Flow-injection-electrochemical oxidation fluorimetry for determination of methotrexate

A novel fluorimetric model of on-line electrochemical oxidation for the determination of methotrexate (MTX) is described in this paper. The method was based on the oxidation of MTX to a highly fluorescent product, 2,4-diaminopteridine-6-carboxylic acid, by constant potential in the electrochemical oxidation flow cell. Stopped-flow techniques were employed. Under optimal conditions, the fluorescence intensity of 2,4-diaminopteridine-6-carboxylic acid was measured at excitation and emission wavelengths of 380 and 465 nm, respectively. The calibration graph was linear over concentrations of methotrexate in the range 2.0 x 10(-7)-1.0 x 10(-5) g/mL, with a detection limit of 5.2 x 10(-8) g/mL (3sigma). The method has been successfully applied to the determination of methotrexate in human urine samples and showed a percentage recovery in the range 94.3-102.5%.