Amperometric detectors constant potential

These detectors are based upon the ability of the compound to undergo oxidation or reduction at the surface of a carbon or platinum electrode, which has an applied constant potential. The resulting current is measured and is proportional to the concentration of the electrode species present. There are two types of EC detectors commonly available, amperometric and coulometric. 

Electrochemical detectors can be broadly classified as either amperometric or voltammetric. An amperometric detector is one in which the potential applied to the detecting electrode is held constant and the resulting current is measured as a function of time. A voltammetric detector is one in which the applied potential is varied with time and the current response is measured as a function... 

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. 

Electrochemical Detectors. In amperometric electrochemical detectors (see Chapter 4), an electroactive analyte enters the flow cell, where it is either oxidized or reduced at an electrode surface under a constant potential. Electroactive compounds of clinical interest conveniently analyzed by HPLC with electrochemical detection include the urhiary catecholamines (see Chapter 29). In addition, electrochemi-cally active tags (e.g., bromine) are added to compounds such as unsaturated fatty acids or prostaglandins. 

The constant potential amperometric detector determines the current generated by the oxidation or reduction of electoactive species at a constant potential in an electrochemical cell. Reactions occur at an electrode surface and proceed by electron transfer to or from the electrode surface. The majority of electroactive compounds exhibit some degree of aromaticity or conjugation with most practical applications involving oxidation reactions. Electronic resonance in aromatic compounds functions to stabilize free radical intermediate products of anodic oxidations, and as a consequence, the activation barrier for electrochemical reaction is lowered significantly. Typical applications are the detection of phenols (e.g. antioxidants, opiates, catechols, estrogens, quinones) aromatic amines (e.g. aminophenols, neuroactive alkaloids [quinine, cocaine, morphine], neurotransmitters [epinephrine, acetylcoline]), thiols and disulfides, amino acids and peptides, nitroaromatics and pharmaceutical compounds [170,171]. Detection limits are usually in the nanomolar to micromolar range or 0.25 to 25 ng / ml. 

Coulometric Detectors. Coulometric detectors are based on potentiostatic coulometry [30]. The signal of the constant-potential measurements, as in amperometric detection, is the current resulting from an electron-transfer process (reduction or oxidation of the analyte arriving with the eluent) while the working electrode is held at constant potential. 

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. 

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. 

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. 

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