Electrochemical detection analytical measurements

Figure 3.21 Properties measured during electrochemical detection. Amperometry measures the current or charge transferred between neutral or ionic analytes and the electrode. Conductivity measures the mobility of ions in an electric field. (Reprinted from Ref. 49 with permission.)...

Scale of Operation Voltammetry is routinely used to analyze samples at the parts-per-million level and, in some cases, can be used to detect analytes at the parts-per-billion or parts-per-trillion level. Most analyses are carried out in conventional electrochemical cells using macro samples however, microcells are available that require as little as 50 pL of sample. Microelectrodes, with diameters as small as 2 pm, allow voltammetric measurements to be made on even smaller samples. For example, the concentration of glucose in 200-pm pond snail neurons has been successfully monitored using a 2-pm amperometric glucose electrode. ... 

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. 

Electrochemical detection is based on measurement of the current resulting from oxidation/reduction reactions of analytes at a suitable electrode as a function of the applied voltage. Since the level of the current is directly proportional to the analyte concentration, these detectors are used for both qualitative and quantitative purposes (127). 

Benzidines (Method 605). The method chosen for the determination of benzidine and 3,3 -dichlorobenzidine uses HPLC. Lichrosorb RP-2 (5 jLtm) is used as the analytical column, and acetonitrile and an acetate buffer are used as the mobile phase. A relatively selective electrochemical detector is used to detect and measure the benzidines. The instability of 1,2-diphenylhydrazine, which decomposes to azobenzene, caused it to be eliminated from consideration. 

Many specific and highly sensitive fluorometric and electrochemical detection methods for various analytes are available. The combination of such detection schemes with a DS-based collection system provides a combination of sensitive and affordable instrumentation for atmospheric measurements. A step-by-step construction and operation manual for a DS-based fluorometric H202 analyzer is available (94). With a change in the reagents, the calibration source, and the conditions of the fluorometric measurement, such an instrument is readily reconfigured for a different analyte. The 1992 fabrication cost of a complete DS instrument that utilizes fluorometric detection and includes a thermostated calibration source from commercially available components is approximately 12,000. 

This configuration based on the use of two surfaces, magnetic beads for immunoassay and screen-printed electrodes for electrochemical detection, allows to obtain a faster and a more sensitive detection of the immunoreaction than using a unique surface (screen-printed electrode) in this case it is possible to perform the electrochemical measurement in faster times (less then 30 min) and improve the sensitivity (around two magnitude orders). For this reason, this approach is advised in the development of an electrochemical immunosensor specific to any analyte. 

In the last step of an assay, i.e. during the electrochemical detection of the electroactive species of interest, a new window appears in order to let the experimenter follow the detection on-line. As presented in Fig. 36.8, the software has been designed to perform chrono-amperometric measurements of the desired analyte. On the left part of the window the raw measurement data that provide the evolution of the current for each microchannel that is measured during a time period of generally 2 s appears. With the multi-potentiostat used, the eight channels are... 

Potentiodynamic gas sensors have a schematic structure that is practically equal to that of amperometric gas sensors. They are -> electrochemical cells that measure a -> current signal directly related to the concentration of the analyte, but are not necessary operated in a region where -> mass transport is limiting. They are typically employed to detect less reactive species such as benzene and halogenated hydrocarbons that require a previous accumulation step at a suitable -> adsorption potential to be then reduced or oxidized according to a given potential scan [iii]. The adsorption time can be automat-... 

It is well established that electrochemical techniques are useful for determining ions and molecules in solution and gas phases. Oxidizable and reducible chemical species dissolved in solution can be detected by measuring oxidation or reduction current that is produced upon electrolysis of the species on the surface of electrodes. Usually, the output current of the system depends on the concentration of redox species in the solution. Thus, one can quantitatively determine the concentration of the analyte. Another merit of electrochemical techniques is easy identification of analyte... 

The selectivity inherent to electrochemical detection is derived from the differences between the oxidation or reduction half-wave potentials exhibited by different analytes. Even when two or more analytes have nearly the same half-wave potentials, complexing agents or alterations in mobile-phase composition can be used to differentiate between analytes. In order to carry out the electrochemical quantitation of an analyte, the potential difference between the working microelectrode and the reference electrode is maintained at a value that lies on the plateau of the oxidation or reduction wave (voltamogram) of the analyte of interest. The diffusion current thus measured, which is due to the oxidation or reduction of the analyte, is proportional to the area under the analyte peak eluted. In order for absolute quantitation to be effected, the diffusion current of a standard sample of the analyte must also be measured for comparison with that of the unknown sample. 

Electrochemical detection is based on the electrical signal arising between two electrodes immersed in a sample solution. Electroanalytical techniques fall into two main categories, potentiometric and Faradaic techniques. Potentiometry is the measurement of a potential difference between two electrodes under equilibrium conditions (i.e., no current flow). The potential is then related to concentration of the analyte species. Faradaic processes are based on the oxidation or reduction of the analyte, where a specific potential waveform is applied and the current is used to extract information about the sample. Many different techniques have been developed to gain quantitative and... 

Electrochemical immunoassays include a wide variety of devices based on the coupling of immunological reactions with electrochemical transduction. All of them involve the immobilization of an immunoreagent component on the surface of the electrode transducer. Electrochemical detection is based on the direct intrinsic redox behavior either of an analyte species or of some reporter molecule. For the detection no expensive equipment is needed, with the measurement of either a simple current or a voltage charge. Different electrochemical detection strategies are used, but ampero-metric detection is most widely used. Potentiometric and conductometric detection are applied in different assays as well. 

The determination of catecholamines requires a highly sensitive and selective assay procedure capable of measuring very low levels of catecholamines that may be present. In past years, a number of methods have been reported for measurement of catecholamines in both plasma and body tissues. A few of these papers have reported simultaneous measurement of more than two catecholamine analytes. One of them utilized Used UV for endpoint detection and the samples were chromatographed on a reversed-phase phenyl analytical column. The procedure was slow and cumbersome because ofdue to the use of a complicated liquid-liquid extraction and each chromatographic run lasted more than 25 min with a detection Umit of 5-10 ng on-column. Other sensitive HPLC methods reported in the literature use electrochemical detection with detection limits 12, 6, 12, 18, and 12 pg for noradrenaline, dopamine, serotonin, 5-hydroxyindoleace-tic acid, and homovanillic acid, respectively. The method used very a complicated mobile phase in terms of its composition while whilst the low pH of 3.1 used might jeopardize the chemical stability of the column. Analysis time was approximately 30 min. Recently reported HPLC methods utilize amperometric end-point detection. 

Research Needs. Over the years L-ascorbic acid has been shown to be an essential nutrient for many insects including species of Lepidoptera, Orthoptera, Coleoptera, and Diptera. Others such as cockroaches, houseflies, and mealworms are reared on simple diets without added ascorbic acid. Perhaps those insects require very low levels of vitamin C in their diets. A sensitive analytical method is needed to measure levels of L-ascorbic acid and dehydroascorbic acid in insect tissue and food. Such a method, which is likely to be developed using HPLC with electrochemical detection, could be used to monitor vitamin C levels in feed ingredients as well as in tissues during an insect s life cycle. This information is needed to determine whether ascorbic acid is used to... 

Most amino acids react with ninhydrin at ambient temperatures to form a blue color that becomes purple on heating. However, proline and hydroxyproline yield yellow compounds that are measured at a different wavelength. Other postcolumn derivatizations use fluorogenic reagents, such as o-phthaldialdehyde or fluorescamine. Precolumn derivatization techniques using o-phthaldialdehyde, dansyl, phenyl isothiocyanate, or 9-fluorenylmethyl chloroformate derivatives have been used with reversed-phase HPLC. Electrochemical detection has also been coupled with derivatization methods to enhance analytical sensitivity. 

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