Reduction, of analyte

The use of robotics can be adopted also in sample preparation steps, in particular on-line SPE [7], This necessity is particular evident when small quantity of starting materials is available and the target molecules are present at low concentration levels. With the advent of miniaturization and automated procedures for samples handling, treatments and analysis, the lost of analytes due to a laboratory steps can be reduced. The reduction of analyte losses and the possibility to analyze even a total sample (no loss) leads to lower limits of detection (and consequently lower limits of quantification). Smaller volumes bring to obtain adequate sensitivity and selectivity for a large variety of compounds. In addition, on-line SPE requires low solvent consumption without the need to remove all residual water from cartridges, since elution solvents are compatible with the separation methods. 

Reduction of analyte occurs at the cathode (on the right-hand side of the cell). Once formed, however, the reduced form of the analyte couple diffuses across the cell - it may also be swept along by the stirred solution - and/or be re-oxidized again at the anode. Clearly, a single molecule of analyte could be oxidized and reduced many times, thus leading to an artificially high charge at the coulometer. For this reason, the two halves of the coulometry cell should be separated if possible, e.g. with a semipermeable membrane or frit, or we should ensure that the product of electron transfer should be a solid, i.e. it is immobilized as soon as it is formed. 

Direct current (DC) amperometry is used for the analysis of catecholamines, phenols, and anilines, which are easy to oxidize. A single potential is applied, and the current is measured. The current resulting from the oxidation or reduction of analyte molecules is dependent on many factors, including the concentration of the analyte, temperature, the surface area of the working electrode, and the linear velocity of the flowing stream over the surface of the working electrode. 

There are a number of electrochemical interactions which may be useful as the basis for detection in HPLC the most commonly used electrochemical detectors are based on amperometric measurements. The principle of operation of an amperometric detector is the oxidation or reduction of analyte in a flow-through electrolysis cell with a constant applied electrical potential, e.g. the oxidation of hydroquinone. 

Faradaic techniques are those in which oxidation or reduction of analyte species occurs at the electrodes and therefore a measurable current is passed through the electrochemical cell. This discussion will be limited to controlled-potential techniques, primarily volta-metry and amperometry, coupled to liquid chromatography. While other Faradaic electrochemical techniques have been developed and electrochemical techniques in bulk solution are common, the use of liquid chromatography employing these two detection strategies is by far the most common electroanalytical technique in pharmaceutical studies. 

Figure 17-6 shows reduction of analyte at the working electrode, which is therefore the cathode in this figure. In other cases, the working electrode could be the anode. The working electrode is always the indicator electrode at which analyte reacts. 

---