Typical calibration graph for nitrate samples injected into the optical fiber is shown in Fig. 5. Normalized fluorescence intensity at 575 nm detected from the optical fiber with suspended core is plotted against the concentration of nitrate. From the response curve, we find the intensity is obviously decreased when the concentration of nitrate increases. The fluorescence response of the ASA404 is nearly linear depending on the concentration of nitrate in the range 0.1–2.6 mmol/L with a linear regression equation is y = 0.0697 + 0.2295x (R2 = 0.9851). The limit of detection is about 0.05 mmol/L. Moreover, the dynamic response shows the intensity present a quickly change when filling the channel of the optical fiber within 5 s. In a fluorescence quenching reaction, the extent of mixing can greatly influence the intensity of the emission. This result reveals that the sample solutions are uniformly mixed and form steady microflows around the core of the optical fiber. In addition, we have determined the interference of some other contents during testing. We choose about 10 ions to do the experiments. Results show that Na+, K+, Mg2+, NH4+, Zn2+, NO3−, Cl− (500 times of the nitrite concentration) and Cu2+, Fe2+, Ni2+, Hg2+ (after complex with ethylene damine tetraacetic acid, 50 times of the nitrite concentration) have no obvious influence on the testing. The error can be controlled in less than 5%.