Detection amperometric

Background noise is a problem in electrochemical detection, mainly in the reduction mode. Dissolved oxygen must be removed by both bubbling helium through the mobile phase and using steel connections, and trace metals may also give a background. 

In the oxidation mode, there are less problems with noise. 

The working electrode will gradually become poisoned and thereby needs frequent cleaning, depending on the sample quality. Glassy carbon electrodes need mechanical cleaning. 

The flow cells are small, convenient for narrow-bore columns. The working potential is from 0 to 2 V. 

For detection of DNA fragments, Fe(phen)32+ was used as the electrochemi-cally active intercalation reagent. The constant background current from free Fe(phen)32+ decreased in the presence of the DNA-Fe(phen)32+ complexes. Therefore, this is an indirect amperometric detection method. It was found that a distance of 300 pm, instead of 600 pm, between the working electrode and reference electrode has produced less electrical interference (in the form of a sloping baseline), allowing the use of a separation voltage up to 1200 V (240 V/cm) [745]. 

There are numerous advantages of amperometric detection on-chip. These include (1) the detector has a minimal dead volume (2) preparation of electrodes (working) is compatible with the planar micromachining technology (3) the electrodes can be miniaturized without compromising LOD and (4) the EC detection has a short response time [746]. 

In another report, unlike CE/EC devices previously reported, not only the working electrode was integrated, but the reference and counter electrodes were also patterned (Pt/Ti) on the glass chip. No external wire electrodes were used. This method has minimized dispersion at the column exit [747]. The electrodes were also situated under a shelf (due to the cover plate) so that the detection volume was restricted and the dispersion was further reduced. The stability of the chip was found to be more than 2 months. LOD of dopamine and catechol were reported to be in the 4-5 pM range [747]. 

In amperometric detection, a reference electrode was usually employed. However, in one report, a platinized Au electrode was used as a pseudoreference electrode in a three-electrode system for amperometric detection. The operation principle follows that of the hydrogen reference electrode [242]. 

Counter electrode Working electrode Sample reservoir 

Two developmental difficulties existed in the application of electrochemical methods to CE detection. The first involved the cross-talk or interference that resulted from the high voltages used in CE separations. These dc voltages may be as high as 50 kV, and the resulting electroosmotic currents can be up to six orders of magnitude greater than the faradaic currents measured at an amperometric detector poised 

To date, amperometric detectors have only been applied to small, readily oxidized analyte species. A variety of alternate amperometric detection schemes are under development, however, including indirect amperometry. With this technique, the separation buffer contains a redox-active species that is continuously detected by the ultramicroelectrode. The analyte displaces the electroactive species, so that the electrochemical response is decreased during the elution of an analyte peak. 

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HPLC method with amperometric detection was applied for detenuination of phenols in sea sediment and some dmg preparation. Peaks of phenol, guaiacol, cresols, hydroquinon and resorcinol were identified on chromatogram of birch tai. The HPLC method with electrochemical detectors was used for detenuination of some drug prepai ation of aminophenol derivate. So p-acetaminophenol (paracetamol) was determined in some drug. 

Apparatus. Set up the apparatus as in Section 14.10 with two small platinum plates connected to apparatus for the amperometric detection of the end point. 

Pulsed amperometric detection (PAD), introduced by Johnson and LaCourse (64, 65) has greatly enhanced the scope of liquid chromatography/electrochemistry (66). This detection mode overcomes the problem of loss of activity of noble metal electrodes associated with the fixed-potential detection of compounds such as carbohydrates, alcohols, amino acids, or aldehydes. Pulsed amperometric detection couples tlie process of anodic detection with anodic cleaning and cathodic reactivation of a noble metal electrode, thus assuring a continuously cleaned and active... 

FIGURE 3-27 Three-dimensional chromatogram for oxidizable biological compounds at a multichannel amperometric detection system, consisting of an array of 16 carbon-paste electrodes held at different potentials. AA = ascorbic acid NE = norepinephrine DOPAC = 3,4-dihydroxyphenylacetic acid 5-HIAA = 5-hydroxyindole-3-acetic acid DA = dopamine HVA = homovanillic acid. (Reproduced with permission from reference 68.)... 

Potential of zero charge, 20, 23, 25, 66 Potential scanning detector, 92 Potential step, 7, 42, 60 Potential window, 107, 108 Potentiometry, 2, 140 Potentiometric stripping analysis, 79 Potentiostat, 104, 105 Preconcentrating surfaces, 121 Preconcentration step, 121 Pretreatment, 110, 116 Pulsed amperometric detection, 92 Pulse voltammetry, 67... 

Clark GJ, Goodin RR, Smiley JW. 1985. Comparison of ultraviolet and reductive amperometric detection for determination of ethyl and methyl parathion in green vegetables and surface water using high-performance liquid chromatography. Anal Chem 57 2223-2228. 

Sun, X. et al., Capillary electrophoresis with amperometric detection of curcumin in Chinese herbal medicine pretreated by solid-phase extraction, J. Chromatogr. A, 962, 117, 2002. 

Oligosaccharides were isolated preparatively by high-pH anion exchange chromatography carried out on a LC-system (Dionex Corporation, Sunnyvale CA) equipped with a CarboPac PA-1 column (9 x 250 mm), coupled to a Spectra System AS 3500 auto sampler. The detection was carried out using Pulsed Amperometric Detection (PAD-II). 

Oligalacturonides were separated on a Dionex Bio LC with a CarboPac PA 1 column and pulsed amperometric detection using a linear Na-acetate-gradient in 0.1 M NaOH. 

While the terms amperometric detection and coulometric detection have come into use to describe detectors of less than 100% efficiency and 100% efficiency respectively, these terms are actually misnomers. An amperometric detector is any electrochemical detector where current is plotted as a function of time, regardless of the conversion efficiency. A coulometric detector is any electrochemical detector where charge is plotted as a function of time, again regardless of the conversion efficiency. Preferred terminology should be high efficiency and low efficiency detectors to describe the two situations. 

The simplest homogeneous ECIA formats have employed an amperometrically detectable antigen label. The pioneering work for this approach was again provided by Breyer and Radcliff vide supra. Since this early work, ferrocene has been... 

Based on many of the advances described above in electrochemical approaches to immunoassay, it is tempting to conclude that commercialization of some of the approaches is imminent. This may be true, but the historical use of optical methods for many clinical chemistry tests coupled with their rapidly growing use in immunoassay is a difficult barrier for any radically different method to overcome, though electrochemical sensors have become more important in the clinical chemistry laboratory over the last decade. In any event, to be successful ECIA methods will have to demonstrate clear superiority over existing and emerging technologies in both cost and performance. Some of the more recently described approaches such as those using enzyme amplified amperometric detection and ecLIA appear... 

To ensure that the detector electrode used in MEMED is a noninvasive probe of the concentration boundary layer that develops adjacent to the droplet, it is usually necessary to employ a small-sized UME (less than 2 /rm diameter). This is essential for amperometric detection protocols, although larger electrodes, up to 50/rm across, can be employed in potentiometric detection mode [73]. A key strength of the technique is that the electrode measures directly the concentration profile of a target species involved in the reaction at the interface, i.e., the spatial distribution of a product or reactant, on the receptor phase side. The shape of this concentration profile is sensitive to the mass transport characteristics for the growing drop, and to the interfacial reaction kinetics. A schematic of the apparatus for MEMED is shown in Fig. 14. 

Most aliphatic compounds are not considered to be amenable to amperometric detection at a constant potential. Free-radical... 

The use of non-inert and chemically modified electrodes and other strategies for the detection of species that are difficult to analyze with the normal electrode materials have been reviewed.55 Photosensitization prior to amperometric detection is another tactic that has proved useful for the analysis of substances that are normally considered to be electrochemically inert.56 The use of pulsed amperometry has recently been reviewed.57... 

SCX column was used to separate oxalate and urate.140 In this separation, differential pulse and DC amperometric detection were compared. Differential pulse detection was found to allow better selectivity in detection. Anion exchange on Diaion CA08 was used to separate 20 carboxylic acids in the analysis of white wine, as shown in Figure 10.141 Because many carboxylic acids have a relatively weak absorbance, detection is difficult. The colorimetric detection scheme shown in the figure may be useful in some applications. 

Hydrophilic interaction chromatography on Asahipak NH2P or Excel-pak CHA-P44 with pulsed amperometric detection has been used to fractionate malto-oligosaccharides.266 The Asahipak NH2P is a polyvinyl alcohol support with a polyamine bonded phase, and the Excelpak is a sulfonated polystyrene in the Zn+2 form. Amine adsorption of sialic acid-containing oligosaccharides was performed on a Micropak AX-5 column (Varian) using acetonitrile-water-acetic acid-triethylamine.267... 

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