Study On The Method Of Self-absorption Effect Inhibition In Laser-induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy(LIBS)is a rapid elemental analysis technique,which uses a high-energy pulsed laser to focus onto samples to generate plasmas.The elemental species type and contents of the samples are deduced according to the characteristic wavelength and spectral wavelength and intensity of the plasma emission spectrum.LIBS has become a research hotspot due to its inherent advantages such as fast,in-situ,microdestruction,no sample preparation,multi-elemental simultaneous analysis,and remote detection.LIBS have been applied in metallurgical industry,agricultural production,environmental monitoring,food safety,biomedical and space exploration.However,the emission spectrum is seriously affected by self-absorption in LIBS,and the linear mapping relationship between spectral intensity and element concentration is destroyed,which leads to poor quantitative analysis accuracy when LIBS is compared with traditional elemental analysis techniques.Thus,thise disadvantage hinders the further development and application of LIBS.Based on the study of the self-absorption effect in LIBS,this thesis mainly explores two complementary self-absorption reduction methods,namely self-absorption reduction of a single element and self-absorption reduction of multi-elements.For the self-absorption reduction of a single element,according to the principle of stimulated absorption,a wavelength tunable laser is used to selectively excite the energy transition of low-level particles in the plasma to greatly reduce the number of low-level particles in the outer layer of the plasma,thereby effectively inhibiting self-absorption of the emission spectrum from the plasma center.For the self-absorption reduction of multi-element,is to use of microwave radiation to excite low-level particles in laser-induced plasma to achieve simultaneous reduction of multi-elemental self-absorption.The main research results and innovations obtained in this paper are as follows:(1)According to the physical mechanism of the self-absorption effect and the time evolution of the self-absorption effect by the exponential curve fitting method,the results show that due to the high plasma temperature and strong ionization at the early stage of plasma evolution,the number density of the lower-level particles is lower,and the selfabsorption effect is weaker.Therefore,a method for acquireding spectra at the early stage of plasma evolution to reduce the self-absorption effect is proposed.Quantitative analysis of Cu and Mn elements by this method showed that the average relative error decreased by 75% and 91%,respectively.(2)Based on the intrinsic physical properties of the plasma,a method of laser-stimulated absorption to reduce the self-absorption effect of LIBS is was proposed.The method uses a wavelength tunable OPO laser to irradiate the entire plasma,selectively exciting low-level particles to a high energy level matched to the wavelength of the OPO laser,and reducing the number of low-level particles,thereby reducing absorption of the emitted light from the plasma center.The method can effectively reduce the self-absorption effect while eliminating the spectral interference of the matrix.The experimental results show that the average relative error of Mn element determination in microalloyed steel and Cr element in alloy structural steel is reduced by 93% and 83%,respectively.(3)Based on the laser-stimulated absorption,a method of spatially selective laserstimulated absorption is was proposed.The spatial distribution of each element in the plasma was studied.It is found that the distribution of the Cu and Cr element lines of the microalloyed steel sample are very different in the plasma,and the absorption region of the spectral line in the plasma is stimulated by spatial selection.The self-absorption effect is effectively reduced.Compared with the performance when using the laser-stimulated absorption method,the self-absorption factor ? of the Cu I 324.75 nm and Cr I 425.43 nm lines in the microalloyed steel samples is improved by 54% and 11%,respectively.(4)A new method of multi-elemental self-absorption reduction by using microwaveassisted excitation(MAE-LIBS)in a wide spectral range(200~900 nm)is proposed.In MAELIBS,the electrons in the outer layer of the plasma absorb the microwave energy and accelerate the movement,and the low-level particles are excited by the collision,so that the low-level particles are transitioned to the excited state,and the low-level particles density is greatly reduced,thereby it effectively reduces the self-absorption effect.Further studies have shown that this method has no wavelength selectivity and therefore can simultaneously reduce the self-absorption effect of multiple elements.The experimental results show that the self-absorption effect of multi-elemental spectral lines is significantly reduced.For example,in potassium feldspar,the full width at half maximum of Na I 589.0 nm,Na Iand 589.6 nm,K I 766.5 and 769.9 nm lines decreased by 43%.,43%,53% and 47%,respectively.Based on the intrinsic physical properties of plasma,this thesis attempts to reduce the self-absorption effect from its source.The method of acquireding spectroscopy at early plasma evolution,laser-stimulated absorption method,space selective laser-stimulated absorption method and microwave assisted excitation method are studied.The reduction of self-absorption effect is significantly effective,which greatly improves the precision accuracy of LIBS,promotes the development of LIBS,and has broad application prospects.