Abstract: To construct solid-state sensors for NH4+, a layer (~7 mm) of epoxy-resin, rendered conductive by mixing it with twice its wt. of graphite powder, was applied to one end of an epoxy or PVC tube, and a shielded lead was fixed to this layer while it was soft. The resin was dried at 40°C for 24 h, then a flat cavity 0.3 mm deep was drilled in it from outside and layers of sensing material [nonactin - tris-(2-ethylhexyl) phosphate - PVC (1:15:4) in THF] were applied to the cavity and allowed to dry to form a convex membrane. The air-dried membrane was conditioned in aqueous 0.1 M NH4+. The sensors could be used in a conventional configuration or a flow-through sandwich arrangement (illustrated). Potentiometric urea sensors were produced by covering an NH4+ sensor with urease or Proteus vulgaris immobilized on nylon net or a polycarbonate filter, respectively. The sensors were evaluated under batch and flow injection conditions. For each, details of potential response, linear response range, response time, lifetime and electrode buffer solution are tabulated. Interference by K+ could be avoided by interposing a gas-permeable hydrophobic membrane between the biocatalytic and the NH4+-selective membranes. Advantages of the solid-state construction are stressed.

Abstract: The electrode previously described (Anal. Abstr., 1989, 51, 6J177) is mounted in a flow-through wall-jet cell and applied in a single-line flow injection system. The carrier solution (0.8 mL min-1) is 10 mM glucose in 0.1 M phosphate buffer of pH 7.0 and is deoxygenated online with use of a reactor containing glucose oxidase and catalase. The electrode is operated at +0.45 V vs. a SCE. For a 14 µL sample solution, the calibration graph is rectilinear up to ~9 ppm of O. With the proposed system a throughput of 360 samples per hour is achieved with a precision of 0.4% (n = 17).

Abstract: Xanthine oxidase (I) and purine-nucleoside phosphorylase (II) were separately incorporated (details given) into cellulose triacetate membranes. To a Clark-type O electrode were applied, in order, the II-containing membrane, three sheets of unimpregnated cellulose triacetate, the I-containing membrane and a dialysis membrane. With use of apparatus similar to that described earlier (cf. J. Food Sci., 1983, 48, 496), 0.05 M phosphate buffer (pH 7.8) containing 0.1 mM cysteine was pumped to the sensor, and when the output current became steady a 50 µL portion of a solution of hypoxanthine and/or inosine was injected into the flow-line. The current decrease was recorded. Measurable responses were obtained for both analytes, with rectilinear ranges of 0.1 to 0.4 mM for hypoxanthine and 1.0 to 4.0 mM for inosine at 30°C. Storage of the sensor for 5 days at 25°C did not appreciably affect the response.