Abstract: The concentration of a selected alkali metal cation in an aqueous sample is determined using an aqueous fill solution containing a dissolved polymeric cationic material (e.g., copper--PEI) and a dissolved fluorescent anionic material (e.g., ANS) and, in contact with the fill solution, an ionophore selective for the selected alkali metal cation. The aqueous fill solution is contacted with an aqueous sample through a membrane, the membrane being permeable to alkali metal cations, but impermeable to the polymeric cationic material. Fluorescence is detected either from molecules of fluorescent anionic material which have migrated from adjacent to the polymeric cationic material to adjacent to ionophore-complexed alkali metal cations or from molecules of fluorescent anionic material which have remained adjacent to the polymeric cationic material, or from both, especially through a fiber optic.