Study On Application Of Laser-induced Breakdown Spectroscopy In Gas Detection

Laser induced breakdown spectroscopy?LIBS?is a detection and analysis method based on emission spectrum.The local high temperature of the laser focused region will atomize and ionize the sample to form plasma.A light with characteristic wavelength is emitted when an atom or molecule undergoes a transition from an excited level to a lower level.The qualitative information of samples can be obtained using the relationship between wavelength and specific elements,and the quantitative information can be obtained using the relationship between spectral intensity and element concentration.In principle,LIBS can analyze any matter regardless of its physical state,be it solid,liquid or gas and can detect all elements.Therefore,LIBS is widely used in many fields such as material processing,environmental monitoring,industrial production,biomedicine,biology and archeology.In this paper,the application research of gas detection based on LIBS technology is studied in UV-VIS range using a Nd:YAG pulse laser.The main work as follows:?1?The LIBS signal processing methods are studied.Firstly,an improved background removal method based on iterative discrete wavelet transform is developed to eliminate the interference from continuous background radiation in the LIBS spectrum.Secondly,the application of mathematical morphology in LIBS signal processing is explored,then a background removal algorithm combined with discrete wavelet transform and mathematical morphology is proposed.Thirdly,by analyzing the noise source of the LIBS signal,except Gauss white noise,a noise related to the intensity of the signal in the LIBS spectral is found.Then the noise is effectively suppressed by a method combined with stationary wavelet transform and sliding window.Finally,to improve the efficiency of LIBS spectral signal processing,an algorithm for LIBS signal automatic fitting based on continuous wavelet transform is proposed,which can adaptively extract peak information from LIBS spectra with different intensity and signal-to-noise ratio.?2?The characteristics of laser induced gas plasma are studied.Firstly,based on the theory of laser interaction with gas,the physical process involving in gas breakdown is determined by comparing the experimental threshold and the theoretical breakdown threshold which is calculated by considering different physical mechanisms.Secondly,the time resolved spectroscopy is used to study the temporal evolution of laser induced N₂ plasma temperature and electron density,as well as corresponding mechanisms.Results show that plasma temperature decays exponentially due to the radiation loss is less than the collision cooling,and three-body collision recombination is the main mechanism of electron decay.Thirdly,the temporal evolution of laser induced SF₆ plasma temperature is studied by using the time resolved spectroscopy.And then the components of laser induced SF₆ plasma are calculated by combining the ionization equation,the dissociation equation,the Dalton partial pressure law,the electric neutrality principle and the stoichiometry conservation law.The theoretical and experimental electron number density are compared.Finally,the influences of N₂/SF₆ ratio on plasma temperature,temperature change rate and electron number density were studied by measuring the plasma spectra with different N₂/SF₆ ratio.?3?The quantitative analysis method of gas component based on LIBS is also studied.Firstly,the calibration curve of O content in SF₆ is established using the internal standard method.The detection limit of 38ppm is obtained.The partial least squares regression model of O content in SF₆ is also established.Seconedly,using the ratio of OI:777.3nm line strength to continuum emission,the experimental calibration curves are obtained.Detection limits of oxygen concentration are determined to be31ppm in Ar and 41ppm in N₂.The reasons for the difference of O content detection limit in Ar and N₂ are explained by the physical processes of plasma formation and decay.Finally,the quantitative analysis model of the element component of SF₆/Air based on CF-LIBS is established,and the factors that affect the uncertainty of the calculation result are analyzed.