Development and characterization of a highly sensitive fluorometric transducer for ultra low aqueous ammonia nitrogen measurements in aquaculture

The accurate measurement of ammonia nitrogen concentration is of great significance for the study of aquatic ecological processes. Unfortunately, the commonly used colorimetric and ammonia sensitive electrode methods suffer from poor consistency, especially when the concentration of ammonia nitrogen is low. This paper presents the development and characterization of a highly sensitive fluorometric transducer, intended for measuring ultra-low ammonia nitrogen concentration in natural waters. The design of the transducer is based on molecular fluorescence analysis and focused on the ammonia-orthophthaldialdehyde-sulfite complexation reaction in alkaline aqueous solutions. LED excitation and photodiode detection are integrated in a compact pressure hull, with a small dichroic beam-splitter installed on the optical path. The beam-splitter reflects a small percentage of ultraviolet light into the reference photodiode to evaluate variations of the excitation energy and simultaneously guide fluorescence light into the sample photodiode by total internal refection. As a major innovation, pseudo-random sequence modulation and self-calibration are introduced into the amplitude measurement of the fluorescence for the first time. Specifications include a limit of detection (LOD) of 0.16 μg·L−1, a measurement range of 0-419.31 μg·L−1, and the R-square of 0.99. Two sets of algorithms with inputs of pH, temperature, salinity, and atmospheric pressure are embedded into the device for the high-precision measurement.