Abstract: A flow injection method is described for the selective measurement of chromium(VI) in aqueous solutions. This method is based on the dynamic quenching of ruthenium(II) fluorescence. The detection limit is 0.43 ppm and 40 samples can be analyzed per hour. Selectivity is demonstrated over ferrous, nickel, cupric and zinc cations and no effect is observed from sulfate, chloride, berate and phosphate. Some interference quenching was measured for cyanide and nitrate, but the method is more responsive to chromium(VI) by factors of 10.2 and 82, respectively. The effects of solution pH, carrier stream flow rate and ruthenium concentration are demonstrated. Results indicate the method is suitable for measuring chromium(VI) in effluents from electroplating baths.

Abstract: Response properties and selectivity are reported for glutamate biosensors constructed with a film of Nafion between the platinum anode and a layer of immobilized glutamate oxidase. The effects of enzyme loading, sample pH and temperature are established. Operation at pH 7.8 and 37°C results in linearity up to 800 µM and a limit of detection of 0.3 µM. Nafion enhances selectivity for glutamate over test species that include ascorbic acid, uric acid and acetaminophen. Selectivity enhancement was greater over the anionic interferences because of electrostatic repulsion and the extent of this enhancement depends on the thickness of the Nafion layer. Even under ideal conditions, some interfering signal is observed when glutamate levels are ten-times less than ascorbate.

Abstract: Two sensor designs were used. In one, the test solution was contained in a horizontal gas-permeable PTFE tube with an optical probe (four optical fibers within a glass tube) inserted into either end to direct source radiation through the solution In the other, a flow-through arrangement, four plastic optical fibers were inserted into the PTFE tubing, followed by a stainless steel tube and the space between the steel tube and the PTFE tubing was filled with epoxy-resin. The test solution was impelled through the inner chamber by a peristaltic pump. All sensors were enclosed in a thermostatted cell at 25°C with 50 µM-bromothymol blue/4 mM NH4Cl/0.151 M NaCl as internal solution The sensors were calibrated by recording the output during passage of aqueous NH3 through the outer chamber. Operation in stop-flow mode gave response times of 15-25 min for 0.75-11.25 µM-NH3; in continuous-flow mode, the response time was 5 min for 0.7-47 µM-NH3 but measurement sensitivity was impaired. The response was linear up to 2.3 µM-NH3 with a detection limit of 47 nM or better.

Abstract: A flow injection system is described for the determination of H2O2 based on the chemiluminescent reaction with 10-methyl-9-phenoxycarbonylacridinium (I). The carrier stream is pumped at 4 mL min-1 and consists of a standard H2O2 solution in boric acid buffer solution of pH 9; the concentration. of the standard solution cover the range 0.1 µM to 1 mM H2O2. I reagent (20 µL) is injected into the carrier stream and the solution are mixed by passage through a coiled incubation tube. The chemiluminescence reaction is initiated by adding a base just before the sample passes in front of a photomultiplier tube. The detection limit is 0.25 µM. By coupling this system to an enzyme column it is possible to determine glutamate, the H2O2 formed as glutamate passes through a reaction column of L-glutamate oxidase being determined by the above procedure. The detection limit is 0.5 µM with a throughput of 300 samples h-1. An automated anal. system is described for the measurement of hydrogen peroxide based on a chemiluminescence reaction with Ph 10-methylacridinium-9-carboxylate (PMAC). A reversed FIA experimental arrangement is used to establish the operating conditions for the measurement of submicromolar levels of hydrogen peroxide. The carrier stream consists of hydrogen peroxide standards prepared in a pH 9.0, boric acid buffer and the flow rate for this carrier/sample stream is 4 mL/min. Twenty microliters of a 10 mM PMAC solution, prepared in a pH 3 phosphate buffer, are injected into the carrier/sample stream. Hydrogen peroxide mixes with the PMAC reagent in an incubation coil that is constructed by wrapping 107 cm of polyethylene tubing around a 1 cm o.d. plastic rod. The chemiluminescence reaction is then initiated by adding a base just before the sample passes in the front of a photomultiplier tube detector. The calculated limit of detection (S/N = 3) for hydrogen peroxide is 0.25 µM. In addition, the pH dependent hydrolysis of the PMAC reagent is characterized by an HPLC method which has been specifically developed for the separation and detection of the hydrolysis products of PMAC. Results indicate that a pH of 3.0 is required for long term stability of the PMAC reagent. Finally, this system has been successfully extended to the measurement of glutamate by coupling a bioreactor column of glutamate oxidase with the hydrogen peroxide detection scheme. A detection limit (S/N = 3) of 0.5 µM has been established for glutamate with a throughput of 200 samples per h.

Abstract: An absorbance based flow-through fiber-optic probe is developed for measurements of ammonia in neurochemical samples. The probe is constructed by trapping a small volume of an internal indicator solution between two sets of optical fibers. This indicator solution is separated from a flowing sample stream by a microporous gas permeable membrane. The flow-through design permits analysis of small aliquots of sample which are injected and pass across the gas permeable membrane. Ammonia in the sample diffuses across this membrane, enters the internal solution, and alters the distribution of a pH sensitive chromophore. The resulting change in absorbance is measured and related to the sample ammonia concentration. The final ammonia sensor possesses a 0.2-20 µM dynamic range with a detection limit of 0.2 ± 0.1 µM and response time of 8 min. The utility of this ammonia sensor is illustrated by measuring extracellular ammonia levels from perfused rat hippocampal tissue. These measurements indicate extracellular ammonia levels fluctuate during tissue depolarization, which is consistent with our findings with other neurological tissues.