An inexpensive flow injection analysis system for determining low concentration levels of nitrite employing the Griess reagent spectrophotometric method is reported. The novel photometric detector applied within this manifold is a highly sensitive, low cost, miniaturized light emitting diode (LED) based flow detector. This colorimetric detector employs two LEDs, operating one as a light source and the other as a light detector. The emitter LED is forward biased and the detector reverse biased. The emitter and detector LED had a λmax of 530 nm and 623 nm respectively. The emission spectrum of the emitter LED efficiently overlapped with the absorbance spectrum of 9 µM NO2 and Griess reagent complex. A simple timer circuit measures the time taken for the photocurrent generated by the emitter LED to discharge the detector LED from 5 V (logic 1) to 1.7 V (logic 0). The Griess reagent method employed for nitrite determination is based on the formation of an azo dye, the intensity of which is directly related to nitrite concentration. The linear range, reproducibility and limit of detection were investigated. Detection limits in the nanomolar range were achieved using the Paired Emitter-Detector Diode (PEDD) flow analysis device. For a comparative study the linear range and limit of detection were also investigated using a platewell reader. Higher sensitivity and improved precision were obtained from the PEDD compared to the commercially available plate well reader.