Theory And Technology Of Self-Absorption-Free Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy(LIBS)is a new kind of laser-induced emission spectroscopy analysis technique.The qualitative and quantitative analysis of sample components is realized by analyzing the atomic,ion or molecular-based radiations in plasma generated by the interaction between the high-energy pulsed laser and sample.It has a great application potential in industrial production,environmental detection and biomedicine and other fields due to its advantages of rapid,simultaneous multi-element analysis,no or minimal need for sample preparation,real-time,in situ and contactless detection.The LIBS analysis of sample components depends on the wavelength and intensity of plasma radiation spectrum,that is,the wavelength characterizes the element composition of the measured sample,and the intensity determines the content of each component after calibration.Actually,the plasma generated by the high-energy pulse laser is a bulk light source with a certain optical size.When the photons generated by the internal spontaneous emission propagate to the outside,they will be absorbed by the same kind of atoms or ions that cause the emission in the moving path.This phenomenon is the socalled self-absorption effect(SA),which not only weakens the spectral line intensity and increases its full width at half maximum,but also causes saturation effects in univariate calibration,thus affecting the final quantitative analysis accuracy and limit of detection.Fundamentally,only when the radiated light passes through the plasma and escapes without obvious attenuation or scattering,the plasma can be considered as optically thin.In such a state,the self-absorption effect in spectral emission could be neglected,so as to achieve the ideal accuracy and limit of detection(LOD)of LIBS.In most practical applications,especially in real-time online conditions,the complexity of the interaction between laser and matter,the inhomogeneity of plasma and the instantaneity of plasma evolution make self-absorption a very complicated phenomenon,which has a great negative impact on the accuracy of the analysis results and directly affects the quality control in the production process.There are some theoretical models and precise experimental measurements to explain the self-absorption mechanism,and some methods have been proposed for SA effect correction and elimination,but these explanations and methods have their own limitations.Therefore,it is of great significance to further research on the physical mechanism of self-absorption effect and develop a more universal and reliable self-absorption-free technology for improving the precision and accuracy of LIBS quantitative analysis.This can further develop the application of LIBS in industrial production and environmental detection,and improve the economic and social benefits of the current industry.This article focuses on the key scientific and technical issues of self-absorption-free in LIBS measurement,mainly from the following two aspects.Firstly,the mechanism research of self-absorption generation and evolution,the quantitative relationship between self-absorption and parameters of corresponding transition spectral lines and plasma characteristic parameters.Secondly,developed the theory and technology of self-absorption-free laser-induced breakdown spectroscopy(SAF-LIBS),and evaluated its quantitative analysis performance.The main contents of this paper include:1.Introduced the development of LIBS,its basic principle and the bottlenecks and reasons for bottlenecks encountered in current applications.The generation of self-absorption effect in LIBS,its adverse effects and existing elimination methods were analyzed.2.The mechanism of self-absorption generation and evolution,and the quantitative relationship between self-absorption and parameters of corresponding transition spectral lines,plasma characteristic parameters were researched by exploring the emission and absorption processes of plasma radiation.Combining this quantitative relationship with dual-wavelength differential imaging technique and Abel inversion theory,the quantification of self-absorption of plasma radiation lines has been obtained.The evolution and quantification of the self-absorption effect in plasma of aluminium and copper alloys were studied experimentally.The results showed that self-absorption is positively correlated with transition probability,degeneracy of upper level,wavelength,number density and length of the absorption path,and is negatively correlated with lower energy level.Note that the relationship between self-absorption and temperature varies with energy level.SA increases with the decrease of plasma temperature if the lower level is in the ground state,while decreases with the decrease of plasma temperature if the lower level is in a relatively high excited state.Based on the above theoretical research,a self-absorption quantification method was proposed to quantify the self-absorption degree of spectral lines,in which plasma characteristics including electron temperature,elemental concentration ratio,and absolute species number density can be deduced directly.Experimental results performed on aluminum-lithium alloy showed that the average electron temperatures was 0.965 e V,which is close to 0.990 e V that obtained by the traditional Boltzmann plot with self-absorption correction.The weight ratio wMg/wAl in the plasma was calculated as 0.0171,which is approximately coincident with the nominal value of 0.0168.These showed that the self-absorption used for practical quantitative analysis and plasma characteristic diagnosis.3.The theory and technology of self-absorption-free laser-induced breakdown spectroscopy(SAF-LIBS)were developed to directly capture the optically thin spectral line by matching the measured intensity ratios of doublet atomic lines with the theoretical one,which avoid introducing any extra modeling errors or experimental setups.The optically thin conditions are validated by comparing the linearity of Boltzmann plots with the traditional self-absorption correction method and evaluating the SA coefficients.Univariate quantitative analysis results showed that,compared with ordinary LIBS,the linearity of the calibration curve was improved from 0.86 to 0.98 and the average absolute error of composition measurement was reduced from 1.2% to 0.13%.For SAF-LIBS,the limitations of concentration and laser energy were certified to be 19.5% and larger than 21 m J,respectively.4.A resonance/non-resonance doublet-based SAF-LIBS technique was developed.The quantitative analysis of Cu element shows that this technique can effectively avoid the influence of self-absorption effect and expand the measurement range of quantitative elemental analysis.The nonlinear LIBS calibration and the linear multi-segment SAF-LIBS calibration were established for the samples with Cu content in the range of 0.01-0.05%,0.1-1%,3-30%,and the correlation coefficients R2 of all these curves were greater than 0.99.Univariate quantitative analysis results showed that the absolute measurement error values of tablets with copper content of 0.25% and 10% were 0.01% and 0.1%,respectively.The applicable measurement range for copper was estimated to be within 50.7%,while the LOD was of 1.35 ppm.5.A rapid spectral line selection criterion for SAF-LIBS was proposed.The theoretical analysis shows that the evolution trend of doublet intensity ratio is monotonous under the assumptions that the plasma is uniform and in local thermal equilibrium and the area density is a constant,which was also confirmed by the experimental results of Cu and Al.Thus,a rapid spectral line selection criterion for SAF-LIBS applications was derived.Only when the doublet intensity ratios measured at the initial and final stages of plasma induced by the boundary sample with the highest element content lie on both sides of the theoretical ratio,the doublet lines can reach quasi-optically thin during plasma evolution and are suitable for SAF-LIBS measurements.The main innovations of this thesis include:Principle innovation:1.The mechanism of self-absorption generation and evolution,the quantitative relationship between self-absorption and parameters of corresponding transition spectral lines and plasma characteristic parameters were investigated.Combining this quantitative relationship with dual-wavelength differential imaging technique and Abel inversion theory,the quantification of self-absorption of plasma radiation lines has been obtained.2.The theory of SAF-LIBS and the rapid selection of spectral lines were proposed based on the theoretical research of the optically thin plasma criterion and the evolution trend of the double-line intensity ratio.Technological innovation:1.A self-absorption quantification method was proposed to quantify the self-absorption degree of spectral lines,in which plasma characteristics including electron temperature,elemental concentration ratio,and absolute species number density can be deduced directly by quantifying the degree of self-absorption through the full width at half maximum of spectral lines.2.The technology of SAF-LIBS was developed to directly capture the quasi-optically thin spectral line by matching the measured intensity ratios of doublet atomic lines with the theoretical one.3.A resonance/non-resonance doublet-based SAF-LIBS technique was developed.This technique can expand the measurement range of quantitative elemental analysis and realizes the accurate content analysis of trace elements by combining the choice of resonance and non-resonance doublet lines.