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Microtiter plate-based assays are a promising technique for toxicity assessment of substances. Chemicals with physicochemical properties such as high volatility and/or high lipophilicity, however, can be lost from the exposure solution during an experiment, so that exposure concentrations are not consistent. The aim of the present study was to determine and reduce the proportion of the reference compounds phenanthrene and phenanthridine lost during exposure in the zebra fish (Danio rerio) embryo test regime. It could be shown that under the standard exposure regime (48 h), the concentration of phenanthrene decreased strongly, by more than 99%, whereas that of phenanthridine decreased by 17% during a 48-h experiment. After modifications to the microtiter plate exposure regime, the phenanthrene concentration showed a decrease of only 40%, while the phenanthridine concentration remained unchanged. The major processes of substance loss could be assigned to accumulations of these substances into the glue of commercially available adhesive foils and the polystyrene walls of the microtiter plates. Furthermore, by investigating the sorption capacity of different plastics, it was found that the phenanthrene concentration decreased less when using a plexiglass specimen (28%) compared with the same-sized polystyrene specimen (94%). Moreover, it was found, for a constant exposure regime, that concentration profiles of different phenanthrene concentrations in the microtiter plate assay during an experiment were similar. A mathematical method is proposed to predict concentration profiles in an exposure solution by scaling a determined profile.

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