A flow system with a multi-channel peristaltic pump placed before the solenoid valves is proposed to overcome some limitations attributed to multi-commuted flow injection systems: the negative pressure can lead to the formation of unwanted air bubbles and limits the use of devices for separation processes (gas diffusion, dialysis or ion-exchange). The proposed approach was applied to the colorimetric determination of ammonium nitrogen. In alkaline medium, ammonium is converted into ammonia, which diffuses over the membrane, causing a pH change and subsequently a colour change in the acceptor stream (bromothymol blue solution). The system allowed the re-circulation of the acceptor solution and was applied to ammonium determination in surface and tap water, providing relative standard deviations lower than 1.5%. A stopped flow approach in the acceptor stream was adopted to attain a low quantification limit (42 microgL(-1)) and a linear dynamic range of 50-1000 microgL(-1) with a determination rate of 20 h(-1).

The separations of peptide and protein mixtures in capillary zone electrophoresis (CZE) at various solution conditions were studied with the direct control of electroosmosis. The zeta potential at the aqueous/capillary interface and the resulted electroosmosis in the presence of an electric field were directly controlled by using an additional electric field applied from outside of the capillary. The controlled electroosmotic flow affected the migration time and zone resolution of peptide and protein mixtures. The changes in the magnitude and polarity of the zeta potential caused the various degrees of peptide and protein adsorption onto the capillary through the electrostatic interactions. The separation efficiencies of peptide and protein mixtures were enhanced due to the reduction in peptide and protein adsorption at the capillary wall. The direct manipulations of the separation efficiency and resolution of peptide and protein mixtures in CZE were demonstrated by simply controlling the zeta potential and the electroosmotic flow with the application of an external electric field.

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A picrate ion-selective electrode without inner reference solution and based on bis(triphenylphosphoranylidene)ammonium picrate dissolved in 2-nitrophenyloctyl ether was constructed and its characteristics assessed. With respect to reducing sugar determination in Port wine, a tubular electrode based on the same sensor system was developed and incorporated into a flow-injection manifold devised for this determination. The methodology involves a reaction between the reducing sugar and picric acid, the decrease in the picrate concentration being monitored with the tubular electrode. A linear dependence between peak width, at a fixed potential and the logarithm of sugar concentration in the range 25-200 g 1~ ' was obtained. The proposed system allows wine samples to be injected with no previous treatment and allows a sample throughput of 50 determinations per hour, a relative standard deviation less than 4%, and the results are comparable to the reference procedures.

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