with a.c. impedance measurements. Glarum and Marshall [209] reported that the oxidized PAn behaved like a series combination of resistance and capacitance and obtained very similar potential dependencies of equivalent series conductance to those obtained by in situ measurements by Wrighton et al. [28i]. Two different time constants of the R-C circuit were detected in their work, which they related to ionic and electronic conductivities. Rubinstein et al [210], who used a parallel combination of a capacitor with a... 

The simplest and most common detection mode is current measurement at constant potential. The reference and counter electrodes should be located downstream of the working electrode so that reaction products at the counter electrode or leakage from the reference electrode do not interfere with the working electrode. The current is amplified and plotted versus time to yield a chromatogram. 

For the amperometric detection of arsenite on the platinum working electrode, the applied oxidation potential can be kept constant. Figure 5.12 shows a respective chromatogram of a 10 mg/L arsenite standard that was obtained with an oxidation potential of -iO.95 V. 

Electrochemical detection of ascorbic acid is based on the oxidation of ascorbic acid to DHA (Fig. 2). Amperometry and coulometry are the measurements of current at a constant electrode potential. The main difference between these two measurements is the amount of analyte oxidized in the detector in amperometry the oxidation and current are limited in coulometry, the analyte is totally oxidized. The structure of an amperometric detector is usually a flow-by cell, whereas in coulometry a porous flow-through cell is used. In coulometry, a higher amount of analyte is allowed in contact with the electrode surface and sensitivity increases. Working with electrochemical detector, the components of mobile phase must allow for distinct separation of ascorbic acid and be conductive to carry the charge of the analyte. However, the mobile phase must not yield too high background signal. 

We have already briefly described a popular application of amperometry in Chapter 13. This was the electrochemical detector used in HPLC methods. In this application, the eluting mobile phase flows across the working electrode embedded in the wall of the detector flow cell. With a constant potential applied to the electrode (one sufficient to cause oxidation or reduction of mixture components), a current is detected when a mixture component elutes. This current translates into the chromatography peak... 

---