Measurement Of Dissolved Oxygen Concentration In Seawater Using Time-Domain Fluorescence Lifetime Method

The concentration of dissolved oxygen reflects the health status of marine ecosys-tems.It is an important indicator of the quality of seawater.Therefore,the fast and accurate measurement of dissolved oxygen concentration is of great significance for marine ecological environment safety management.Compared with the traditional measurement method,the fluorescence lifetime detection method does not consume oxygen,and it is easy to maintain and easy to integrate.However,the current method for fluorescence lifetime detection has higher requirements on the performance of the excitation light source,and it takes a long time for stable measurement.Aiming at the technology and application requirements of seawater dissolved oxygen concentration detection,this paper studied the rapid measurement technology of seawater dissolved oxygen concentration based on time-domain fluorescence lifetime detection method.Firstly,the principle of time-domain fluorescence lifetime measurement was ana-lyzed and a fluorescence lifetime experimental measurement system was constructed.The correlation between different excitation light intensity as well as excitation duration and fluorescence lifetime measurement results was studied.The results show that the fluorescence quenching curve tends to be a single exponential decay as the intensity of the excitation light increases or the duration of the excitation rises,and the fluorescence lifetime detection results remain stable,when the excitation light intensity changes within the range of 1037-2690pmol quanta/m2/s or the excitation duration changes within the range of 3?20?s.Secondly,aiming at the requirements of rapid monitoring or vertical profile of dis-solved oxygen in the marine environment,and the requirements for miniaturization and integration of the detection system during in-situ monitoring of complex sea conditions,a two-point integral fluorescence lifetime detection technique was studied.The LED was used as the excitation light source and the MPPC was used as the photodetector to design the detection system circuit.Focusing on the gate signal pulse light emission driving technology,the LED edge acceleration circuit was designed,decreasing the time of rising and falling edges.The fast fluorescence signal integration detection tech-nology was studied.High sensitivity integral amplification of weak fluorescence signals under different dissolved oxygen concentration was achieved by using the MPPC bias temperature compensation circuit,hardware integral and variable gain circuit.The re-sults show that the standard deviation of the two-point integral fluorescence lifetime test results decreases with the increase of the number of excitations,and the fluores-cence lifetime can be stably obtained within 2.5s.The relative standard deviation of the 10 consecutive measurements is 0.38%.On this basis,the inversion method of fluorescence lifetime and dissolved oxygen concentration as well as the effects of changes in typical marine environmental factors such as salinity,pH and temperature on dissolved oxygen and compensation methods were studied and a miniaturized time domain fluorescence lifetime detection system was developed.The results show that there is a fixed delay of 10.38?s of hardware system between the measured fluorescence lifetime and theoretical fluorescence life-time.The correlation coefficient of Stern-Volmer equation between the measured fluo-rescence lifetime and dissolved oxygen concentration increases from 0.946 to 0.998 after system delay compensation.The concentration of dissolved oxygen concentration is not affected by salinity and pH,but is linearly related to temperature.By temperature compensation of dissolved oxygen concentration,the linear correlation coefficient with actual dissolved oxygen is above 0.995.The relative standard deviation of continuously measurement in a saturated dissolved oxygen solution for 2 hours of the small-scale seawater dissolved oxygen concentration meter is 2.6%.The detection limit at a low concentration of 0.17mg/L is 0.03mg/L.The results provide a technical basis for the rapid measurement of dissolved oxygen content in seawater.