The Study On Mechanisms Of Signal Enhancement By Spatial Confinement And Discharge Assisted In Laser Induced Breakdown Spectroscopy

As a method of atomic spectrum analysis,laser induced breakdown spectroscopy(LIBS)has been widely studied and applied in quantitative analysis of elements and quantification of unknown components in materials.However,the detection limit is still not satisfied and the accuracy of quantitative analysis is not high enough due to the low reproducibility of spectral signals and the influence of matrix effect and self-absorption effect,which still hinders the development of LIBS technology.In this paper,several methods of spectral signal enhancement were studied based on LIBS,including the signal enhancement by spatial confinement and discharge assisted.Firstly,the confinement enhancement experiment was carried out by using the confinement rings with different relative permittivity and different size.Secondly,the spectral signal was enhanced by using the method of high-voltage discharge,the spectral data with abundant spectral lines and strong signal intensity are obtained.Finally,the abnormal signal point of the spectral signal was corrected by statistical and physical method,the influence of the peak shift and the abnormal intensity of peaks on the quantitative analysis were resolved,and a standardized data processing criterion was established.The partial least squares regression(PCA-PLS)method based on principal component analysis(PCA)was used to establish the mathematical model of fixed carbon,ash,volatile and other industrial parameters in standard coal samples.The detail studies are listed as follows:(1)For the first time,different confinement rings with various relative permittivity are utilized to enhance the spectra signal,and the relationship between the electromagnetic energy dissipation of the plasma with different relative permittivity confinement rings is investigated,experiments on different samples,such as silicon and coal,show that the enhancement of spectral signal is much more obvious when the high relative permittivity material(Nitrile-Butadiene Rubber)is used as the confinement ring,the enhancement factor of silicon and standard coal samples is about 2,the fluctuation of spectral signal RSD is reduced by 57%approximately,and the stable and repeatable spectral signals can be obtained.(2)The physical mechanisms of the interaction between the shockwave and the plasma are studied by using the confinement rings with different diameters and heights,the results show that the maximum enhancement factor reaches 2.06 when the height of the confinement ring is 3 mm and the diameter is 5 mm.Based on these phenomenon,the deep physical mechanism was discussed,the physical reasons for the signal enhancement related to the confinement size are investigated from the time scale.(3)The high-voltage pulse discharge assistant LIBS signal enhancement was studied;the enhancement factor and the number of spectral characteristic peaks are explored under different discharge voltage.Under the discharge voltage of 1 KV,1.5 KV,1.75 KV,2 KV,2.5 KV,2.8 KV,the spectral enhancement gradually increased,the number of characteristic peaks increases gradually.Using the enhanced spectral data,the prediction model was established.The regression coefficients R~2of ash,carbon content and volatile matter in coal samples are 0.99,0.99 and 0.98,respectively.(4)Combined with physics and statistics methods for preprocessing spectral data,it includes background elimination and abnormal peak position correction to reduce the effect of quantitative analysis results caused by experimental and systematic errors.Furthermore,the partial least squares regression method based on principal component analysis(PLS-PCA)was proposed,in the training of regression model,the methods of random extraction of data with return and multiple regression are used.Finally,the prediction model is determined.In this work,the strong and relatively stable spectral signals are obtained by using the method of the plasma spatial confinement and the confinement rings with different relative permittivity and size to restrict the energy dissipation of the plasma,the decrease of spectral RSD shows that repeatability of spectral signal has been improved.Finally,the LIBS signal is enhanced by high-voltage pulse discharge assistant,and the spectral signal with abundant characteristic lines and strong signal intensity are obtained.Considering the effect of self-absorption in the plasma,spectral analysis of all peak utilization by partial least squares(PLS)method based on principal component analysis(PCA-PLS)prediction model was established,prediction precision,quantitative analysis results shows good robustness.