Flow-through fiber-optic ammonia sensor for analysis of hippocampus slice perfusates

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.