Sensing gases

Gas-sensing electrodes have been developed for a variety of gases, the characteristics for which are listed in Table 11.4. The composition of the inner solution changes with use, and both it and the membrane must be replaced periodically. Gas-sensing electrodes are stored in a solution similar to the internal solution to minimize their exposure to atmospheric gases. 

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. 

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. 

Comparing the two optical transduction techniques (absorption or SPR) used in this work, we can conclude that SPR technique appears to be more suitable for gas sensing even if it presents some limitation regarding the suitable film thickness for SPR excitation. Moreover, the response and recovery times during the anal5fle/sensing layer interaction appears shortest in the case of optical absorption measurements. Further investigations are in... 

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,... 

Biocatalytic membrane electrodes have an ISE or a gas sensing electrode in contact with a thin layer of biocatalytic material, which can be an immobilized enzyme, bacterial particles or a tissue slice, as shown in Fig. 3 The biocatalyst converts substrate (the analyte) into product, which is measured by the electrode. Electrodes of this type are often referred to as biosensors . 

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... 

Foil of PTFE or polythene Chemical reagent + glass membrane Gas-sensing electrode H+ (or OH -) as an indirect measure of NH3, S02, nitrous vapours, H2S, HCN, CMXa), C02... 

Gas-sensing electrodes. A gas-sensing electrode consists of a combination electrode that is normally used to detect a gas in its solution by immersion. The sensor contains the inner sensing element, usually a glass electrode or another ISE, and around this a layer of a 0.1 Af electrolyte, surrounded by a gas-permeable membrane. On immersion of the sensor this membrane contacts the solution of the gas which diffuses through it until an overall equilibrium is established, i.e., the partial pressure of the gas attains an equilibrium between sample solution and membrane and between membrane and sensor electrolyte. For a better understanding of the interaction between this electrolyte and the... 

Orion Model 95-64). In practice, one simply determines E ntot by calibration with a standard solution without the necessity of knowing the various constants mentioned. The S02 electrode allows the determination of concentrations down to 10 8 Af with a response time of a few minutes. From the above it appears that the gas-sensing electrodes show Nemstian behaviour provided that the concentrations to be measured are not high there is little or no interference by other components in the sample solution. 

Neither the usual membrane ISEs nor the gas-sensing electrodes, in which their internal indicator electrode functions as a zero-current potentiometric half-cell, are under consideration here. 

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