Carbon paste

Production of carbon electrodes is a capital-intensive business. Two suppHers dominate the prebaked market. Carbon paste producers are more numerous and tend to serve local markets. There is no international standard for the threaded joints on carbon electrodes. Manufacturers of straight pin carbon electrodes have followed the physical specifications adopted for graphite electrodes (37). Unified standards do not exist for pinless joints resulting in limited interchangeability among brands. Electrode diameters are offered in both English and metric sizes with no restrictions on new or unique diameters. 

Of recent years the use of mercury film electrodes based on substrates other than platinum has been explored, and increased sensitivity is claimed for electrodes based on wax-impregnated graphite, on carbon paste and on vitreous carbon a technique of simultaneous deposition of mercury and of the metals to be determined has also been developed. 

FIGURE 3-21 Typical amperometric readout during automated flow injection assays of ethanol at an enzyme carbon-paste electrode. Peaks a through h 2 x 10 5 M to 1.6 x 10 4 M ethanol. 

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.)... 

FIGURE 4-15 Cyclic voltanmiograms for 1.5 x 10 3 M ribose (a), glucose (b), galactose (c), and fructose (d) recorded at a Ru02-modified carbon-paste electrode. Dotted lines were obtained in carbohydrate-free solutions. (Reproduced with permission from reference 50.)... 

Practical examples of using preconcentrating CMEs include the use of a mixed 2,9-dimcthyl-1.10-phenanthrolinc/carbon-pastc electrode for trace measurements of copper (55), the use of clay-containing carbon pastes for voltammetric measurements of iron (56), the use of polyelectrolyte coatings for the uptake and... 

Tissue and Bacteria Electrodes The limited stability of isolated enzymes, and the fact that some enzymes are expensive or even unavailable in the pure state, has prompted the use of cellular materials (plant tissues, bacterial cells, etc.) as a source for enzymatic activity (35). For example, banana tissue (which is rich with polyphenol oxidase) can be incorporated by mixing within the carbon paste... 

FIGURE 6-12 The mixed tissue (banana) carbon-paste sensor for dopamine. PPO = polyphenol oxidase. (Reproduced with permission from reference 36.)... 

FIGURE 6-24 Response pattern of an amperometric sensor array for various carbohydrates. The array comprised carbon-paste electrodes doped with CoO (1), Cu20 (2), NiO (3) and Ru02 (4). (Reproduced with permission from reference 84.)... 

A carbon paste made of Acheson graphite and bromonaphthalene was used. 

Preconcentration of metal cations is also achieved by providing ligand binding sites in polymer layers, e.g. 4-methyl-4 -vinyl-2,2 -bipyridine for Fe " and Cu ". Carbon paste electrodes containing dimethylglyoxime or o-phenanthroline... 

Homogeneous electrochemical enzyme immunoassays for both phenytoin and digoxin have been developed. In both cases the label was glucose-6-phosphate dehydrogenase, which catalyzes the reduction of NAD to NADH. The NADH produced was detected by LCEC at a carbon paste electrode. 

A second surface modification has been reported by Yamamoto et al. These workers added stearic acid to their carbon paste mixture. This produced an electrode which was relatively insensitive to ascorbic acid and DOPAC relative to dopamine. It is theorized that this electrode works because of electrostatic repulsion of the anionic ascorbate and DOPAC by surface stearate groups. Ionic repulsion has also been employed by covering the surface of the working electrode with an anionic polymer membrane. Gerhardt et al. used Nafion, a hydrophobic sulfonated perfluoro-polymer, to make a dopamine selective electrode. This electrode exhibited selectivity coefficients as large as 250 1 for dopamine and norepinephrine over ascorbic acid, uric acid, and DOPAC. 

If the electrochemical sensor does not require a permselective membrane, immobilization of the enzyme onto the surface of the electrode is possible. Glassy carbon graphite reticulated vitreous carbon and carbon paste electrodes... 

By covering the Si02 insulating layer with a thin film of ion-selective carbon paste one would expect interesting possibilities for determining other ions such as halides and sulphide. ... 

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