Study On The Calibration-free Laser-induced Breakdown Spectroscopy

Laser-induced Breakdown Spectroscopy(LIBS)is an elemental analysis method which obtains the qualitative and quantitative information of the sample by analyzing the wavelength and intensity of the plasma emission spectrum generated by converting laser pulses to ablate and atomize a small amount of sample.Since LIBS uses pulsed lasers as excitation sources,it has many advantages,such as no sample preparation,near lossless,remote analysis and multi element simultaneous analysis.In recent years,LIBS has been widely used in many areas of analysis.It is sometimes difficult to find a matrix matching standard sample or to ensure that the experimental conditions are constant,which greatly limits the application of LIBS.Based on this,the Calibration-Free Laser-Induced Breakdown Spectroscopy(CF-LIBS)method,which does not require the use of standard samples to establish calibration curves,is proposed and widely studied by researchers.Since the CF-LIBS is to describe the physical state of the laser induced plasma by a simple mathematical model,so that it can overcome the adverse effects of matrix effects.However,the laser-induced plasma is very complex and can not be described with a simple physical formula.Therefore,the analytical accuracy of CF-LIBS has not yet reached the level of standard LIBS or other elemental analysis techniques.In view of the unique advantages of CF-LIBS,understanding and improving the CF-LIBS algorithm has important theoretical and practical value.In this thesis,the factors influencing the accuracy of CF-LIBS analysis are analyzed from the aspects of experiment setup and basic theory hypothesis,and the algorithm is improved from the aspects of acquisition delay and self-absorption correction.The main research contents are as follows:1.The basic principles and characteristics of LIBS are reviewed;The development course of LIBS is introduced according to the experimental sequence:the interaction between laser and matter,the spectral pretreatment,qualitative and quantitative analysis;The application of LIBS,related academic conferences and existing equipment is introduced;The domestic and foreign research progress of CF-LIBS analysis method is introduced in detail.2.The experimental system for CF-LIBS analysis is set up and the main broadening mechanism of plasma emission spectrum is reviewed.The influence of the laser pulse number on the spectral stability is studied;The importance of wavelength calibration and relative efficiency correction of the spectrometer is discussed;The main spectral broadening mechanisms are analyzed:natural broadening,Doppler broadening,Van der Waals broadening,resonance broadening,Stark broadening,self-absorption broadening and instrumentation broadening.3.Based on the theory of thick and inhomogeneous one-dimensional plasma,the thin and uniform plasma,which is required by CF-LIBS,is derived.The algorithm flow and the basic theory of CF-LIBS are introduced in detail.The main factors affecting the accuracy of CF-LIBS calculation and the improvement of the algorithm in recent years are reviewed.4.The time-resolved CF-LIBS is studied.The interaction between laser and matter and the evolution of plasma emission spectra is introduced;The laser-induced slag plasma emission spectra at different delay times are collected;The find peaks,identify peaks and peak fitting algorithm of linear spectrum are introduced in detail;The different methods to calculate the plasma temperature is introduced;The influence of delay time on the accuracy of CF-LIBS is studied;The calculation results are explained from the aspects of self-absorption and local thermodynamic equilibrium.5.In order to overcome the influence of the main elements self-absorption and the trace elements scarce spectrum,the standard reference line method is proposed based on the local thermodynamic equilibrium assumption that all species present in the plasma have the same temperature value.The electron density and the plasma temperature is calculated based on the main element spectrum,and the standard reference line which has the best signal-to-noise ratio is used to calculate the corresponding standard reference concentration.Compare the results of the standard reference lines with and without self-absorption.The selection method of the standard reference line is optimized by calculating self-absorption coefficient of the main element line.The main innovations of this thesis are as follows:(1)The time-resolved CF-LIBS study was carried out successfully.The accuracy of CF-LIBS quantitative analysis with the experimental spectrum acquisition delay was obtained.The results are explained from self-absorption and local thermodynamics equilibrium.(2)In order to overcome the influence of the main element line's self-absorption and trace elements scarce spectrum,the standard reference line method is proposed.(3)Because the selection of the standard reference line is easily influenced by the subjective factors,theoretical calculated self-absorption coefficient is used to optimize the selection of standard reference lines.