A simply fabricated microfluidic device integrated with a fluorescence detection system has been developed for on-line determination of ammonium in aqueous samples. A 365-nm light-emitting diode （LED） as an excitation source and a minor band pass filter were mounted into a polydimethylsiloxane （PDMS）-based microchip for the purpose of miniaturization of the entire analytical system. The ammonium sample reacted with o-phthaldialdehyde （OPA） on-chip with sodium sulfite as reducing reagent to produce a fluorescent isoindole derivative, which can emit fluorescence signal at about 425 nm when excited at 365 nm. Effects of pH, flow rate of solutions, concentrations of OPA-reagent, phosphate and sulfite salt were investigated. The calibration curve of ammonium in the range of 0.018- 1.8 μg/mL showed a good linear relationship with R2 = 0.9985, and the detection limit was （S /N = 3） 3.6 × 10 4 μg/mL. The relative standard deviation was 2.8% （n = 11） by calculating at 0.18 μg/mL ammonium for repeated detection. The system was applied to determine the ammonium concentration in rain and river waters, even extent to other analytes fluorescence detection by the presented device.
