Individual and joint stopped flow kinetic spectrofluorimetric determination of neomycin and tyrothricin

Kinetic methodology was used for the first time to develop a simple, fast spectrofluorimetric method for the determination of neomycin and tyrothricin using the stopped-flow mixing technique. The proposed method involves reaction of these antimicrobial substances with o-phthaldialdehyde in the presence of ndashacetylcysteine, which is used as a thiol reagent. Neomycin and tyrothricin can thus be determined simultaneously by applying the proportional-equation method to the initial rate and signal increment of the kinetic curves obtained for each analyte. The reaction rate can be determined in ca. 1 s and the signal increment in ca. 3 s. The calibration graph was linear over the range 0.07–70 μg ml−1 neomycin and 0.08–40 μg ml−1 tyrothricin. The mid-range relative standard deviation was less than 3%. The proposed method was applied to the individual determination of these compounds in pharmaceutical samples, with a recovery of 98.1–103.6% for neomycin and 92.0–108.0% for tyrothricin. Synthetic mixtures of neomycin and tyrothricin in ratios between 1:4 and 4:1 were also satisfactorily analysed.