Development Of A WMS-based Methyl Mercaptan Analyzer Using Multicomponent Spectral Fitting

Methyl mercaptan?CH₃SH?is a colorless,high toxic gas with a foul odor,which is produced in both industrial and agricultural activities,and is also a metabolic byproduct.It is also the main component in bad breath caused by periodontal disease.However,due to the band absorption of CH₃SH and gas interference in the actual test conditions,it is urgent to develop a related technology and instruments with high selectivity,high sensitivity and low detection limit for methyl mercaptan monitoring.Firstly,an algorithm based on wavelength modulation spectroscopy and multicomponent spectral fitting was proposed.The mathematical model was established to give a detailed theoretical derivation and the algorithm flow was designed.Then the algorithm proposed was verified by both simulation and experiments.The simulation was performed under different signal to noise ratio levels to evaluate the capacity of resisting disturbance of the algorithm.The algorithm was applied to the simultaneous detection of CH₃SH,CH₄ and H₂O,while the weight of the fitting process was selected.The results showed that the uncertainties of CH₃SH,CH₄ and H₂O detection were no more than 2%.The detection limit of CH₃SH were6.3 ppb.Thus the algorithm is valid for simultaneous detection of multicomponent and can achieve the CH₃SH detection with high sensitivity.Secondly,a methyl mercaptan analyzer was integrated and debugged.The optical block,the gas block and circuit block were considered to integrate the methyl mercaptan analyzer.The human-computer interaction function was achieved by touch-screen programming.To reduce the disturbance of optical fringes,the machine structure of the laser mount was adjusted;then the modulation parameter was optimized to restrain the remaining etalon fringes.Thirdly,the performance of the analyzer was evaluated by a series of experiments.The result of the experiments was used to evaluate the analyzer`s repeatability,linearity,.zero draft,detection limit et al.The sources of the analyzer`s measurement uncertainty were also discussed.The result showed the linear fitting correlation of99.97%,detection limit of 10.6 ppb,uncertainty of measurement of 0.14%FS,which fitted for the applications of CH₃SH monitoring.The algorithm based on wavelength modulation spectroscopy and multicomponent spectral fitting was suitable for the simultaneous detection of components in mixture and band absorption measuring with a good antijamming capability.The methyl mercaptan gas analyzer developed in the paper was of high sensitivity,specific detecting ability,and easy operation.The analyzer can meet the requirement of real time and online methyl mercaptan monitoring.