Abstract: The fluorescence characteristics of complexes of nalidixic acid with alkali earth metals(Mg,Ca,Sr and Ba) have been studied in this paper.It was found that in the pH 7～9 media the alkali earth metals ions could form stable fluorescent complexes.Their fluorescence intensities enhanced with the decrease of ionic radii of alkali earth metals.Among them the fluorescence intensity of magnesium-nalidixic acid system was 58 folds more than that of nalidixic acid itself.Anew method for the determination of nalidixic acid using the first order derivative fluorescence excitation spectrum has been developed.The detection limit was found to be 0.015μg/mL.This method has been used to determine nalidixic acid in human urine with recoveries of 96.6%～102.6%.

Key words: Nalidixic acid, Magnesium ions, First order derivative excitation spectrum fluorimetry