RESUMO
The high sensitivity that can be attained by enzymatic amplification via substrate cycling has been verified by on-line interfacing of a rotating bioreactor and continuous-flow/stopped-flow/continuous-flow processing [Raba, J.; Mottola, H. A. Anal. Biochem. 1994, 220, 297-302]. The determination of glucose levels was possible with a limit of detection of 0.2 fmol·L(-)(1) in the processing of as many as 30 samples per hour. Determination at such low levels is of interest in several situations encountered in fermentation biotechnology and clinical chemistry, and this determination in culture broths illustrates the capabilities of the proposed approach. The glucose oxidase/glucose dehydrogenase coupled system was used by immobilizing glucose oxidase (EC 1.1.3.4) on the top of a rotating disk while glucose dehydrogenase (EC 1.1.1.47) was immobilized on the top part of the flow-through cell. Substrate cycling was realized via NADH/NAD(+) that, in conjunction with glucose dehydrogenase, regenerates glucose, the substrate in the glucose oxidase-catalyzed reaction. This cycling permits generation of H(2)O(2) (detected at Pt ring electrode concentric to the rotating disk) beyond stoichiometric limitations. This permits a 100-fold increase in the sensitivity for glucose determination when compared with the determination involving no substrate cycling.
RESUMO
A simple and sensitive potentiometric sensor for end-point detection in the automatic titration of Zn(II) with EDTA was prepared and studied. The sensor was based on a conventional carbon paste which was mixed during preparation with 4-(3,5-dichloro-2-pyridil-azo)-1,3-diaminobenzene (3,5-Cl2PADAB). pH effects, buffer concentration, reagent content into carbon paste and the presence of foreign salts on the electrode response were studied. Titration curves with sharp end-point breaks were obtained in Zn(II) concentration range from 0.5 to 6550 ppm. The electrode was easily made and with very inexpensive materials. A titrimetric method for the determination of zinc in insulin by automatic potentiometric end-point detection is described. It was applied to the determination of the metal in two commercial pharmaceutical preparations. The results were in good agreement with those obtained by the U.S. Pharmacopeia standard method.
RESUMO
A titrimetric method for determination of sulfide and of sulphur in steels with chloranilic acid by biamperometric and automatic potentiometric end-point detection is described. The construction of the sensor for potentiometric indication is also described. The results obtained agree with those of the iodine-thiosulfate method and with the certified values for the steels.