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USA - Trace metal speciation in natural waters
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CTS
USA - Trace metal speciation in natural waters In WATER AIR SOIL POLLUT. (90/1-2 (257-267) 1996) D.K. Nordstrom of the U.S. Geological Survey reports on 'Trace metal speciation in natural waters: Computational vs. analytical'. Improvements in the field sampling, preservation, and determination of trace metals in natural waters have made many analyses more reliable and less affected by contamination. The speciation of trace metals, however, remains controversial. Chemical model speciation calculations do not necessarily agree with voltammetric, ion exchange, potentiometric, or other analytical speciation techniques. When metal-organic complexes are important, model calculations are not usually helpful and on-site analytical separations are essential. Many analytical speciation techniques have serious interferences and only work well for a limited subset of water types and compositions. A combined approach to the evaluation of speciation could greatly reduce these uncertainties. The approach proposed would be to (1) compare and contrast different analytical techniques with each other and with computed speciation, (2) compare computed trace metal speciation with reliable measurements of solubility, potentiometry, and mean activity coefficients, and (3) compare different model calculations with each other for the same set of water analyses, especially where supplementary data on speciation already exist. A comparison and critique of analytical with chemical model speciation for a range of water samples would delineate the useful range and limitations of these different approaches to speciation. Both model calculations and analytical determinations have useful and different constraints on the range of possible speciation such that they can provide much better insight into speciation when used together. Major discrepancies in the thermodynamic databases of speciation models can be evaluated with the aid of analytical speciation, and when the thermodynamic models are highly consistent and reliable, the sources of error in the analytical speciation can be evaluated. Major thermodynamic discrepancies also can be
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Biosensors & Bwelectronics Vol. 12No. 5 (1997)
evaluated by simulating solubility and activity coefficient data and testing various chemical models for their range of applicability. Until a comparative approach such as this is taken, trace metal speciation will remain highly uncertain and controversial. Contact: U.S. Geological Survey, 3215 Marine Street, Boulder, CO 80303, USA.
Italy Peroxidase based amperometric biosensors In ANAL. CHIM. ACTA (328/1 (41-46) 1996) F. Mazzei, F. Botre, G. Lorenti & F. Porcelli of Universita 'La Sapienza' report on 'Peroxidase based amperometric biosensors for the determination of gamma-ammobutyric acid' This work presents the realization of enzymatic bioelectrodes, suitable for the determination of gamma-aminobutyric acid (GABA). The biosensors are based on the catalytic activity of the enzymes gamma-aminobutyric glutamic transaminase (GABA-T), succmic semialdehyde dehydrogenase (SSDH) and horseradish peroxidase (HPO). The fu'st two enzymes are usually indicated by the general term 'GABASE'. All the biosensors presented in this work are realized by immobilizing the enzyme HPO on the tip of an amperometric oxygen selective electrode: the resulting NADPH-sensitive biosensor is used in combination with GABASE to determine the concentration of GABA in aqueous samples. Since SSDH depends on the NADP+/NADPH equilibrium, it follows that, in the presence of HPO, the NADPH formed is oxidized to NADP, and the decrease in the concentration of dissolved oxygen is proportional to the concentration of NADPH and, in turn, to that of GABA. The experiments were performed either with GABASE free in solution or co-immobilized with HPO on the surface of the oxygen electrode. In the latter case, the immobilization of the three enzymes has been performed either on a single membrane or on two separated membranes. In both cases there is an almost perfect linearity between the electrode signal and GABA concentration in the range 5.0 x 105-1.2 x 103 M, with a lower
CTS
USA - Trace metal speciation in natural waters In WATER AIR SOIL POLLUT. (90/1-2 (257-267) 1996) D.K. Nordstrom of the U.S. Geological Survey reports on 'Trace metal speciation in natural waters: Computational vs. analytical'. Improvements in the field sampling, preservation, and determination of trace metals in natural waters have made many analyses more reliable and less affected by contamination. The speciation of trace metals, however, remains controversial. Chemical model speciation calculations do not necessarily agree with voltammetric, ion exchange, potentiometric, or other analytical speciation techniques. When metal-organic complexes are important, model calculations are not usually helpful and on-site analytical separations are essential. Many analytical speciation techniques have serious interferences and only work well for a limited subset of water types and compositions. A combined approach to the evaluation of speciation could greatly reduce these uncertainties. The approach proposed would be to (1) compare and contrast different analytical techniques with each other and with computed speciation, (2) compare computed trace metal speciation with reliable measurements of solubility, potentiometry, and mean activity coefficients, and (3) compare different model calculations with each other for the same set of water analyses, especially where supplementary data on speciation already exist. A comparison and critique of analytical with chemical model speciation for a range of water samples would delineate the useful range and limitations of these different approaches to speciation. Both model calculations and analytical determinations have useful and different constraints on the range of possible speciation such that they can provide much better insight into speciation when used together. Major discrepancies in the thermodynamic databases of speciation models can be evaluated with the aid of analytical speciation, and when the thermodynamic models are highly consistent and reliable, the sources of error in the analytical speciation can be evaluated. Major thermodynamic discrepancies also can be
iv
Biosensors & Bwelectronics Vol. 12No. 5 (1997)
evaluated by simulating solubility and activity coefficient data and testing various chemical models for their range of applicability. Until a comparative approach such as this is taken, trace metal speciation will remain highly uncertain and controversial. Contact: U.S. Geological Survey, 3215 Marine Street, Boulder, CO 80303, USA.
Italy Peroxidase based amperometric biosensors In ANAL. CHIM. ACTA (328/1 (41-46) 1996) F. Mazzei, F. Botre, G. Lorenti & F. Porcelli of Universita 'La Sapienza' report on 'Peroxidase based amperometric biosensors for the determination of gamma-ammobutyric acid' This work presents the realization of enzymatic bioelectrodes, suitable for the determination of gamma-aminobutyric acid (GABA). The biosensors are based on the catalytic activity of the enzymes gamma-aminobutyric glutamic transaminase (GABA-T), succmic semialdehyde dehydrogenase (SSDH) and horseradish peroxidase (HPO). The fu'st two enzymes are usually indicated by the general term 'GABASE'. All the biosensors presented in this work are realized by immobilizing the enzyme HPO on the tip of an amperometric oxygen selective electrode: the resulting NADPH-sensitive biosensor is used in combination with GABASE to determine the concentration of GABA in aqueous samples. Since SSDH depends on the NADP+/NADPH equilibrium, it follows that, in the presence of HPO, the NADPH formed is oxidized to NADP, and the decrease in the concentration of dissolved oxygen is proportional to the concentration of NADPH and, in turn, to that of GABA. The experiments were performed either with GABASE free in solution or co-immobilized with HPO on the surface of the oxygen electrode. In the latter case, the immobilization of the three enzymes has been performed either on a single membrane or on two separated membranes. In both cases there is an almost perfect linearity between the electrode signal and GABA concentration in the range 5.0 x 105-1.2 x 103 M, with a lower