a bio - selective potentiometric electrode probe for the determination of amino acid concentrations in aqueous liquids comprising an ammonia gas analytical electrode provided at its tip with a closely adjacent thin layer of a fresh animal tissue containing , as a natural constituent thereof , an enzyme effective to catalyze degradation of the specific amino acid in analysis to either ammonia or to an intermediate compound subject to further degradation to ammonia by an additional enzyme , which ammonia is a function of the amino acid concentration in the liquid .