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A semicontinuous microbial assay for the determination of halogenated short-chain hydrocarbons in water samples was developed. The bacterium Xanthobacter autotrophicus GJ 10 forms dehalogenating enzymes, which liberate the halides in 1,2-dichloroethane as halogen ions. Cells of the organism were immobilized in chitosan beads and placed into a tube reactor, whose outlet was connected to a flow-through cell with chloride-selective potentiometric electrodes. Water samples were delivered continuously to the system, and the EMF was recorded. Both the difference in EMF between blank and sample and the velocity of the EMF change were used for calibration. The detection limit for 1,2-dichloroethane was below 0.5 mg/ L; the relative standard deviation was <10%. The effects of several parameters like flow rate and cell density were studied in detail.

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To improve the applicability and efficiency of the microbial assay developed by (1995) a stop-flow-technique was developed for the determination of halogenated hydrocarbons in water samples. Cells of Rhodocuccus sp. DSM 6344 were imniobilized in alginate beads and placed in a stirred flow-through reactor. The time of incubation, the bacterial cell density and the amount of alginate beads in the reactor on the response of the system as determined by the drop in EMF was investigated. Optimal conditions were achieved with 2 g beads containing a bacterial cell concentration of 0,l g cells wet wt/g matrix and an incubation time of 20 min. Calibrations with chlorinated and brominated substrates like ethyl bromide, 1 ,Zdl&romopropane, isobutyl bromide, 1 -chlorobutane and 1,5-dichloropentane showed a non-linear dependence at low substrate concentrations. The detection limits for ethyl bromide and l-chlorobutane were estimated as 0.02 mgA and 0.45 mg/l, respectively; the relative standard deviation was below 10 %. The great advantage of the stop-flow-technique compared to discontinuous measurements can be seen in a simplified handling and an increase of sample capacity.