Gas selective electrodes

Electrochemical Potential Ion-selective electrodes Gas selective electrodes Field effect transistors (ISFET, semiconductors)... 

Gas-selective electrodes are a particularly important application of the glass electrode. For example, the carbon dioxide electrode is a self-contained system with a glass electrode and a concentric silver-silver chloride electrode enclosed by a C02 permeable membrane. The latter holds a thin film of bicarbonate solution in contact with the glass membrane, which provides a junction to the silver/silver chloride reference electrode. The electrode, which is illustrated schematically by Figure 2.2, has found extensive application in monitoring C02 levels in blood and probably will find increasing application in other systems that require continuous measurement of C02 partial pressures. The electrode response is based on the reaction... 

Gas-selective electrodes are used to a hmited extent in food analysis although, in certain areas of food production their use is quite common. The oxygen-selective electrode is often used on the production hnes for fruit and vegetable juices, where a vacuum has been apphed to remove air before packaging, this in order to check the residual oxygen content. S02-selective electrodes are used to determine the SO2 content of wines and other S02-treated liquids either directly or after the acid conversion of a component such as NaHSOa to SO2. Other electrodes are available for similar measurements, such as nitrogen oxide (for nitrite contents) and carbon dioxide (for either CO2 directly or CO2 from acid-converted carbonate or bicarbonate salts). There are perhaps other more accurate ways of determining such analytes, but the gas-selective electrode is... 

Ion-selective membranes attain their permselectivity from ion-exchange, dissolution, or complexation phenomena. Different types of membranes are available for the construction of ion-selective electrodes glass and other solid state rods (crystals), liquid or polymer ion ecchangers, or dissolved ionophores. Many electrodes are commercially available with selec-tivities for different ions, mainly H, alkali metal cations, heavy metal ions, and halides or pseudohalides. Also gas-sensing electrodes may be constructed from an ion-selective electrode and a gas-permeable membrane [182]. Ion selective electrodes and gas-selective electrodes... 

Dichlorodiphenyltrichloroethane (DDT) inhibits carbonic anhydrase at the amount of micrograms at which other inhibitors are inactive. Carbonic anhydrase catalyzes the hydration of carbon dioxide to carbonic acid. The activity of the enzyme is assayed by measuring the CO2 liberated by dehydration in a Warburg manometer or a CO2 gas-selective electrode. DDT inhibits the enzyme, causing a decrease in the rate of CO2 liberation. 

Sensor type Metal electrode Ion selective electrode (ISE) Gas selective electrode (GSE) Enzyme of whole cell electrode... 

A urea sensor has conventionally consisted of a urease-immobilized membrane and electrode for the determination of the metabolites electrodes for ammonium ion (20), pH (27), ammonia gas (22), and carbon dioxide (23) have been used. These sensors were all potentiometric ones to give Nernstian responses, the linear concentration range of which was typically between 10" and 10 M of urea. If one wishes to detect urea with using the ion- or gas-selective electrodes, urease is indispensable not only for the specific recognition based on the bioaffinity interaction but also for chemical conversion of urea to detectable species, since urea is essentially electro-inactive. On the contrary, the polymer-modified electrode described here does not rely upon enzymes the concentration of urea can be determined directly. 

Potcntiomctric Biosensors Potentiometric electrodes for the analysis of molecules of biochemical importance can be constructed in a fashion similar to that used for gas-sensing electrodes. The most common class of potentiometric biosensors are the so-called enzyme electrodes, in which an enzyme is trapped or immobilized at the surface of an ion-selective electrode. Reaction of the analyte with the enzyme produces a product whose concentration is monitored by the ion-selective electrode. Potentiometric biosensors have also been designed around other biologically active species, including antibodies, bacterial particles, tissue, and hormone receptors. 

Few potentiometric biosensors are commercially available. As shown in Figures 11.16 and 11.17, however, available ion-selective and gas-sensing electrodes may be easily converted into biosensors. Several representative examples are described in Table 11.5, and additional examples can be found in several reviews listed in the suggested readings at the end of the chapter. 

Potentiometric electrodes also can be designed to respond to molecules by incorporating a reaction producing an ion whose concentration can be determined using a traditional ion-selective electrode. Gas-sensing electrodes, for example, include a gas-permeable membrane that isolates the ion-selective electrode from the solution containing the analyte. Diffusion of a dissolved gas across the membrane alters the composition of the inner solution in a manner that can be followed with an ion-selective electrode. Enzyme electrodes operate in the same way. 

Methods for iodine deterrnination in foods using colorimetry (95,96), ion-selective electrodes (94,97), micro acid digestion methods (98), and gas chromatography (99) suffer some limitations such as potential interferences, possibHity of contamination, and loss during analysis. More recendy neutron activation analysis, which is probably the most sensitive analytical technique for determining iodine, has also been used (100—102). 

Ammonia.. The most rehable results for ammonia are obtained from fresh samples. Storage of acidified samples at 4°C is the best way to minimi2e losses if prompt analysis is impossible. The sample acidity is neutrali2ed prior to analysis. Ammonia concentrations of 10 -0.5 M can be determined potentiometricaHy with the gas-sensing, ion-selective electrode. Volatile amines are the only known interferents. 

Ion-selective electrodes can also become sensors (qv) for gases such as carbon dioxide (qv), ammonia (qv), and hydrogen sulfide by isolating the gas in buffered solutions protected from the sample atmosphere by gas-permeable membranes. Typically, pH glass electrodes are used, but electrodes selective to carbonate or sulfide may be more selective. 

Gas-sensing electrodes consist of an ion-selective electrode in contact with a thin layer of aqueous electrolyte that is confined to the electrode surface by an outer membrane as shown schematically for a COj electrode in Fig. 2. The outer membrane,... 

Conventional ion-selective electrodes have been used as detectors for immunoassays. Antibody binding measurements can be made with hapten-selective electrodes such as the trimethylphenylammonium ion electrode Enzyme immunoassays in which the enzyme label catalyzes the production of a product that is detected by an ion-selective or gas-sensing electrode take advantage of the amplification effect of enzyme catalysis in order to reach lower detection limits. Systems for hepatitis B surface antigen and estradiol use horseradish peroxidase as the enzyme label and... 

The electrolytes Na", and Cl are second only to glucose in being the most frequently run hospital tests. Many clinical chemistry analyzers now contain an ISE module for electrolyte analysis. Most commonly the module will consist of a Na -glass electrode, a valinomycin/PVC electrode, a Ag/AgCl pellet or a quaternary ammonium ion/PVC electrode and a reference electrode. A selective electrode for the bicarbonate ion continues to elude workers in the field. An indirect measurement of HCOf must be made. The sample is usually reacted with acid to evolve carbon dioxide gas which is measured with a traditional Severinghaus type CO2 electrode. Alternatively, the sample is treated with base to convert HCO to CO3 and a carbonate ion-selective electrode is used In this manner, the complete primary electrolyte profile is obtained electrochemically. 

Principles and Characteristics Combustion analysis is used primarily to determine C, H, N, O, S, P, and halogens in a variety of organic and inorganic materials (gas, liquid or solid) at trace to per cent level, e.g. for the determination of organic-bound halogens in epoxy moulding resins, halogenated hydrocarbons, brominated resins, phosphorous in flame-retardant materials, etc. Sample quantities are dependent upon the concentration level of the analyte. A precise assay can usually be obtained with a few mg of material. Combustions are performed under controlled conditions, usually in the presence of catalysts. Oxidative combustions are most common. The element of interest is converted into a reaction product, which is then determined by techniques such as GC, IC, ion-selective electrode, titrime-try, or colorimetric measurement. Various combustion techniques are commonly used. 

The methods most commonly used to detect hydrogen sulfide in environmental samples include GC/FPD, gas chromatography with electrochemical detection (GC/ECD), iodometric methods, the methylene blue colorimetric or spectrophotometric method, the spot method using paper or tiles impregnated with lead acetate or mercuric chloride, ion chromatography with conductivity, and potentiometric titration with a sulfide ion-selective electrode. Details of commonly used analytical methods for several types of environmental samples are presented in Table 6-2. 

Hydrogen fluoride and inorganic fluorides in air (using an ion selective electrode). Toluene in air (pumped charcoal adsorption tubes, solvent desorption and gas chromatography). 

Metal-air cells are developed with air gas-diffusion cathodes and Mg-anodes. Non-aggressive NaCl-solution is used as electrolyte. Carbon based catalysts for the oxygen reduction are selected and tested in the air gas-diffusion electrodes. Various Mg-alloys are tested as anodes. The V-A, power and discharge characteristics of the Mg-air cells are investigated. 

The described method for the diagnostic of the activity and the transport hindrances in air gas-diffusion electrodes is very useful in the research of porous catalysts for air electrodes. The comparison of the activity and the transport hindrances of air electrodes with catalysts from various types of active carbon allow a proper selection to be accomplished. 

Besides, potentiometric sensors with ion-selective ionophores in modified poly(vinyl chloride) (PVC) have been used to detect analytes from human serum [128], Cellular respiration and acidification due to the activity of the cells has been measured with CMOS ISFETS [129], Some potentiometric methods employ gas-sensing electrodes for NH3 (for deaminase reactions) and C02 (for decarboxylase reactions). Ion-selective electrodes have also been used to quantitate penicillin, since the penicillinase reaction may be mediated with I or GST. 

Membranes. Photopolymer chemistry is being applied to the design and manufacture of a variety of membrane materials. In these applications, photopolymer technology is used to precisely define the microscopic openings in the membrane as it is being formed or to modify an existing membrane. Some of the applications of photopolymer chemistry to membranes include the modification of ultrafiltration membranes (78) and the manufacture of amphiphilic (79), gas permeable (80), untrafiltration (81), ion-selective electrode (82) and reverse osmosis membranes. 

Schematic diagram of a gas-sensing electrode. Selective ion electrode is shown as a glass electrode. The reference electrode is an Ag-AgCl electrode. Other electrode combinations are possible.

---