Basic Electrochemical Detector Electronics

The auxiliary, electrode A is held at a fixed potential by amplifier B the voltage being selected by potentiometer P. The potential near the auxiliary electrode is sensed by the reference electrode R. As stated before, the reference electrode allows the voltage to be compensated for changes in the mobile phase conductivity. On arrival of a solute at the surface of the working electrode that can be either oxidized or reduced, a 

The electrochemical detector is extremely sensitive, but suffers from two main drawbacks. Firstly, the mobile ( ase has to be extremely pure, in particular, free of oxygen and metal ions. Secondly, by-products of the oxidation or reduction processes are often absorbed on the surface of the electrodes and thus, if quantitative activity is required, frequent calibration is necessary. Ultimately the electrodes have to be cleaned usually by mechanical abrasion and replaced in the cell. Electrochemical detection is particularly suitable for small bore columns and possibly, in the future, LC capillary columns, due to the fact that the detector can be made extremely small in size. The detector has had a fairly wide area of application. It has been used under oxidizing conditions for the detection of phenols, hydroquinone and catechols and in particular for many compounds of biological interest including, catecholamines (35). It has been used to determine substances of industrial interest, and agricultural chemicals (36). In its oxidation form, it has been used to detect amines of various types together with i enols and thiols. It has also been used in the analysis of ascorbic acid in food and biological materials, and in the pharmaceutical industry, for the analysis of multivitamin products. 

In the reduction mode, electrochemical detectors can be used for detecting quinones, nitro compounds and consequently for the analysis of various forms of explosives (37). With the use of reagents containing aromatic nitro groups, derivatives of amines, ketones, and acids can also be detected by electrochemical detection. 

Haroon et al. (38) developed a dual electrode system for the detection of vitamin K compounds. The device utilized two sequential generator/detector electrodes. Vitamin K was electrolyzed at the first electrode and the reaction products were then detected electrochemically at a second electrode. The minimum mass of vitamin that they claim could be detected was 100 pg. They applied their system to the analysis of rat liver extracts and claimed it was superior in both sensitivity and specificity to the UV detector. 

The association of a spectrometer with the liquid chromatograph is usually for the purpose of structure elucidation of the eluted solute, a procedure that will be discussed in a later chapter. The association of tui atomic spectrometer with the liquid chromatograph, in contrast, is almost exclusively for the specific detection of the metalic and semi-metalic elements. The atomic spectrometer is a highly specific detector, and for element detection perhaps more so than the electrochemical detector. However, in general, a flame atomic absorption spectrometric (AAS) system is not as sensitive. If an atomic emission spectrometer or an atomic fluorescence spectrometer is employed then multi-element detection is possible. The inductively coupled plasma spectrometer can also, under some circumstances, provide multi-element detection but all three instruments are extremely expensive particularly in terms of an LC detector. It follows that most LC/AAS combinations employ a flame atomic absorption spectrometer or occasionally an atomic spectrometer fitted with a graphite furnace. Furthermore the spectrometer is usually set to monitor one element only, throughout the development of any given separation. 

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