Research On Quantitative Analysis By Calibration-free Laser-induced Breakdown Spectroscopy

Laser induced breakdown spectroscopy(Laser-Induced Breakdown Spectroscopy,LIBS)is a typical optical emission spectroscopy method.It uses laser-induced plasma from samples as the light source.By measuring the emission lines from these plasmas,the qualitative or quantitative element information of the tested samples can be obtained.LIBS has many advantages,such as no need for sample preparation,remote non-contact analysis,multi-element simultaneous analysis,etc.In recent 40 years,LIBS technology has been widely used in metallurgical analysis,material classification,food detection and many other fields.However,due to the influence of matrix effect on LIBS,it is necessary to draw calibration curve by standard samples when using the traditional calibration curve method,which greatly limits the advantage of LIBS.Therefore,when the method of Calibration-Free Laser-induced Breakdown Spectroscopy(CF-LIBS),which does not need calibration curves,was proposed in 1999,it received wide attention immediately.CF-LIBS method is based on an ideal plasma light source model,which is optical thin and in the state of local thermal equilibrium(LTE).In this ideal state,the intensity of emitted spectral line of various species in the plasma is in accordance with Boltzmann distribution.Therefore,intensity ratios of spectral lines of different species can reflect the concentration ratio of corresponding species.CF-LIBS does not need to draw calibration curves,for matrix elements are also included in the calculation.Thus the matrix effect are fundamentally avoided.Because it is not necessary to prepare the standard sample,CF-LIBS has great value in the field of cultural relic identification,radioactive sample detection,melting metal on-line monitoring and so on.CF-LIBS has great advantages in theory,but there are many limitations in practical application.For example,the laser-induced plasma emission has some problems,such as self-absorption,poor spectral repeatability,less identifiable spectral lines for some species and so on.The result is that the accuracy of CF-LIBS is lower than that of traditional calibration method.For these problems,researchers have proposed a number of correction methods.Among them,the standard reference line method is a simple and reliable one.And it is the main research method disscussed in this thesis.This thesis focuses on realizing the rapid quantitative analysis by CF-LIBS,in order to promote the practicality of CF-LIBS method.The development of LIBS and CF-LIBS is briefly reviewed,the characteristics of laser-induced plasma and the calculation method of CF-LIBS are discussed,and the experimental study of CF-LIBS and the corresponding data processing method are carried out.Spectral response coefficient and its influence on quantitative analysis were studied;the branching ratio method was studied,which can quickly screen a large number of spectral lines,deciding whether the spectral line has self-absorption or not;the selection of standard reference lines for various species was studied;the computer program for quantitative analysis of CF-LIBS was written,and the LIBS spectra of Ni-based samples were quantitatively analyzed.Good results were obtained.The main work of this thesis is summarized as follows:(1)The important influence of absolute spectral response efficiency on CF-LIBS method is proposed and a device for measuring the absolute spectral response efficiency of LIBS system is built.(2)A fast method of judging the self-absorption of emission lines by using the spectral line branching ratios is proposed,and the on-line spectral response efficiency of LIBS system measuring method is developed.(3)The LIBS measurement of solid pure metals samples was carried out,which provides standard reference lines for CF-LIBS calculation.(4)A computer program for automatic processing of measured spectral files using CF-LIBS method was developed,from which quantitative analysis results could be obtained quickly.