| The ocean is an important node in the global methane（CH4）cycle.Concentration measurement of dissolved methane plays a crucial role in methane flows research.With the rising international attention to ocean even deep-sea exploration,scientific research such as exploration of deep-sea combustible ice enrichment areas,study of environmental characteristics of deep-sea ecosystems,and the exploration of cold springs and hydrothermal areas are inseparable from the precise detection of dissolved CH4concentration.The infrared absorption spectroscopy based on membrane technology has a good application prospect in in-situ dissolved gas detection,but this technology still has many limitations.To meet the needs of rapid,real-time detection of dissolved CH4,this paper proposes a rapid detection of CH4using Quartz Enhanced Photoacoustic Spectroscopy（QEPAS）technology.In this thesis,by combining the fiber-coupled incident laser with the structure of a half on-beam resonator tube,a sub-milliliter photoacoustic cell is designed,and a CH4rapid detection system based on QEPAS is developed.In this paper,the relevant basic theories of infrared absorption spectroscopy,such as the Beer-Lambert law and the spectral line type function,are expounded,and the photoacoustic effect and the principle of photoacoustic signal generation in photoacoustic spectroscopy（PAS）is introduced.The advantages of QEPAS technology over PAS technology are analyzed.The piezoelectric characteristics and equivalent circuit model of tuning forks are analyzed.The important parameters and related measurement methods of tuning forks are introduced.In terms of QEPAS optical path design,the optical fiber is directly coupled to incident laser to replace the spatial optical path,and the overall optical path is shortened to less than 5 mm.At the same time,a single resonator tube is used to amplify the sound wave,whose amplification reaches 8.4 times.Its total gas consumption is only 300μL.The CH4absorption spectral line at 6046.93 cm-1is selected as the target,corresponding to a wavelength of 1653.73 nm.In terms of circuity,combing the tuning forks parameter calibration with optical acoustic signal detection,a transimpedance preamplifier is designed to convert and amplify the tuning fork signal.Finally,a DFB laser with a central wavelength of 1654 nm is used as the laser source.The scanning and modulation signals are applied to the laser through the Lab VIEW program to detect the second harmonic signal.For the experimental system,the optimized parameters,such as the distance between fiber and the resonator tube,the length of the resonator,the modulation depth and gas flow rate.Based on the QEPAS system,different concentrations of CH4are measured and the linearity between the measured CO2concentration and the QEPAS signal amplitude could reach 0.9989.The system response time is about 1 s.An Allan deviation analysis is performed to evaluate its stability in long-time detection,the minimum detection limit is 1.1 ppm at an integration time of 1245 s.Finally,the sub-m L photoacoustic cell is used to verify the performance of in-situ deep-sea detection.During the 150 min system equilibrium time,the concentration of dissolved CH4near the cold spring vent was measured as 6%.The feasibility of the design in-situ detection is verified. |
