Investigation On The Self-absorption Suppression Of LIBS Based On Several Typical Elements

Laser-induced breakdown spectroscopy has unique advantages such as rapid,in-situ,online,and full-element analysis.It has been initially applied in the fields of metallurgical industry,environmental protection,geological exploration,space exploration and biomedicine.However,the self-absorption effect will cause the spectrum intensity to weaken.The center of the spectrum line will be depressed,forming a double peak in severe cases,which is called"self-reversal".The self-absorption effect causes the linear relationship between the spectral line intensity and the element concentration to deteriorate,which reduces the accuracy and precision of the quantitative analysis of laser-induced breakdown spectroscopy.It seriously affects the analysis performance of the LIBS system and restricts the further development and application of this technology.This paper systematically studies the self-absorption effect.Aiming at the self-absorption effects of several typical elements(K,Na,Ca,Al and Si),a method of selecting the laser wavelength and changing the ambient gas to adjust the plasma excitation and expansion process is proposed to eliminate the self-absorption effect.The main research results and innovations are as follows:(1)Based on the physical mechanism of plasma generation,through the study of the difference between the interaction mechanism of different laser wavelengths and materials,comparative analysis of the self-absorption change law of the plasma generated by the excitation to obtain the optimal laser wavelength.Three typical laser wavelengths were selected-ultraviolet(355 nm),visible(532 nm)and infrared(1064 nm)to study the effect of laser wavelength on the self-absorption effect of LIBS.The study found that when the laser wavelength is 1064 nm,the electron number density of the plasma is lower and the number of ground state particles is less,so that a relatively weak self-absorption plasma can be obtained.Compared with lasers with wavelengths of 355 and 532 nm,when the wavelength is 1064 nm,the FWHM of the K I 766.5 nm line is reduced by 58%and 22%,and that of Na I 396.2 nm is decreased by 39%and 23%,respectively.The FWHMs of the other elements Si,Al,and Ca all decreased.(2)After adopting the excitation wavelength of 1064 nm and obtaining weak self-absorption plasma,in order to further suppress the self-absorption effect of LIBS,it is proposed to control the expansion and evolution process of the plasma by changing the atmosphere environment of the plasma to suppress the self-absorption effect.The effects of argon,air,nitrogen,and helium on the plasma expansion of typical elements K,Na,Si,Al,and Ca were studied.It was found that under the argon atmosphere,the expansion rate of the plasma was significantly reduced.Because argon has a lower ionization energy and a larger molar mass,it effectively reduces the number of ground-state particles around the plasma.Compared with the air condition,the self-absorption coefficients of K,Na,Si,Al,Ca elements under argon atmosphere increased from 0.170,0.181,0.373,0.556,0.224 to 0.377,0.361,0.411,0.685,0.292.(3)On the basis of optimizing the laser wavelength of 1064 nm and the argon environment to dually regulate the plasma generation and evolution process,quantitative analysis of the high content of potassium and sodium elements in the saline-alkaline soil.Compared with traditional LIBS,The argon atmosphere can well restrain the self-absorption effect on the quantitative analysis of LIBS.The experimental results show that when argon gas is used as the protective gas,the correlation coefficient R~2 of the calibration curve of the K and Na elemental spectra in the soil is increased from 0.936,0.959 to 0.994 and 0.995;the RMSECV is improved to 0.75 wt.%And 0.15 wt.%To 0.27 wt.%And 0.14 wt.%;Lo D decreased from 312.33μg/g,287.81μg/g to 123.01μg/g and 48.69μg/g respectively.This paper studies the key parameters that affect the self-absorption effect of LIBS,namely the excitation wavelength and atmosphere environment.The initial plasma of the weak self-absorption effect obtained by the 1064 nm laser,and then the argon atmosphere is used to regulate the plasma evolution process,which effectively eliminates the self-absorption effect and greatly improves the quantitative analysis of potassium and sodium elements in saline and alkaline soils Sensitivity and accuracy.This research provides a new method and idea for overcoming the self-absorption effect of LIBS,and will strongly promote the engineering and application of LIBS technology.