Ma, R.; Xie, Z.-X.; Zhang, S.-W.; Chu, D.-Z.; Cao, X.; Wu, N., and Liu, Y.,2018. The orthogonal reference method for in-situ analysis of ammonia nitrogen content in seawater. In: Wang, D. and Guido-Aldana, P.A. (eds.), Select Proceedings from the 3rd International Conference on Water Resource and Environment (WRE2017). Journal of Coastal Research, Special Issue No. 84, pp. 36–43. Coconut Creek (Florida), ISSN 0749-0208.
A novel orthogonal reference method of ammonia nitrogen in-situ analysis in seawater is proposed. The method is based on the joint application of “water + reagent” blending device and constant-temperature heating device, which enhances the reaction rate of chromogenic agent and water samples. Water samples and ortho-phthalaldehyde (OPA) are fully mixed and heated in the circulation system up to 40°C by the constant temperature heating device. Next, the above mixture is injected into a flow pool, and fluorescence (400 nm to 480 nm) is excited by a 360 nm LED. Two photodiodes of the same type with the wavelength range from 300 nm to 500 nm are used to detect the fluorescent and LED reference signals, while the ammonia nitrogen content is calculated via the orthogonal reference algorithm. A robust reference procedure is proposed to improve the efficiency and accuracy of the operational amplifier algorithm used in the analysis by way of simplifying the microprocessor logical operation process. The embedded processing and microfluidic analysis technology used in the proposed method offers a good portable alternative to a fluorescence spectrophotometer. The results of performed in situ tests and their analysis are closely correlated with those obtained in laboratory conditions by the conventional method (R2 = 0.997). Within the salinity range of 24 to 32, a linear relationship between the fluorescent intensity and ammonia concentration from 3 to 500 μg/L is observed, with the standard deviation not exceeding 5%. The most critical advantage of the proposed detection method over the available ones is its real-time data acquisition and processing: the time required for a single measurement is less than 10 minutes. The results obtained strongly suggest that the proposed method has good feasibility and practicability, which can be used for in-situ measurement of ammonia nitrogen.
