Ammonia sensing electrode

Similar equations for an ammonia-sensing electrode whose internal electrolyte is an ammonium salt lead to the relation... 

Bacterial electrodes using whole cell suspensions and an ammonia sensing electrode also have been reported by Rechnitz et al (7,8). 

Similar arguments apply with an ammonia-sensing electrode. 

E. An ammonia gas-sensing electrode gave the following calibration points when all solutions contained 1 M NaOH. 

As a second example, a biosensor for urea could be constructed using an ammonia gas-sensing electrode [10] or a biocomposite containing soybean powder, polymer matrix and manganese dioxide as pH-sensing electrode [5]. 

Ammonia 368, 540 gas sensing electrodes 366 Ammonium 540-541 Amorphous silicon (a-Si) 99 Amperometric 907, 913 biosensors 359 detection 876, ell9 no sensor 930 oxygen electrodes 915 ultramicroelectrode 907 Amyloglucosidase 676 Analytical approximation 912 Anatoxin-a(s) 335 Aniline 985... 

The gas-sensing electrodes also are used for the potentiometric measurement of biologically important species. An enzyme is immobilized at or near the gas probe. The gas sensor measures the amount of characteristic gas produced by the reaction of the analyzed substance with the enzyme. For example, an enzyme electrode for urea [NH2C(0)NH2] determination is constructed by the immobilization of urease onto the surface of an ammonia-selective electrode. When the electrode is inserted into a solution that contains urea, the enzyme catalyzes its conversion to ammonia ... 

Several sensors for L-alanine are obtained by immobilizing L-alanine dehydrogenase over an ammonia gas-sensing electrode (288) or over an O2 sensor (280). The enzyme catalyzes the specific deamination of alanine in the presence of the coenzyme NAD ... 

Glutamine is selectively determined using a glutamine-selective electrode. Comparing several biocatalysts (isolated enzyme, bacteria cells, kidney mitochondrial fractions) immobilized at an ammonia gas-sensing electrode shows... 

Bovine serum albumin (BSA) and cyclic AMP (cAMP) are determined by a competitive binding enzyme immunoassay (315). With urease as label, an ammonia gas-sensing electrode is used to measure the amount of urease-labeled antigen bound to a double-antibody solid phase by continuously measuring the rate of ammonia produced from urea as substrate. The method yields accurate and sensitive assays for proteins (BSA less than 10 ng/mL) and antigens (cAMP less than 10 nM), with fairly good selectivity over cGMP, AMP, and GMP. 

An enzyme immunoassay using adenosine deaminase as the enzyme label has been described (318). Potentiometric rate measurements were made with an ammonia gas-sensing electrode. The immunoassay system employed is based on competition between a model haptenic group, dinitrophenyl (DNP) covalently coupled to adenosine deaminase, and free DNP hapten for the available binding sites on the anti-DNP antibody molecules. The detection limit is 50 ng antibody. 

Activity assays of enzymes bound to solid phases in EIA systems have previously been limited to fixed-time spectrophotometric methods following incubation of substrate and solid phase for extended periods of time. Kinetic assays of enzyme activity have not been used to date because of the difficulty in directly monitoring initial rates of enzyme reactions in a turbid solid phase suspension. With urease as the label, an ammonia gas sensing electrode can be used to directly quantitate the amount of urease-labeled antigen or hapten bound to a double-antibody solid phase by continuously measuring the initial rate of ammonia produced from urea as a substrate. 

Equipment. All potentiometric measurements were taken on a Coming Model 12 research pH meter in conjunction with a Heath-Schlum-berger Model SR-255B strip chart recorder. An Orion Model 95-10 ammonia gas sensing electrode was used for all assays of urease activity. Potentiometric activity measurements were made at 25° in a 10-ml glass thermostatted cell. 

Fig. 2. Typical potential ( ) vs time recording obtained for assay of urease with an ammonia gas sensing electrode. Curve shown is for addition of IS /tl of 6 Af urea to 1 ml of 0.4 nM urease in 0.1 M Tris-HCl-1 mM EDTA, pH 7.5.

Gas-sensing electrodes are examples of multiple membrane sensors these contain a gas-permeable membrane separating the test solution from an internal thin electrolyte film in which an ion-selective electrode is immersed. For example, for the ammonia sensor, the pH of the recipient layer is determined by the Henderson-Hasselbach equation [Eq. (18)], derived from the chemical equilibrium between solvated ammonia and ammonium ions ... 

The conditions under which chemical indicator reactions are used often necessitates the use of postcolumn addition, however. Figure 4.11 shows an experimental setup for urea assays using an immobilized urease reactor.30 The postcolumn addition of sodium hydroxide allows the NH produced by the reactor to be detected as NH3 at an ammonia gas-sensing electrode placed in a flow cell. 

Kobos et al. (1988) described the adsorption of urease on a fluorocarbon membrane for the construction of a urea sensor. The spontaneous adsorption was enhanced by a factor of 7 by perfluoroalkylation of the amino groups of the enzyme. The enzyme membrane was attached to an ammonia gas-sensing electrode. The urea sensor thus prepared exhibited a sensitivity of 50 mV per decade of urea concentration and a response time of 3 min. Only a small amount of enzyme could be adsorbed on the limited membrane surface, so the sensor was stable for only 7 days. 

The cells were immobilized on an ammonia gas-sensing electrode. The potential of the resulting microbial sensor was linearly dependent on the concentration of NTA between 0.1 and 1 mmol/1. Intermediates of NTA metabolism, such as glycine and serine, were measured with the same sensitivity. 

A heterogeneous EIA coupled with a potentiometric electrode permitted the assay of BSA down to 10 ng/ml and cAMP down to 10 nmol/1 (Meyerhoffand Rechnitz, 1979). Urease was used as the marker enzyme and its activity was measured by means of an ammonia gas-sensing electrode. The equilibrium of the immunological reaction at the sensor was reached rather slowly. The advantage of the rapid response of biosensors could not therefore be exploited. 

The resulting ammonium ions can be detected with a cation-sensitive glass membrane. Alternatively, one could use a gas-sensing electrode for ammonia in place of the glass electrode, so that interferences from H , Na", and K are reduced. 

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